Shia–Sunni divide
After the death of Muhammad in 632, a group of Muslims, who would come to be known as the Sunnis, believed that Muhammad's successor as caliph of the Islamic community should be Abu Bakr, whereas a second group of Muslims, who would come to be known as the Shias, believed that his successor should have been Ali. This dispute spread across various parts of the Muslim world, which eventually led to the Battle of Jamal and Battle of Siffin. Sectarianism based on this historic dispute intensified greatly after the Battle of Karbala, in which Husayn ibn Ali and some of his close partisans, including members and children of the household of prophet (Ahl al bayt), were killed by the ruling Umayyad Caliph Yazid I, and the outcry for revenge divided the early Islamic community, albeit disproportionately, into two groups, the Sunni and the Shia. This is known today as the Islamic schism. The present demographic breakdown between the two denominations is difficult to assess and varies by source, with most approximations stating that roughly 90% of the world's Muslims are Sunni and 10% are Shia; with about 85% of Shias belonging to the Twelver  tradition, and the rest divided between other small groups. Sunnis are a majority in almost all Muslim communities around the world. Shia make up the majority of the citizen population in Iran, Iraq, Bahrain, and Azerbaijan, as well as being a minority in Pakistan, Lebanon, Saudi Arabia, Syria, Yemen, Nigeria, Afghanistan, Chad, Turkey and Kuwait. Today there are differences in religious practice, traditions, and customs, often related to jurisprudence. Although all Muslim groups consider the Quran to be divine, Sunni and Shia have different opinions on hadith. In recent years, the Sunni–Shia divide has been increasingly marked by conflict, particularly the Iran–Saudi Arabia proxy conflict. Sectarian violence persists to this day from Pakistan to Yemen and is a major element of friction throughout the Middle East and South Asia. Tensions between communities have intensified during power struggles, such as the Bahraini uprising, the Iraqi Civil War, the Syrian Civil War, as well as the War in Iraq (2013–2017), during which the self-styled Islamic State of Iraq and Syria launched a persecution of Shia. Numbers
Sunni Muslims are the vast majority of Muslims in most Muslim communities in Central Asia (including China), Europe (including Russia and the Balkans), South Asia, Southeast Asia, Africa, Arab World, Turkey and among Muslims in the United States. Shia Muslims make up approximately 10% of the Muslim population. In Shi'i Islam itself, about 85% are Twelver, and in Twelver Shia the overwhelming majority are of the Usuli school. In Iran -- a Shia country since the 16th century -- Shia make up the majority (around 90%). They are also a majority in Azerbaijan (around 65%), Iraq (around 55%) and Bahrain (around 60% of the citizens, excluding expatriates). Shia communities are also found in Yemen where a large minority of the population are Shia (mostly of the Zaidi sect), according to the UNHCR. Sources put the numbers of Shia in Yemen at 25–30%. About 10% of Turkey's population belong to the Alevi sect of Shi'i Islam. The Shia constitute around 20% of Kuwait, 45% of the Muslim population in Lebanon, 10% of Saudi Arabia, 12% of Syria (mostly of the Alawite sect), and 10% of Pakistan. Around 10% of Afghanistan, less than 5% of the Muslims in Nigeria, and around 5% of population of Tajikistan are Shia. India has as many Shia ("potentially") as there are in Iraq. Scholar Vali Nasr argues that numbers/percentages of how many Sunni and Shia there are, are not exact because "in much of the Middle East it is not convenient" to have exact numbers, "for ruling regimes in particular". Differences in beliefs and practices

Successors of Muhammad

Mahdi

The Mahdi is the prophesied redeemer of Islam. While Shia and Sunnis differ on the nature of the Mahdi, many members of both groups believe that the Mahdi will appear at the end of the world to bring about a perfect and just Islamic society. In Shia Islam, "the Mahdi symbol has developed into a powerful and central religious idea." Twelvers believe the Mahdi will be Muhammad al-Mahdi, the twelfth Imam returned from the Occultation, where he has been hidden by Allah since 874. Mainstream Sunnis beliefs are somewhat different. The Mahdi forms an important component of Sunni eschatology, his appearance being considered the last of the minor signs of the Day of Judgment before its major signs. They believe the Mahdi will be a descendant of Muhammad named Muhammad, and will revive the faith. Hadith
The Shia accept some of the same hadiths of Muhammad used by Sunnis as part of the sunnah and the basis of divine law and religious practice. In addition, they consider the sayings of Ahl al-Bayt that are not attributed directly to Muhammad as hadiths. Shia do not accept many Sunni hadiths unless they are also recorded in Shia sources or the methodology of how they were recorded can be proven. Also, some Sunni-accepted hadiths -- for example by Aisha or Abu Hurairah -- are less favored by Shia (Aisha's opposed Ali and Abu Hurairah is considered an enemy of Ali and according to Shia, only a Muslim for four years of his life before Muhammad's death. Although he accompanied Muhammad for only four years, he managed to record ten times as many hadiths as Abu Bakr and Ali each). Shiism and Sufism
Shiism and Sufism are said to share a number of hallmarks: Belief in an inner meaning to the Quran, special status for some mortals (saints for Sufi, Imams for Shia), as well as veneration of Ali and Muhammad's family. Pillars of faith
The Five Pillars of Islam (Arabic: أركان الإسلام) is the term given to the five duties incumbent on every Muslim, and are held by both Sunni and Shia. These duties are Shahada (profession of faith), Salat (prayers), Zakāt (giving of alms), Sawm (fasting, specifically during Ramadan) and Hajj (pilgrimage to Mecca). In addition,  Shia theology has two concepts that define religion as a whole. There are Roots of Religion (Usūl al-Dīn) and Branches of Religion (Furu al Din). Practices
Many distinctions can be made between Sunnis and Shiaīs through observation alone:

Salat

When prostrating during ritual prayer known as Salah (one of the five pillars of Islam), Shia place their forehead onto a piece of naturally occurring material -- most often a clay tablet (mohr), soil (turbah) from Karbala, the place where Hussein ibn Ali was martyred -- instead of directly onto a prayer rug. There are five salat prayers at different times of the day, but unlike Sunni, some Shia combine two sets of the prayers, (1+2+2, i.e. fajr on its own, Dhuhr with Asr and Maghrib with Isha')  praying five times a day but with a very small break in between the prayer, instead of five prayers with some gap between them as required by Sunni schools of law. Shia and the followers of the Sunni Maliki school hold their hands at their sides during prayer while Sunnis of other schools cross their arms (right over left) and clasp their hands; it is commonly held by Sunni scholars (especially of the Maliki school) that either is acceptable. Mut'ah and Misyar

The Twelver branch of Shia Islam permits Nikah mut‘ah—fixed-term temporary marriage. The practice is not allowed within the Sunni community, nor within the Ismaili Shia or the Zaidi Shia, who consider it planned and agreed fornication rather than marriage. These schools believe that Mutah was permitted until Umar forbade it during his rule. (Mutah is not the same as Misyar marriage or  'Arfi marriage, which has no date of expiration and is permitted by some Sunnis. A Misyar marriage differs from a conventional Islamic marriage in that the man does not have financial responsibility of the woman by her own free will. The man can divorce the woman whenever he wants to in a Misyar marriage.) Hijab and dress

Both Sunni and Shia women wear the hijab. Devout women of the Shia traditionally wear black as do some Sunni women in the Persian Gulf. Some Shia religious leaders also wear a black robe. Mainstream Shia and Sunni women wear the hijab differently. Some Sunni scholars emphasize covering of all body including the face in public whereas some scholars exclude the face from hijab. Shia believe that the hijab must cover around the perimeter of the face and up to the chin. Like Sunnis, some Shia women, such as those in Iran and Iraq, use their hand to hold the black chador, in order to cover their faces when in public. Given names
Muslim are often named after famous early Muslims, so that given names of Shia are often derived from the names of Ahl al-Bayt. In particular, the names Fatema, Zaynab, Ali, Abbas, Hassan and Hussain are disproportionately common among Shia; while Umar, Uthman, Abu Bakr, Aisha are very common among Sunnis, but very rare -- if not virtually absent -- among Shia. Pilgrimages
The pilgrimage to Mecca, known as hajj, is one of the pillars of Islam for both Sunnis and Shi‘ites, but Shia have many other holy sites they make pilgrimages (ziyarat) to. Among them are  Al-Baqi Cemetery near Medina, Cairo, in Egypt, Najaf and Karbala, in Iraq, and Qom and Mashhad, in Iran. Early and pre-modern history

The origin of Shia Islam initially arose in response to the succession to Muhammad, and whether Ali ibn Abi Talib or a more experienced member of the Quraysh tribe should succeed. The concept of Shi'ism further crystallized around events at the Battle of Karbala (680 CE), where Husayn ibn Ali, the son of Ali and grandson of the prophet, was killed alongside many of his supporters. Thus a political split became a far more personal one, marked by blood feud, and a cause for further divergence. Even so, by the thirteenth to fourtheenth century, Sunni and Shiite practices remained highly intertwined and figures today commonly associated with Shia Islam, such as Ali and Jafar al-Sadiq, played an almost universal role for Muslim believers to understand the unseen (Al-Ghaib). Abbasid era

The Umayyads were overthrown in 750 by a new dynasty, the Abbasids. The first Abbasid caliph, As-Saffah, recruited Shia support in his campaign against the Umayyads by emphasising his blood relationship to Muhammad's household through descent from his uncle, ‘Abbas ibn ‘Abd al-Muttalib. The Shia also believe that he promised them that the Caliphate, or at least religious authority, would be vested in the Shia Imam. As-Saffah assumed both the temporal and religious mantle of Caliph himself. He continued the Umayyad dynastic practice of succession, and his brother al-Mansur succeeded him in 754. Ja'far al-Sadiq, the sixth Shia Imam, died during al-Mansur's reign, and there were claims that he was murdered on the orders of the caliph. (However, Abbasid persecution of Islamic scholars was not restricted to the Shia. Abū Ḥanīfa, for example, was imprisoned by al-Mansur and tortured.) Shia sources further claim that by the orders of the tenth Abbasid caliph, al-Mutawakkil, the tomb of the third Imam, Hussein ibn Ali in Karbala, was completely demolished, and Shia were sometimes beheaded in groups, buried alive, or even placed alive within the walls of government buildings still under construction. The Shia believe that their community continued to live for the most part in hiding and followed their religious life secretly without external manifestations. Iraq

Many Shia Iranians migrated to what is now Iraq in the 16th century. "It is said that when modern Iraq was formed, some of the population of Karbala was Iranian". In time, these immigrants adopted the Arabic language and Arab identity, but their origin has been used to "unfairly cast them as lackeys of Iran". However many of these Shia come from Sayyid families with origins in tribes from Iraq, Lebanon and Bahrain, one of said tribes being al-Musawi, and two prevalent families that are descended from it and lived in Iran for some time before settling in Iraq are the al-Qazwini and al-Shahristani families. Other Iraqi Shia are ethnic Arabs with roots in Iraq as deep as those of their Sunni counterparts. Persia

Shafi'i Sunnism was the dominant form of Islam in most of Iran until rise of the Safavid Empire although a significant undercurrent of Ismailism and a very large minority of Twelvers were present all over Persia. The Sunni hegemony did not undercut the Shia presence in Iran. The writers of the Shia Four Books were Iranian, as were many other scholars. According to Morteza Motahhari:

Pre-Safavid
The domination of the Sunni creed during the first nine Islamic centuries characterizes the religious history of Iran during this period. There were some exceptions to this general domination which emerged in the form of the Zaidis of Tabaristan, the Buwayhid, the rule of the Sultan Muhammad Khudabandah (r. 1304–1316) and the Sarbedaran. Nevertheless, apart from this domination there existed, firstly, throughout these nine centuries, Shia inclinations among many Sunnis of this land and, secondly, Twelver and Zaidi Shiism had prevalence in some parts of Iran. During this period, the Shia in Iran were nourished from Kufa, Baghdad and later from Najaf and Al Hillah. Shia were dominant in Tabaristan, Qom, Kashan, Avaj and Sabzevar. In many other areas the population of Shia and Sunni was mixed. The first Zaidi state was established in Daylaman and Tabaristan (northern Iran) in 864 by the Alavids; it lasted until the death of its leader at the hand of the Samanids in 928. Roughly forty years later the state was revived in Gilan (north-western Iran) and survived under Hasanid leaders until 1126. After which from the 12th–13th centuries, the Zaidis of Daylaman, Gilan and Tabaristan then acknowledge the Zaidi Imams of Yemen or rival Zaidi Imams within Iran. The Buyids, who were Zaidi and had a significant influence not only in the provinces of Persia but also in the capital of the caliphate in Baghdad, and even upon the caliph himself, provided a unique opportunity for the spread and diffusion of Shia thought. This spread of Shiism to the inner circles of the government enabled the Shia to withstand those who opposed them by relying upon the power of the caliphate. Twelvers came to Iran from Arab regions in the course of four stages. First, through the Asharis at the end of the 7th and during the 8th century. Second through the pupils of Sabzevar, and especially those of Al-Shaykh Al-Mufid, who were from Rey and Sabzawar and resided in those cities. Third, through the school of Hillah under the leadership of Al-Hilli and his son Fakhr al-Muhaqqiqin. Fourth, through the scholars of Jabal Amel residing in that region, or in Iraq, during the 16th and 17th centuries who later migrated to Iran. On the other hand, the Ismaili da‘wah ("missionary institution") sent missionaries (du‘āt, sg. dā‘ī) during the Fatimid Caliphate to Persia. When the Ismailis divided into two sects, Nizaris established their base in northern Persia. Hassan-i Sabbah conquered fortresses and captured Alamut in 1090. Nizaris used this fortress until the Mongols finally seized and destroyed it in 1256. After the Mongols and the fall of the Abbasids, the Sunni Ulama suffered greatly. In addition to the destruction of the caliphate there was no official Sunni school of law. Many libraries and madrasahs were destroyed and Sunni scholars migrated to other Islamic areas such as Anatolia and Egypt. In contrast, most Shia were largely unaffected as their center was not in Iran at this time. For the first time, the Shia could openly convert other Muslims to their movement. Several local Shia dynasties like the Marashi and Sarbadars were established during this time. The kings of the Kara Koyunlu dynasty ruled in Tabriz with a domain extending to Fars and Kerman. In Egypt the Fatimid government ruled. Muhammad Khudabandah, the famous builder of Soltaniyeh, was among the first of the Mongols to convert to Shiaism, and his descendants ruled for many years in Persia and were instrumental in spreading Shī‘ī thought. Sufism played a major role in spread of Shiism in this time. Post-Safavid
Ismail I initiated a religious policy to recognize Shiism as the official religion of the Safavid Empire, and the fact that modern Iran and Azerbaijan remain majority-Shia states is a direct result of Ismail's actions. Unfortunately for Ismail, most of his subjects were Sunni. He thus had to enforce official Shiism violently, putting to death those who opposed him. Under this pressure, Safavid subjects either converted or pretended to convert, but it is safe to say that the majority of the population was probably genuinely Shia by the end of the Safavid period in the 18th century, and most Iranians today are Shia, although there is still a Sunni minority. This led to a wide gap between Iran and its Sunni neighbors, particularly its rival, the Ottoman Empire, in the wake of the Battle of Chaldiran. This gap continued until the 20th century. Hejaz
In the holy cities of Mecca and Medina, where Muslims, including Shia, perform Hajj (one of the pillars of Islam), tensions between Shia and Sunni have waxed and waned. Historian Martin Kramer writes that both Sunni and Shia spread "farfetched" libelous rumours about the other sect — Sunnis that Shia defiled the Ka‘bah with excrement, and Shia that Sunni considered he lives of Shi‘ite pilgrims to be "forfeit" in a holy shrine where in fact "all forms of strife and bloodshed are forbidden". According to English explorer Richard Francis Burton, a non-Muslim who journeyed to Mecca in disguise in 1853, when a Shi‘ite performs hajj, 

But in "the late Ottoman years" toleration had reached a level were Shi‘ites observed Muharram in Jidda (65 km from Mecca) openly. An Iranian Shi‘ite on hajj in 1885 reported:

Levant

The Shia faith in the Levant started spreading during the Hamdanid rule, which commenced in the start of the 10th century. It was followed by the Mirdasid Shi'ite emirate in the 11th century, with both the emirates centered at Aleppo. The general observations recorded by Muslim travellers passing through the Levant during the tenth and eleventh centuries, notably al-Maqdisi in his geographical works, “The best divisions in the knowledge of the regions”, as well as Ibn Jubayr, indicate that Shia Muslims made up the majority of the populations of the regions of the Levant during this era, notably in the cities of Damascus, Tiberias, Nablus, Tyre, Homs and Jabal Amel. In addition to the account of Nasir Khusraw who visited Jerusalem in the year 1045 AD and reported: "The population of Jerusalem is about 20,000, the populace being mostly Shi'a Muslims". However, with the advent of the Zengids and Ayyubids, the population of Shia dwindled greatly due to conversion and migrations. In 1305, the Sunni Mamelukes carried out a grand campaign to erase the Shiite dominance in the coastal mountains of Lebanon. This campaign forced most of the Shiites to disperse, with some fleeing south to Jabal Amel and some to the Bekaa, while a very small portion of them took on the practice of Taqiyya until the Ottomans came in 1517. Many Shia in the Levant were killed for their faith. One of these was Muhammad Ibn Makki, called Shahid-i Awwal (the First Martyr), one of the great figures in Shia jurisprudence, who was killed in Damascus in 1384. Shahab al-Din Suhrawardi was another eminent scholar, killed in Aleppo on charges of cultivating Batini teachings and philosophy. On 21 April 1802, about 12,000 Wahhabi Sunnis under the command of Abdul-Aziz bin Muhammad, the second ruler of the First Saudi State attacked and sacked Karbala, killed between 2,000 and 5,000 inhabitants and plundered the tomb of Husayn ibn Ali, grandson of Prophet Muhammad and son of Ali ibn Abi Talib, and destroyed its dome, seizing a large quantity of spoils, including gold, Persian carpets, money, pearls, and guns that had accumulated in the tomb, most of them donations. The attack lasted for eight hours, after which the Wahhabis left the city with more than 4,000 camels carrying their plunder. Caucasus region

The Sack of Shamakhi took place on 18 August 1721, when 15,000 Sunni Lezgins, of the Safavid Empire, attacked the capital of Shirvan province, Shamakhi (in present-day Azerbaijan Republic), massacred between 4,000 and 5,000 of its Shia population and ransacked the city. India

Kashmir

Sunni razzias (raids) which came to be known as Taarajs, virtually devastated the Shi'i community. History records 10 such Taarajs, or Taraj-e-Shia, between the 15th and 19th centuries in 1548, 1585, 1635, 1686, 1719, 1741, 1762, 1801, 1830, 1872. During these raids,  the Shia habitations of the Kashmir region of India were slaughtered and their libraries burnt,  their sacred sites desecrated and plundered. Mughal Empire
Shia in India faced persecution by some Sunni rulers and Mughal Emperors which resulted in the killings of Shia scholars like Qazi Nurullah Shustari (also known as Shaheed-e-Thaalis, the third Martyr) and Mirza Muhammad Kamil Dehlavi (also known as Shaheed-e- Rabay, the fourth Martyr) who are two of the five martyrs of Shia Islam. Shia in Kashmir in subsequent years had to pass through the most atrocious period of their history. 20th century
Sunni–Shia clashes also occurred occasionally in the 20th century in India, particularly between 1904 and 1908. These clashes revolved around the public cursing of the first three caliphs by Shia and the praising of them by Sunnis. To put a stop to the violence, public demonstrations were banned in 1909 on the three most sensitive days: Ashura, Chehlum and Ali's death on 21 Ramadan. Intercommunal violence resurfaced in 1935–36 and again in 1939 when many thousands of Sunni and Shia defied the ban on public demonstrations and took to the streets. Shia are estimated to be 10–15% of the Muslim population in India and Pakistan and less than 1% of Muslim population in Bangladesh, although the total number is difficult to estimate due to the intermingling between the two groups and practice of taqiyya by Shia. Modern history

1919–1979
At least one scholar sees the period from the collapse of the Ottoman Empire through the decline of Arab nationalism as a time of relative unity and harmony between traditionalist Sunni and Shia Muslims. A unity brought on by a feeling of being under siege from a common threat, i.e. secularism -- first of the European colonial variety and then Arab nationalist. An example of Sunni–Shia cooperation was the Khilafat Movement which swept South Asia following the defeat of the Ottoman Empire, the seat of the Caliphate, in World War I. Shia scholars "came to the caliphate's defence" by attending the 1931 Caliphate Conference in Jerusalem. This was despite the fact that  theologically Shia
held that Imams, not caliphs, were the  successors to Muhammad, and that the caliphate was "the flagship institution" of Sunni, not Shia, authority. This has been described as unity of traditionalists in the face of the twin threats of "secularism and colonialism." In 1938,  Allama Muhammad Taqi Qummi travelled to Cairo for the purpose of rebuilding/strengthening Islamic unity at Al-Azhar University. His efforts,  including connecting with scholars such as Mahmud Shaltut and Seyyed Hossein Borujerdi, led to the founding of Dar-al-Taghrib (community for reforming unity between Sunni and Shia Muslims). Another example of unity was a fatwā issued by the rector of Al-Azhar University, Mahmud Shaltut, recognizing Shia Islamic law as the fifth school of Islamic law. In 1959, al-Azhar University in Cairo, the most influential center of Sunni learning, authorized the teaching of courses of Shia jurisprudence as part of its curriculum. Post-Iranian Revolution era

The leader of the Islamic revolution in Iran, Ayatollah Ruhollah Khomeini saw the revolution as an Islamic, not a Shi'i Islamic revolution. His revolution was (he hoped) just the first and would spread throughout the Muslim world, with Iran serving as "the base for a global Islamic movement", and himself as the leader, just as Lenin and Trotsky had hoped the Bolshevik Revolution would be only the first communist revolution. The year of the Revolution was "one of great ecumenical discourse", and shared enthusiasm by both Shia and Sunni Islamists. Khomeini endeavored to bridge the gap between Shiites and Sunnis by declaring it permissible for Twelvers to pray behind Sunni imams and by forbidding criticizing the Caliphs who preceded Ali—an issue that had caused much animosity between the two groups. He focused on issues that united Muslims — anti-Imperialism, anti-Zionism, anti-Americanism, and "the battle against outsiders" — rather than "religious questions that were likely to divide them". In addition, Khomeini designated the period of Prophet's Birthday celebrations from 12th to the 17th of Rabi Al-Awwal as the Islamic Unity Week, (there being a gap in the dates of when Shiites and Sunnis celebrate Muhammad's birthday). Outbreak of sectarianism
Sunni–Shia unity did not last long after the Iranian Revolution, and strife between the two sects took a major upturn, the "Shia awakening and its instrumentalisation by Iran" as leading to a "very violent Sunni reaction", starting first in Pakistan before spreading to "the rest of the Muslim world, without necessarily being as violent." As of 2008, "Azerbaijan is probably the only country where there are still mixed mosques and Shia and Sunnis pray together." Discord manifested itself in major and minor ways, from bombings that killed thousands, to cultural changes. Among the immediate causes of the violence was: 
the Islamic revolution in Iran
the 2003 American military intervention in Iraq 
These led to antipathy between the Islamic Republic of Iran and the Kingdom of Saudi Arabia, who mobilized supporters against the other, between Sunni  Muhammad Zia-ul-Haq (president of Pakistan, the country with the second largest Muslim population in the world and neighbor to Iran) and Shia Iranian supreme leader Ruhollah Khomeini, growth of sectarian militias, and the change in attitude of Sunni towards Shia from misguided brethren to heretics, a viewpoint spread not by "marginal extremists" but "senior Sunni Ulama". Examples
Hate speech against both Sunni and Shia began to be spread on satellite television and high-speed Internet starting in the mid-1990s. Fundamentalist Sunni clerics popularized slurs against Shia such as "Safawis" (from the Safavid empire, thus implying their being Iranian agents), or even worse rafidha (rejecters of the faith), and majus (Zoroastrian or crypto Persian). Militant Sunnis began naming their sons after historic enemies of Shi'i heroes (Muawiya -- enemy of the first Shi'i Imam Ali, and Yazid -- held responsible by Shia for killing Husayn ibn Ali) ("Breaking taboos against honoring the caliphs who had persecuted and killed members of the Prophet’s family";  "Eulogies" for these two Umayyad caliphs "became an important part of the new anti-Shia discourse".) Ashura was condemned as "a heathen spectacle" and an "affront to the memory" of the rightful caliphs;
and Shi'i Imams as "un-Islamic historical figures" whom all Sunnis should "actively reject". In turn, Shia religious scholars have "mocked and cursed" the first three caliphs and Aisha (Mohammed's youngest wife who fought against Ali). Explanations for growth in sectarianism 
Among the explanations for the increase are conspiracies by outside forces to divide Muslims, the recent Islamic revival and increased religious purity and consequent takfir, upheaval, destruction and loss of power of Sunni caused by the US invasion of Iraq, and sectarianism generated by Arab regimes defending themselves against the mass uprisings of the Arab Spring. Outside conspiracies
Many in the Muslim world explain the bloodshed as the work of conspiracies by outside forces—"the forces of hegemony and Zionism which aim to weaken [Arabs]" (Akbar Hashemi Rafsanjani and Yusuf al-Qaradawi), unspecified "enemies" (Iran president Mahmoud Ahmadinejad), or "oppressive pressure by the imperialist front." (Mahmoud Ahmadinejad). Some Western analysts assert that the US is practicing divide and rule strategy through the escalation of Sunni-Shia conflict. Nafeez Ahmed cites a 2008 RAND Corporation study for the American military which recommended "divide and rule" as a possible strategy whereby the US takes "the side of the conservative
Sunni regimes ... working with them against all Shiite empowerment movements in the Muslim world". On the other hand, the Pakistani Sunni jihadist organization Lashkar-e-Jhangvi has declared that it is the Shia of Pakistan and Iraq who are "'American agents' and the 'near enemy' in the global jihad against America". Christopher Davidson argues that the current crisis in Yemen is being “egged on” by the US, and could be part of a wider covert strategy to “spur fragmentation in Iran allies and allow Israel to be surrounded by weak states”. Islamic revival
Others (Martin Seth Kramer, Vali Nasr) lay the blame for the strife at a very different source, the unintended effects of the Islamic revival. Historian Martin Seth Kramer (writing circa mid-1990s), argues that the focus on alleging "plots" by outsiders and/or claims by one side that the issue is only with an extremist group on the other side (for example Wahhabism or Khomeinism), distracts from the seriousness of the problem: 

According to scholar Vali Nasr, as the Muslim world was decolonialised and Arab nationalism lost its appeal, religion filled its place. As religion became more important, so did a return to its fundamentals and a following of its finer points; differences once overlooked became deviations to be denouncing and fought, and there were many differences between Sunni and Shia. Fundamentalism blossomed and conflicts reasserted, in particular when Sunni followed the strict teachings of Sunni scholar Ibn Taymiyyah, who considered Shia apostates and who is held in high regard by Sunni Salafi. Iranian Islamic revolution
An indirect way the Islamic revival led to discord between the two major schools of Islam was through the Iranian Islamic revolution. The revolution was a direct result of the Islamic revival, led by an Islamist, Ayatollah Ruhollah Khomeini, who was very much in favor of Islamic unity, and "the leadership position that went with it". At first the revolution inspired and energized Islamist Muslims (both Shia and Sunni) everywhere, but it was a revolution in a predominantly Shi'i Muslim country, led by Shi'i Muslims, and serious rifts with Sunni Muslims soon developed. The revolution changed the Shia–Sunni power equation in Muslim countries "from Lebanon to India". It aroused the traditionally subservient Shia, to the alarm of traditionally dominant and very non-revolutionary Sunni. "Where Iranian revolutionaries saw Islamic revolutionary stirrings, Sunnis saw mostly Shia mischief and a threat to Sunni predominance." Notwithstanding the desire of Iran's leader, Khomeini, for Shia–Sunni unity, as an Islamist revolutionary flush with success that had surprised Iranians as well as the rest of the world, Khomeini now sought the overthrow of unworthy governments in Muslim-majority countries (which were all Sunni regimes, Khomeini's government being the only Shi'i-led country at the time). Pro-American monarchies in particular were high on that list, and at the very top was the Kingdom of Saudi Arabia, not only a Wahhabi state with a long tradition of anti-Shi'ism, but an "American lackey" and "unpopular and corrupt dictatorship" (in his view, especially after seeing the 1979 Grand Mosque seizure), ripe for revolution, like "a ripe apple ready to fall into" their hands. But Saudi Arabia was also spending billions of dollars every year funding Islamic schools, scholarships, and fellowships, mosques around the Sunni world. "Thousands of aspiring preachers, Islamic scholars, and activists from Nigeria to Indonesia went to Saudi Arabia to study, and many more joined Saudi-funded think tanks and research institutions." They "then spread throughout the Muslim world to teach" what they had learned  and "work at Saudi-funded universities, schools, mosques, and research institutions." Khomeini's attack was opposed not only by the Saudi royal family but by its many (Sunni) fundamentalist allies and benefactors throughout the Arab world. For them the House of Saud was very popular, a leader of Islamic revival. Saudi propaganda efforts proceeded to go after both Khomeini's Shia identity, and to "drive all possible wedges between Sunnism and Shiism". Another indirect effect (noted by political scientist Gilles Kepel), was that however religious the Saudi regime was already, in the immediate wake of Iran's Revolution it was motivated to further shore up its "religious legitimacy" with more strictness in religion (and with jihad in Afghanistan) to compete with the grassroots enthusiasm for Iran's Islamism. But this also meant moving in a more anti-Shia religious direction because (as mentioned above) Saudi's own native Sunni school of Islam (Wahhabism, like that of Ibn Taymiyyah), did not consider Shiism part of the diversity of Islam, but a heresy to be fought. This new strictness was spread among the thousands of students in Saudi funded schools and more importantly among the international Islamist volunteers who came to training camps in Peshawar Pakistan in the 1980s to learn to fight jihad against Marxist secularists in Afghanistan and went home to fight jihad in the 1990s. Both groups (especially in Iraq and Pakistan) saw Shia as the enemy. Other Sunni Muslim states -- Indonesia, Egypt -- also "quickly moved" to bolster their Islamic credentials and inoculate themselves from the fate of the shah, aware of the plans the Iranian Islamist revolutionaries had for their downfall. A number of incidents  convinced several Muslim heads of state (again, all Sunni) of Khomeini's contempt for  them and the need to "contain" him:  A delegation of Muslim heads of state that came to Tehran to mediate an end to the Iran-Iraq war was kept waiting for two hours before Khomeini appeared to make a ten minute untranslated statement seated while his visitors stood -- and then leaving;  a street in Tehran was named after the killer of the President of Egypt, Anwar Sadat; a threat by Khomeini to do to Pakistani President Zia ul-Haq-- a pious conservative Muslim seeking to Islamize Pakistan -- "what he had done to the Shah" if Zia mistreated the Shia in Pakistan, and on another occasion mockery of Zia's warning not to provoke a superpower by saying he, (Khomeini), had his own superpower – his being God while Zia's was the United States. Following the Iranian Revolution, "avowedly Shia political movements", often getting funding from the IRI, and  "pushing  specifically Shia political agendas", emerged in to 2015, Shia groups in Lebanon, Iraq, Syria, Yemen, supported by Iran. By 2015 they had won "important political victories" which have boosted Iran's regional influence. In Lebanon, Hezbollah, the Lebanese Shia militia and political movement is the "strongest political actor" in the country. Since the 2003 invasion of Iraq removed Saddam Hussein from power and instituted elected government, the Shia majority has dominated the parliament and its prime ministers have been Shia. In Syria, a Shia minority—the heterodox Alawi sect that makes up only about 13 percent of the population—dominate the upper reaches of the government, military and security services in Syria, and are the "backbone" of the forces fighting to protect the Bashar al-Assad regime in Syria's civil war. In Yemen, Houthi rebels have expanded their territory south of Saudi Arabia, and become the country's "dominant power". US invasion of Iraq

Among those blaming the US invasion of Iraq for the growth in sectarianism are Fawaz Gerges, who writes in his book ISIS: A History,

By destroying state institutions and establishing a sectarian-based political system, the 2003 US-led invasion polarized the country along Sunni-Shia lines and set the stage for a fierce, prolonged struggle driven by identity politics. Anger against the United States was also fueled by the humiliating disbandment of the Iraqi army and the de-Baathification law, which was first introduced as a provision and then turned into a permanent article of the constitution. Malise Ruthven writes that the post invasion de-Ba'athification by the US occupiers deprived Iraq of "the officer class and administrative cadres that had ruled under Saddam Hussein, leaving the field to sectarian-based militias". Many of officers joined the anti-Shia takfiri ISIL group. The US-led invasion also “tilted the regional balance of power decisively" in favor of Shia Iran, alarming Sunni and leading to talk of a “Shia Crescent”. Counter-revolutionary tactic

Marc Lynch in his book The New Arab Wars: Uprisings and Anarchy in the Middle East, argues that as old regimes or political forces sought to control "the revolutionary upsurge" of the Arab Spring, sectarianism became “a key weapon” to undermine unity among the anti-regime masses. Christians were pitted "against Muslims in Egypt, Jordanians against Palestinians in Jordan, and, above all, Sunnis against Shi’ites wherever possible.”

Relations by country and region

Iraq 

Shia–Sunni discord in Iraq starts with disagreement over the relative population of the two groups. The governing regimes of Iraq were composed mainly of Sunnis for nearly a century until the 2003 Iraq War, but according to most sources, the majority of the population is Shia. The CIA's World Factbook, estimates  Shia Arab Muslims as making up 60% of Iraqis, and Sunni muslims 37%. However, Sunni are split ethnically among Arabs, Kurds and Turkmen. Many Sunnis hotly dispute their minority status, (including ex-Iraqi Ambassador Faruq Ziada), and many believe Shia majority is "a myth spread by America". One Sunni belief shared by Jordan's King Abdullah as well as his then Defense Minister Shaalan is that Shia numbers in Iraq were inflated by Iranian Shia crossing the border. Shia scholar Vali Nasr believes the election turnout in summer and December 2005 confirmed a strong Shia majority in Iraq. The British, having put down a Shia rebellion against their rule in the 1920s, "confirmed their reliance on a corps of Sunni ex-officers of the collapsed Ottoman empire". The British colonial rule ended after the Sunni and Shia united against it. The Shia suffered indirect and direct persecution under post-colonial Iraqi governments since 1932, erupting into full-scale rebellions in 1935 and 1936. Shia were also persecuted during the Ba'ath Party rule, especially under Saddam Hussein. It is said that every Shia clerical family of note in Iraq had tales of torture and murder to recount. In 1969 the son of Iraq's highest Shia Ayatollah Muhsin al-Hakim was arrested and allegedly tortured. From 1979 to 1983 Saddam's regime executed 48 major Shia clerics in Iraq. They included Shia leader Mohammad Baqir al-Sadr and his sister. Tens of thousands of Iranians and Arabs of Iranian origin were expelled in 1979 and 1980 and a further 75,000 in 1989. The Shia openly revolted against Saddam following the Gulf War in 1991 and were encouraged by Saddam's defeat in Kuwait and by simultaneous Kurdish uprising in the north. However, Shia opposition to the government was brutally suppressed, resulting in some 50,000 to 100,000 casualties and successive repression by Saddam's forces. Iraq War
Some of the worst sectarian strife has occurred following the start of the Iraq War, and continues at least as of 2016. The war has featured a cycle of Sunni–Shia revenge killing—Sunni often used car bombs, while Shia favored death squads. As part of its rivalry with Iran, Saudi Arabia spent "tens of billions of dollars" helping Saddam Hussein's war effort. According to one estimate, as of early 2008, 1121 suicide bombers have blown themselves up in Iraq. Sunni suicide bombers have targeted not only thousands of civilians, but mosques, shrines, wedding and funeral processions, markets, hospitals, offices, and streets. Sunni insurgent organizations include Ansar al-Islam. Radical groups include Al-Tawhid Wal-Jihad, Jeish Muhammad, and Black Banner Organization. Takfir motivation for many of these killings may come from Sunni insurgent leader Abu Musab al-Zarqawi. Before his death Zarqawi was one to quote Muhammad ibn Abd al-Wahhab, especially his infamous statement urging followers to kill the Shia of Iraq, and calling the Shia "snakes". Another explanation found in his February 2004 open letter to supporters is that by attack  Shia he would provoke them to attack Sunnis and thus "awaken" Sunnis  who previously had not wanted a sectarian war to join his side. The "cunning" Shia planned to build a state "stretching from Iran through Iraq, Syria and Lebanon" to the Gulf kingdoms, but by attacking Shia in their "religious, political, and military depth" his jihadis would "drag" the Shia "into the arena of sectarian war", and leading them to "bare the teeth of the hidden rancor working in their breasts" and so "awaken the inattentive Sunnis as they feel imminent danger and annihilating death at the hands of theses Sabeans", i.e. Shia. An al-Qaeda-affiliated website posted a call for "a full-scale war on Shiites all over Iraq, whenever and wherever they are found." Suicide bombers continue to attack Iraqi Shia civilians, and the Shia ulama have in response declared suicide bombing as haraam (against God, or "forbidden"):

Some believe the war has strengthened the takfir thinking and may spread Sunni–Shia strife elsewhere. On the Shia side, in early February 2006 militia-dominated government death squads were reportedly "tortur[ing] to death or summarily" executing "hundreds" of Sunnis "every month in Baghdad alone," many arrested at random. According to the British television Channel 4, from 2005 through early 2006, commandos of the Ministry of the Interior which is controlled by the Badr Organization, and

The violence shows little sign of getting opposite sides to back down. Iran's Shia leaders are said to become "more determined" the more violent the anti-Shia attacks in Iraq become. One Shia Grand Ayatollah, Yousef Saanei, who has been described as a moderate, reacted to the 2005 suicide bombings of Shia targets in Iraq by saying the bombers were "wolves without pity" and that "sooner rather than later, Iran will have to put them down". In addition to Iran, Iraq has emerged as a major Shia government when the Twelvers achieved political dominance in 2005 under American occupation. The two communities have often remained separate, mingling regularly only during the Hajj pilgrimage in Mecca. In some countries like Iraq, Syria, Kuwait and Bahrain, communities have mingled and intermarried. Some Shia have complained of mistreatment in countries dominated by Sunnis, especially in Saudi Arabia, while some Sunnis have complained of discrimination in the Twelver-dominated states of Iraq and Iran. Iran 

Iran is unique in the Muslim world because its population is overwhelmingly more Shia than Sunni (Shia constitute 95% of the population) and because its constitution is theocratic republic based on rule by a Shia jurist. The founder of the Islamic Republic, Ayatollah Ruhollah Khomeini, supported good Sunni–Shia relations. However tension developed between Sunnis and Shia as a result of clashes over Iranian pilgrims and Saudi police at the hajj. Millions of Saudi adhere to the school of Salafism which is a branch of Sunni Islam. Inside Iran there have been complaints by Sunni of discrimination, particularly in important government positions. In a joint appearance with former Iranian president Akbar Hashemi Rafsanjani calling for Shia-Sunni unity, Sunni Shiekh Yusuf al-Qaradawi complained that no ministers in Iran have been Sunni for a long time, that Sunni officials are scarce even in the regions with majority of Sunni population (such as Kurdistan, or Balochistan) and despite the presence of Christian churches, as a prominent example of this discrimination. Although reformist President Mohammad Khatami promised during his election campaign to build a Sunni mosque in Tehran, none was built during his eight years in office. The president explained the situation by saying Supreme Leader Ayatollah Ali Khamenei would not agree to the proposal. As in other parts of the Muslim world, other issues may play a part in the conflict, since most Sunnis in Iran are also ethnic minorities. Soon after the 1979 revolution, Sunni leaders from Kurdistan, Balouchistan, and Khorassan, set up a new party known as Shams, which is short for Shora-ye Markaz-e al Sunaat, to unite Sunnis and lobby for their rights. But six months after that they were closed down, bank accounts suspended and had their leaders arrested by the government on charges that they were backed by Saudi Arabia and Pakistan. A UN human rights report states that:

Members of the 'Balochistan Peoples Front' claim that Sunnis are systematically discriminated against educationally by denial of places at universities, politically by not allowing Sunnis to be army generals, ambassadors, ministers, prime minister, or president, religiously insulting Sunnis in the media, economic discrimination by not giving import or export licenses for Sunni businesses while the majority of Sunnis are left unemployed. There has been a low level resistance in mainly Sunni Iranian Balouchistan against the regime for several years. Official media refers to the fighting as armed clashes between the police and "bandits," "drug-smugglers," and "thugs," to disguise what many believe is essentially a political-religious conflict. Revolutionary Guards have stationed several brigades in Balouchi cities, and have allegedly tracked down and assassinated Sunni leaders both inside Iran and in neighboring Pakistan. In 1996 a leading Sunni, Abdulmalek Mollahzadeh, was gunned down by hitmen, allegedly hired by Tehran, as he was leaving his house in Karachi. Members of Sunni groups in Iran however have been active in what the authorities describe as terrorist activities. Balochi Sunni Abdolmalek Rigi continue to declare the Shia as Kafir and Mushrik. These Sunni groups have been involved in violent activities in Iran and have waged terrorist attacks against civilian centers, including an attack next to a girls' school according to government sources. The "shadowy Sunni militant group Jundallah" has reportedly been receiving weaponry from the United States for these attacks according to the semi-official Fars News Agency. The United Nations and several countries worldwide have condemned the bombings. (See 2007 Zahedan bombings for more information)

Following the 2005 elections, much of the leadership of Iran has been described as more "staunchly committed to core Shia values" and lacking Ayatollah Khomeini's commitment to Shia–Sunni unity. Polemics critical of Sunnis were reportedly being produced in Arabic for dissemination in the Arab Muslim world by Hojjatieh-aligned elements in the Iranian regime. Sunni mosques are not allowed in the capital city of Tehran, and a number of Sunni mosques in other cities have been demolished, Sunni literature and teachings are banned in public schools and construction of new Sunni mosques and schools are banned. Syria 

Syria is approximately three quarters Sunni, but its government is predominantly Alawite, a Shia sect that makes up less than 13% of the population. Under Hafez al-Assad, Alawites dominated the Arab Socialist Ba'ath Party, a secular Arab nationalist party which had ruled Syria under a state of emergency from 1963 to 2011. Alawites are often considered a form of Shia Islam, that differs somewhat from the larger Twelver Shia sect. During the 20th century, an Islamic uprising in Syria occurred with sectarian religious overtones between the Alawite-dominated Assad government and the Islamist Sunni Muslim Brotherhood, culminating with the 1982 Hama massacre. An estimated 10,000 to 40,000 Syrians, mostly civilians, were killed by Syrian military in the city. During the uprising, the Sunni Muslim Brotherhood attacked military cadets at an artillery school in Aleppo, performed car bomb attacks in Damascus, as well as bomb attacks against the government and its officials, including Hafez al-Assad himself, and had killed several hundred. How much of the conflict was sparked by Sunni versus Shia divisions and how much by Islamism versus secular-Arab-nationalism, is in question, but according to scholar Vali Nasr the failure of the Ayatollah Khomeini and the Islamic Republic of Iran to support the Muslim Brotherhood against the Baathists "earned [Khomeini] the Brotherhood's lasting contempt." It proved to the satisfaction of the Brotherhood that sectarian loyalty trumped Islamist solidarity for Khomeini and eliminated whatever appeal Khomeini might have had to the MB movement as a pan-Islamic leader. Syria Civil War

The Syrian Civil War, though it started as a political conflict, developed into a struggle between the Alawite-dominated Army and government on the one hand, and the mainly Sunni rebels and former members of the regular army on the other. The casualty toll of the war's first three years has exceeded that of Iraq's decade-long conflict, and the fight has "amplified sectarian tensions to unprecedented levels". Rebel groups with 10,000s of Sunni Syrian fighters such as Ahrar ash-Sham, the Islamic Front, and al-Qaeda's al-Nusra Front, employ anti-Shia rhetoric and foreign Arab and Western Sunni fighters have joined the rebels. On the other side Shia from Hezbollah in Lebanon and from Asaib Ahl al-Haq and Kata'ib Hezbollah militias from Iraq have backed the Syrian government. "Even Afghan Shia refugees in Iran", driven from Afghanistan by Sunni extremism, have "reportedly been recruited by Tehran for the war in Syria". According to some reports, as of mid-2013, the Syrian Civil War has become "overtly sectarian" with the "sectarian lines fall most sharply" between Alawites and Sunnis. With the involvement of Lebanese Shia paramilitary group Hezbollah, the fighting in Syria has reignited "long-simmering tensions between Sunnis and Shi’ites" spilling over into Lebanon and Iraq. Bulgaria's ex-Ambassador Dimitar Mihaylov further claims that the current post-Arab Spring situation (encompassing ISIS, the Syrian civil war, Yemen, Iraq and others) represents a "qualitatively new" development in the history of Shi'a-Sunni dynamics. Historically, the inner rifts within Islamic ideology were to be hidden from the public sphere, while the new violent outbreaks highlight said rift in an obvious manner and is nourished by the two extremes of their mutual rivalry which will strongly affect both globally and regionally. Saudi Arabia 

While Shia make up roughly 10% of Saudi Arabia's population, they form a large portion of the residents of the Eastern Province—by some estimates a majority—where much of the petroleum industry is based. Between 500,000 and a million Shia live there, concentrated especially around the oases of Qatif and al-Hasa. The Majority of Saudi Shia belong to the sect of the Twelvers. The Saudi conflict of Shia and Sunni extends beyond the borders of the kingdom because of international Saudi "Petro-Islam" influence. Saudi Arabia backed Iraq in the 1980–1988 war with Iran and sponsored militants in Pakistan and Afghanistan who—though primarily targeting the Soviet Union, which had invaded Afghanistan in 1979—also fought to suppress Shia movements. Relations between the Shia and the Wahhabis are inherently strained because the Wahhabis consider the rituals of the Shia to be the epitome of shirk, or polytheism. In the late 1920s, the Ikhwan (Ibn Saud's fighting force of converted Wahhabi Bedouin Muslims) were particularly hostile to the Shia and demanded that Abd al Aziz forcibly convert them. In response, Abd al Aziz sent Wahhabi missionaries to the Eastern Province, but he did not carry through with attempts at forced conversion. In recent decades the late leading Saudi cleric, Abd al-Aziz ibn Abd Allah ibn Baaz, issued fatwa denouncing Shia as apostates, and according to Shia scholar Vali Nasr "Abdul-Rahman al-Jibrin, a member of the Higher Council of Ulama, even sanctioned the killing of Shia, a call that was reiterated by Wahhabi religious literature as late as 2002." Government policy has been to allow Shia their own mosques and to exempt Shia from Hanbali inheritance practices. Nevertheless, Shia have been forbidden all but the most modest displays on their principal festivals, which are often occasions of sectarian strife in the Persian Gulf region, with its mixed Sunni–Shia populations. According to a report by the Human Rights Watch:

And Amnesty International adds:

As of 2006 four of the 150 members of Saudi Arabia's "handpicked" parliament were Shia, but no city had a Shia mayor or police chief, and none of the 300 girls schools for Shia in the Eastern Province had a Shia principal. According to scholar Vali Nasr, Saudi textbooks "characterize Shiism as a form of heresy ... worse than Christianity and Judaism." Forced into exile in the 1970s, Saudi Shia leader Hassan al-Saffar is said to have been "powerfully influenced" by the works of Sunni Islamists of the Muslim Brotherhood and Jamaat-e-Islami and by their call for Islamic revolution and an Islamic state. Following the 1979 Iranian Revolution, Shia in Hasa ignored the ban on mourning ceremonies commemorating Ashura. When police broke them up three days of rampage ensued—burned cars, attacked banks, looted shops—centered around Qatif. At least 17 Shia were killed. In February 1980 disturbances were "less spontaneous" and even bloodier. Meanwhile, broadcasts from Iran in the name of the Islamic Revolutionary Organization attacked the monarchy, telling listeners, "Kings despoil a country when they enter it and make the noblest of its people its meanest ... This is the nature of monarchy, which is rejected by Islam." By 1993, Saudi Shia had abandoned uncompromising demands and some of al-Saffar's followers met with King Fahd with promises made for reform. In 2005 the new King Abdullah also relaxed some restrictions on the Shia. However, Shia continue to be arrested for commemorating Ashura as of 2006. In December 2006, amidst escalating tensions in Iraq, 38 high ranking Saudi clerics called on Sunni Muslims around the world to "mobilise against Shiites". A year later, Shia Grand Ayatollah Naser Makarem Shirazi is reported to have responded:

 Saudi Sunni
Another reflection of grassroots Wahhabi or Saudi antipathy to Shia was a statement by Saudi cleric Nasir al-Umar, who accused Iraqi Shia of close ties to the United States and argued that both were enemies of Muslims everywhere. Al-Qaeda
Some Wahabi groups, often labeled as takfiri and sometimes linked to Al-Qaeda, have even advocated the persecution of the Shia as heretics. Such groups have been allegedly responsible for violent attacks and suicide bombings at Shi'a gatherings at mosques and shrines, most notably in Iraq during the Ashura mourning ceremonies where hundreds of Shia were killed in coordinated suicide bombings, but also in Pakistan and Afghanistan. However, in a video message, Al-Qaeda deputy Dr Ayman al-Zawahiri directed Abu Musab al-Zarqawi, of Al-Qaeda in Iraq, not to attack civilian targets but to focus on the occupation troops. His call seems to have been ignored, or swept away in the increasing tensions of Iraq under occupation. Hajj
Every year, Muslims from all over the world attend the hajj pilgrimage in Mecca in Western Saudi Arabia. Shia had complained off and on of mistreatment by the Sunnis who ran Mecca and the hajj ceremonies. Following the advent of Saudi-Wahhabi rule over Mecca in 1924 tensions between Shia and Sunni increased. To the fury of Shia Muslims, the Wahhabi Sunnis demolished domes in the cemetery of Al-Baqi, near the Medina, "the reputed resting place of the Prophet Muhammad’s daughter Fatima and four of the Twelve Imams". In 1943, a Saudi religious judge ordered the beheading of an Iranian pilgrim "for allegedly defiling the Great Mosque with excrement" smuggled into "the mosque in his pilgrim’s garment". Saudi public opinion considered the crime unsurprising and the punishment just, Iranian were furious and demanded payment of an indemnity. Tensions lowered again during the 1960s, when pious/tradionalist Muslims set aside differences in the face of the rising popularity of Nasser's leftist Arab nationalism. Pilgrims from Iran (mostly Shia) rose in number from 12,000 in 1961 to 57,000 in 1972. In 1987, about seven years after the Iranian Revolution, Mecca became a site "of unprecedented carnage" when demonstrating Shia Iranian pilgrims clashed with Saudi security forces and over four hundred were killed. The Saudis and their supporters claimed violent Iranian demonstrators crushed themselves to death in a stampede of their own making. The Iranians and their sympathizers claimed the Saudis had conspired to provoke and then shoot Iranian pilgrims. The pilgrimage to Mecca, where violence is forbidden, had itself become a point of confrontation between rival visions of Islam. Lebanon 
Though sectarian tensions in Lebanon were at their height during the Lebanese Civil War, the Shia–Sunni relations were not the main conflict of the war. The Shia party/militia of Hezbollah emerged in Lebanon during the Lebanese Civil War as one of the strongest forces following the Israeli withdrawal in the year 2000, and the collapse of the South Lebanese Army in the South. The tensions blew into a limited warfare between Shia dominated and Sunni dominated political alliances in 2008. With the eruption of the Syrian Civil War, tensions increased between the Shia-affiliated Alawites and Sunnis of Tripoli, erupting twice into deadly violence—in June 2011, and the second time in February 2012. The Syrian war has affected Hezbollah, which was once lauded by both Sunnis and Shi'ites for its battles against Israel, but now has lost support from many Sunnis for its military assistance to Syrian President Bashar al-Assad. The bombings are thought to be in retaliation for a large car bomb which detonated on 15 August 2013 and killed at least 24 and wounded hundreds in a part of Beirut controlled by the Hezbollah

Jordan 

Although the country of Jordan is 95% Sunni and has not seen any Shia–Sunni fighting within, it has played a part in the recent Shia-Sunni strife. It is the home country of anti-Shia insurgent Raed Mansour al-Banna, who died perpetrating one of Iraq's worst suicide bombings in the city of Al-Hillah. Al-Banna killed 125 Shia and wounded another 150 in the 2005 Al Hillah bombing of a police recruiting station and adjacent open air market. In March 2005 Salt, al-Banna's home town, saw a three-day wake for al-Banna who Jordanian newspapers and celebrants proclaimed a martyr to Islam, which by definition made the Shia victims "infidels whose murder was justified." Following the wake Shia mobs in Iraq attacked the Jordanian embassy on 20 March 2005. Ambassadors were withdrawn from both countries. All this resulted despite the strong filial bonds, ties of commerce, and traditional friendship between the two neighboring countries. Egypt 
According to Pew, roughly 99% of Egyptian Muslims regarded themselves as Sunni Muslims. others put the number of Shia somewhere between 800,000 to about two to three million. The Syrian Civil War has brought on an increase in anti-Shia rhetoric, and what Human Rights Watch states is "anti-Shia hate speech by Salafis". In 2013 a mob of several hundred attacked a house in the village of Abu Musallim near Cairo, dragging four Shia worshipers through the street before lynching them. Yemen 

Muslims in Yemen include the majority Shafi'i (Sunni) and the minority Zaidi (Shia). Zaidi are sometimes called "Fiver Shia" instead of Twelver Shia because they recognize the first four of the Twelve Imams but accept Zayd ibn Ali as their "Fifth Imām" rather than his brother Muhammad al-Baqir. Shia–Sunni conflict in Yemen involves the Shia insurgency in northern Yemen. Both Shia and Sunni dissidents in Yemen have similar complaints about the government—cooperation with the American government and an alleged failure to following Sharia law—but it's the Shia who have allegedly been singled out for government crackdown. During and after the US-led invasion of Iraq, members of the Zaidi-Shia community protested after Friday prayers every week outside mosques, particularly the Grand Mosque in Sana'a, during which they shouted anti-US and anti-Israeli slogans, and criticised the government's close ties to America. These protests were led by ex-parliament member and Imam, Bader Eddine al-Houthi. In response the Yemeni government has implemented a campaign to crush to the Zaidi-Shia rebellion" and harass journalists. These latest measures come as the government faces a Sunni rebellion with a similar motivation to the Zaidi discontent. A March 2015 suicide bombing of two mosques (used mainly by supporters of the Zaidi Shia-led Houthi rebel movement), in the Yemeni capital of Sanaa, killed at least 137 people and wounded 300. The Sunni Islamic State of Iraq and the Levant movement claimed responsibility, issuing a statement saying: "Let the polytheist Houthis know that the soldiers of the Islamic State will not rest until we have uprooted them." Both the Sunni al-Qaeda and "Islamic State" consider Shia Muslims to be heretics. Bahrain 

The small Persian Gulf island state of Bahrain has a Shia majority but is ruled by Sunni Al Khalifa family as a constitutional monarchy, with Sunni dominating the ruling class and military and disproportionately represented in the business and landownership. According to the CIA World Factbook, Al Wefaq the largest Shia political society, won the largest number of seats in the elected chamber of the legislature. However, Shia discontent has resurfaced in recent years with street demonstrations and occasional low-level violence." Bahrain has many disaffected unemployed youths and many have protested Sheikh Hamad bin Isa Al Khalifa's efforts to create a parliament as merely a "cooptation of the effendis", i.e. traditional elders and notables. Bahrain's 2002 election was widely boycotted by Shia. Mass demonstrations have been held in favor of full-fledged democracy in March and June 2005, against an alleged insult to Ayatollah Khamenei in July 2005. Pakistan 

Pakistan's citizens have had serious Shia-Sunni discord. Almost 90% of Pakistan's Muslim population is Sunni, with 10% being Shia, but this Shia minority forms the second largest Shia population of any country, larger than the Shia majority in Iraq. Until recently Shia–Sunni relations have been cordial, and a majority of people of both sects participated in the creation the state of Pakistan in the 1940s. Despite the fact that Pakistan is a Sunni majority country, Shia have been elected to top offices and played an important part in the country's politics. Several top Pakistani military and political figures such as General Muhammad Musa, and Pakistan's President Yahya Khan were Shia, as well as Former President Asif Ali Zardari was a Shia. There are many intermarriages between Shia and Sunnis in Pakistan. Unfortunately, from 1987 to 2007, "as many as 4,000 people are estimated to have died" in Shia-Sunni sectarian fighting in Pakistan", another estimate is nearly 4,000 people have been killed and 6,800 injured from the beginning of 2000 to 2013. Amongst the culprits blamed for the killing are Al-Qaeda working "with local sectarian groups" to kill what they perceive as Shia apostates, and "foreign powers ... trying to sow discord." Most violence takes place in the largest province of Punjab and the country's commercial and financial capital, Karachi. There have also been conflagrations in the provinces of Khyber Pakhtunkhwa, Balochistan and Azad Kashmir, with several hundreds of Shia  killed in Balochistan killed since 2008. Shia have responded to attacks creating a classic vicious cycle of "outrages and vengeance". Arab states especially Saudi Arabia and GCC states have been funding extremist Deobandi Sunnis and Wahhabis in Pakistan, since the Afghan Jihad. Whereas Iran has been funding Shia militant groups such as Sipah-e-Muhammad Pakistan, resulting in tit-for-tat attacks on each other. Pakistan has become a battleground between Saudi Arabia-funded Deobandi Sunni and Wahhabis and Iran-funded Shia resulting in the deaths of thousands of innocent Muslims. Background
Some see a precursor of Pakistani Shia–Sunni strife in the April 1979 execution of deposed President Zulfikar Ali Bhutto on questionable charges by Islamic fundamentalist General Muhammad Zia-ul-Haq. Ali Bhutto was Shia, Zia ul-Haq a Sunni. Zia-ul-Haq's Islamization that followed was resisted by Shia who saw it as "Sunnification" as the laws and regulations were based on Sunni fiqh. In July 1980, 25,000 Shia protested the Islamization laws in the capital Islamabad. Further exacerbating the situation was the dislike between Shia leader Imam Khomeini and General Zia ul-Haq. Shia formed student associations and a Shia party, Sunni began to form sectarian militias recruited from Deobandi and Ahl al-Hadith madrasahs. Preaching against the Shia in Pakistan was cleric Israr Ahmed. Manzoor Nomani, a senior Indian cleric with close ties to Saudi Arabia published a book entitled Iranian Revolution: Imam Khomeini and Shiism. The book, which "became the gospel of Deobandi militants" in the 1980s, attacked Khomeini and argued the excesses of the Islamic revolution were proof that Shiism was not the doctrine of misguided brothers, but beyond the Islamic pale. Anti-Shia groups in Pakistan include the Lashkar-e-Jhangvi and Sipah-e-Sahaba Pakistan, offshoots of the Jamiat Ulema-e-Islam (JUI). The groups demand the expulsion of all Shia from Pakistan and have killed hundreds of Pakistani Shia between 1996 and 1999. As in Iraq they "targeted Shia in their holy places and mosques, especially during times of communal prayer." From January to May 1997, Sunni terror groups assassinated 75 Shia community leaders "in a systematic attempt to remove Shia from positions of authority." Lashkar-e-Jhangvi has declared Shia to be "American agents" and the "near enemy" in global jihad. An example of an early Shia–Sunni fitna shootout occurred in Kurram, one of the tribal agencies of the Northwest Pakistan, where the Pushtun population was split between Sunnis and Shia. In September 1996 more than 200 people were killed when a gun battle between teenage Shia and Sunni escalated into a communal war that lasted five days. Women and children were kidnapped and gunmen even executed out-of-towners who were staying at a local hotel. "Over 80,000 Pakistani Islamic militants have trained and fought with the Taliban since 1994. They form a hardcore of Islamic activists, ever-ready to carry out a similar Taliban-style Islamic revolution in Pakistan. ", according to Pakistani journalist Ahmed Rashid. Afghanistan 

Shia–Sunni strife in Pakistan is strongly intertwined with that in Afghanistan. The anti-Shia Afghan Taliban regime helped anti-Shia Pakistani groups and vice versa. Lashkar-e-Jhangvi and Sipah-e-Sahaba Pakistan, have sent thousands of volunteers to fight with the Taliban regime and "in return the Taliban gave sanctuary to their leaders in the Afghan capital of Kabul." Shia–Sunni strife inside of Afghanistan has been  between the Sunni Taliban and Shia Afghans, primarily the Hazara ethnic group -- a function of the puritanical religious character of the Taliban and their "traditional Pashtun biases against Shias". In 1998 more than 8,000 noncombatants were killed when the Taliban attacked Mazar-i-Sharif and Bamiyan where many Hazaras live. Some of the slaughter was indiscriminate, but many were Shia targeted by the Taliban. Taliban commander and governor Mullah Niazi banned prayer at Shia mosques and expressed takfir of the Shia in a declaration from Mazar's central mosque:

Assisting the Taliban in the murder of Iranian diplomatic and intelligence officials at the Iranian Consulate in Mazar were "several Pakistani militants of the anti-Shia, Sipah-e-Sahaba party." There were other pogroms of Shia as well in the first Taliban reign prior to the U.S. invasion. In 2021 Human Rights Watch warned on a "surge in Islamic State Attacks on Shia" in Afghanistan "that amount to crimes against humanity". Attacks on the Hazara Shia community include 
suicide bombings that killed at least 72 people at the Sayed Abad mosque in Kunduz on October 8 2021, 
a bombing that killed at least 63 people at the Bibi Fatima mosque in Kandahar on October 15 2021. In a statement ISIS declared it would target Shia  “in every way, from slaughtering their necks to scattering their limbs… and the news of [ISIS’s] attacks…in the temples of the [Shia] and their gatherings is not hidden from anyone, from Baghdad to Khorasan.”

Nigeria 

In Nigeria—the most populous country in Africa—until recently almost all Muslims were Sunni. As of 2017, estimates of the number of Nigeria's 90–95 million Muslims who are Shia vary from between 20 million (Shia estimate), to less than five million (Sunni estimate) but according to Pew research center, less than 5% of the Muslim population in Nigeria are Shia. In the 1980s, Ibrahim El-Zakzaky—a Nigerian admirer of the Iranian Revolution who lived in Iran for some years and converted to Shia Islam—established the Islamic Movement of Nigeria. The movement has established "more than 300 schools, Islamic centers, a newspaper, guards and a `martyrs’ foundation`". Its network is similar to that of Hezbollah in Lebanon, with a focus on Iran, its Supreme Leader, and fighting America as the enemy of Islam. According to a former U.S. State Department specialist on Nigeria, Matthew Page, the Islamic Movement receives "about $10,000 a month" in Iranian funding. Many of the converted are poor Muslims. The Shia campaign has clashed with Saudi Arabian, which also funds religious centers, school, and trains students and clerics, but as part of an effort to spread its competing Wahabbi interpretation of Islam. According to Wikileaks, "Saudi cables" released in 2015 "reveal concern" about "Iran-driven Shiite expansion from Mali, Mauritania, Burkina Faso and Nigeria" to Shia Islam has taken place in Nigeria since the Iranian Revolution. Shia Muslims protest that they have been persecuted by the Nigerian government. In 1998 Nigerian President General Sani Abacha accused Ibrahim El-Zakzaky  of being a Shia. In December 2015 the Nigerian government alleged that the Islamic Movement attempted to kill Nigeria's army chief-of-staff. In retaliation, troops killed more than 300 Shiites in the city of Zaria. Hundreds of El-Zakzaky's followers were also arrested. As of 2019, El-Zakzaky was still imprisoned. South East Asia 
Islam is the dominant religion in Indonesia, which also has a larger Muslim population than any other country in the world, with approximately 202.9 million identified as Muslim (88.2% of the total population) as of 2009. The majority adheres to the Sunni Muslim tradition mainly of the Shafi'i madhhab. Around one million are Shias, who are concentrated around Jakarta. In general, the Muslim community can be categorized in terms of two orientations: "modernists," who closely adhere to orthodox theology while embracing modern learning; and "traditionalists," who tend to follow the interpretations of local religious leaders (predominantly in Java) and religious teachers at Islamic boarding schools (pesantren). In Indonesia, in 2015, Sunni clerics denounced the Shia as "heretics", and the mayor of Bogor proposed banning the Shia Ashura holy day. The Shia community (which makes up approximately 1% of Indonesia's Muslims) has also been subject to hate campaigns and intimidation, with fears of this escalating into violence. Malaysia claims to be a tolerant Islamic state, however since 2010 it has banned the preaching of Shia Islam, with a "particular ferocity" and warns against Shiism with its, "evil and blasphemous beliefs". United States 
In late 2006 or early 2007, in what journalist Seymour Hersh called The Redirection, the United States changed its policy in the Muslim world, shifting its support from the Shia to the Sunni, with the goal of "containing" Iran and as a by-product bolstering Sunni extremist groups. Richard Engel, who is an NBC News Chief Foreign Correspondent, wrote an article in late 2011 alleging that the United States Government is pro-Sunni and anti-Shia. During the Iraq War, the United States feared that a Shiite-led, Iran-friendly Iraq could have major consequences for American national security. However, nothing can be done about this as Iraq's Shiite government were democratically elected. Shadi Bushra of Stanford University wrote that the United States’ support of the Sunni monarchy during the Bahraini uprising is the latest in a long history of US support to keep the Shiites in check. The United States fears that Shiite rule in the Persian Gulf will lead to anti-US and anti-Western sentiment as well as Iranian influence in the Arab majority states. One analyst told CNN that the US strategy on putting pressure on Iran by arming its Sunni neighbors is not a new strategy for the United States. Europe 
In Europe Shia-Sunni acrimony is part of life for tens of millions of European Muslims. Australia 
Conflict between religious groups in the Middle East have spread to the Australian Muslim community and within Australian schools. ISIL and the 2013–2017 war in Iraq

Growing out of the 2003 US invasion of Iraq and overthrow of Iraqi government of Saddam Hussein, a Salafi jihadi extremist militant group  
led by Sunni Arabs from Iraq and Syria, developed an insurgency that by March 2015 had control over territory  in Iraq and Syria 
occupied by ten million people. It proclaimed itself a worldwide caliphate, 
with religious, political, and military authority over Muslims worldwide. and dubbed itself the Islamic State (, ), 
but by December 2017, it controlled just 2% of the territory it had at the peak of its expansion, and had been driven underground in Iraq. In the few years of its success, it was responsible for human rights abuses and war crimes (United Nations), and  ethnic cleansing  on a "historic scale" (Amnesty International), particularly of Shia Muslims. According to Shia rights watch, in 2014 ISIS forces killed over 1,700 Shia civilians at Camp Speicher in Tikrit Iraq, and 670 Shia prisoners at the detention facility on the outskirts of Mosul. In June 2014, after ISIS had "seized vast territories" in western and northern Iraq, there were "frequent accounts of fighters’ capturing groups of people and releasing the Sunnis while the Shiites are singled out for execution", according to the New York Times. ISIS used a list of questions to "tell whether a person is a Sunni or a Shiite"—What is your name? Where do you live? How do you pray? What kind of music do you listen to? After the collapse of the Iraqi army and capture of the city of Mosul by ISIS in June 2014, the "most senior" Shia spiritual leader based in Iraq, the Grand Ayatollah Ali al-Sistani, who had been known as "pacifist" in his attitudes, issued a fatwa calling for jihad against ISIS and its Sunni allies, which was seen by the Shia militias as a "de facto legalization of the militias’ advance". In Qatari another Shiite preacher, Nazar al-Qatari, "put on military fatigues to rally worshipers after evening prayers," calling on them to fight against “the slayers of Imams Hasan and Hussein” (the second and third Imams of Shia history) and for Iran's supreme leader Ayatollah Ali Khamenei. Shia militias fighting ISIS have also been accused of atrocities. Human Rights Watch has accused government-backed Shia militias of kidnapping and killing scores of Sunni civilians in 2014. Reduced to terror campaigns
By 2019, the group resorted increasingly to terror bombings and insurgency operations, using its scattered underground networks of sleeper cells across regions in the Middle East and various offshoots and adherents. According to military.com, as of May 2023, the Islamic State's Khorasan Province, (ISIS-K), has become "the new boogeyman in the Middle East". CNN also writes that "new data" shows that at least in Afghanistan, the "threat from ISIS is growing". Although the Shia – in particular the ethnic Hazaras – are just one of the targets of ISIS-K, (along with symbolic targets, foreigners, the ruling Taliban itself), they have been targeted, for example in September 2022, when an educational facility, in "a Shiite area" of the Afghan capital of Kabul, was suicide bombed, killing 53 teenage students and injuring 110. Unity efforts
In a special interview broadcast on Al Jazeera on 14 February 2007, former Iranian president and chairman of the Expediency Discernment Council of Iran, Ayatollah Akbar Hashemi Rafsanjani and highly influential Sunni scholar Yusuf al-Qaradawi, "stressed the impermissibility of the fighting between the Sunnis and the Shi’is" and the need to "be aware of the conspiracies of the forces of hegemony and Zionism which aim to weaken [Islam] and tear it apart in Iraq." Even on this occasion there were differences, with Rafsanjani openly asking "more than once who started" the inter-Muslim killing in Iraq, and Al-Qaradawi denying claims by Rafsanjani that he knew where "those arriving to Iraq to blow Shi’i shrines up are coming from". Saudi–Iran summit
In a milestone for the two countries' relations, on 3 March 2007 King Abdullah of Saudi Arabia and President Mahmoud Ahmadinejad held an extraordinary summit meeting. They displayed mutual warmth with hugs and smiles for cameras and
promised "a thaw in relations between the two regional powers but stopped short of agreeing on any concrete plans to tackle the escalating sectarian and political crises throughout the Middle East." On his return to Tehran, Ahmadinejad declared that:

Saudi officials had no comment about Ahmadinejad's statements, but the Saudi official government news agency did say:

Saudi Foreign Minister Prince Saud bin Faisal bin Abdul-Aziz said:

Effort to bring unity between Sunni and Shia Muslims had been attempted by Allama Muhammad Taqi Qummi. Scholarly opinions

Sunni
Sheikh Mahmoud Shaltut (April 1893 – December 1963): In a Fatwa Sheikh Shaltut declared worship according to the doctrine of the Twelve Shia to be valid and recognized the Shiite as an Islamic School. Muhammad Sayyid Tantawy (28 October 1928 – 10 March 2010): "I think that anyone who believes that there is no god but Allah and that Muhammad is his Messenger is definitely a Muslim. Therefore, we have been supporting, for a long time, through Al-Azhar, many calls for the reconciliation of Islamic schools of thought. Muslims should work on becoming united, and protecting themselves from denominational sectarian fragmentation. There are no Shiites and no Sunni. We are all Muslims. Regretfully; the passions and prejudices that some resort to, are the reason behind the fragmentation of the Islamic nation." Sheikh Mohammed al-Ghazali  (1917–1996): "It is the duty of all Muslims to unite against enemies of Islam and their propaganda". Sheikh Abd al-Majid Salim stated in a letter he sent to Ayatollah Borujerdi: "The first thing that becomes obligatory to scholars, Shia or Sunni, is removing dissension from the minds of Muslims." Vasel Nasr, the Grand Mufti of Egypt (Mufti from 1996-2002): "We ask Allah to create unity among Muslims and remove any enmity, disagreement and contention in the ancillaries of Fiqh between them." Shiite
Ayatollah Seyyed Hossein Borujerdi (March 1875 – March 1961) sent a letter to Sheikh Abd al-Majid Salim, the Grand Mufti of Sunnis and former Chancellor of Al-Azhar University and wrote: "I ask Almighty Allah to change ignorance, separation and distribution among different Islamic Schools to each other, to the actual knowledge and kindness and solidarity." Ayatollah Ruhollah Khomeini (May 1900 – June 1989): "We are oneness with Sunni Muslims. We are their brothers" and "It is obligatory for all Muslims that maintain unity." Ayatollah Seyed Ali Khamenei (born April 1939) said in a Fatwa about creating dissension: "In addition to dissension is contrary to the Qur'an and Sunnah, this weakens Muslims. So, creating dissension is forbidden (Haram)." Ayatollah Ali al-Sistani (born August 1930), in answer to the question "is anyone who says Shahadah, prays and follow one of the Islamic Schools, a Muslim? ", Sistani replied: "Every one who says Shahadah, acts as you describe and does not have enmity towards Ahl al-Bayt, is Muslim." See also

 Amman Message
 Anti-Shi'ism
 Criticism of Twelver Shia Islam
 Glossary of Islam
 Index of Islam-related articles
 International Islamic Unity Conference (Iran)
 Islam in Iran
 Kharijite
 Organisation of Islamic Cooperation
 Outline of Islam
 Rafida
 Shia Muslims in the Arab world
 Sunni fatwas on Shias
 The World Forum for Proximity of Islamic Schools of Thought
 Sufi–Salafi relations
 Catholic-Protestant relations – One of the Christian counterparts 
 Catholic-Eastern Orthodox relations – One of the Christian counterparts

References

Notes

Citations

Sources

Further reading

 
 
 
 The Arab Shia: The Forgotten Muslims, by Graham E. Fuller and Rend Rahim Francke. New York: Saint Martin's Press, 1999, 
 Shi'a Islam, by Muhammad Husayn Tabatabaei and Hossein Nasr, SUNY Press, 1979. Saudi Clerics and Shia Islam, by Raihan Ismail, Oxford University Press, 2016. Don't Fear the Shiites: The Idea of a Teheran-Controlled "Shiite Crescent" over the Greater Middle East is at Odds with Reality, by Michael Bröning. In: International Politics and Society, 3 /2008, pp. 60–75. Here Are Some of the Day-To-Day Differences Between Sunnis and Shiites. Azadeh Moaveni. Huffington Post, 25 June 2014
 Opposing the Imam: The Legacy of the Nawasib in Islamic Literature , Nebil Husayn , Cambridge Studies in Islamic Civilization, Cambridge University Press ISBN ebook ebook: 9781108966061

External links
 "The Sunni-Shia Divide", Council on Foreign Relations

 
History of Islam
Shia Islam
Sunni Islam