How did the Islamic conquest of Iran happen?
Question

                I have a Sunni friend from Iran we have a school history project about the history of the Middle East. Can you give us the history behind the Islamic Conquest of Iran and how the people of that land came to Islam? We have heard that they accepted Islam much faster than other people from like Jews and Christians in Holy Land and Syria.
Praise be to Allah.
	Before the Islamic conquest, Iran was under the rule of the 
	Sasanids, and suffered from a great deal of injustice, corruption and 
	oppression. Zoroastrianism was the official religion of Iran until the 
	Islamic conquest. 

	The Islamic conquest of Iran began during the caliphate of 
	Abu Bakr as-Siddeeq (may Allah be pleased with him), when al-Muthanna ibn 
	Haarithah conquered some of the neighbouring lands in the region of 
	Mesopotamia in 13 AH. 

	During the caliphate of ‘Umar (may Allah be pleased with 
	him), Abu ‘Ubayd ath-Thaqafi (may Allah be pleased with him) tried to invade 
	Iran from the south-west, and faced the Persians in the Battle of the Bridge 
	in 13 AH. He met Jaabaan in battle, and Allah defeated the Magians and 
	enabled the Muslims to capture Jaabaan. Then Abu ‘Ubayd marched to Kaskar 
	when he and Nursi met in battle and he defeated him. Then he met Jalinus in 
	battle and defeated him. Then the tide of fortune turned against the 
	Muslims, and many of them were killed and drowned, and Abu ‘Ubayd (may Allah 
	be pleased with him) was killed. 

	Siyar A‘laam an-Nubala’ 
	(2/421) 

	Then the Muslims completed the conquest of Iran under the 
	leadership of Sa‘d ibn Abi Waqqaas in 14 AH, when the Muslim army in Persia 
	inflicted a mighty defeat (on the Persians) in the Battle of al-Qaadisiyyah, 
	which was one of the major battles of the Muslims. 

	The historians stated that the Muslims were between seven and 
	eight thousand strong, and that Rostom, the Persian commander, had sixty 
	thousand troops. 

	On a Monday in Muharram 14 AH, a strong wind blew and lifted 
	the tents of the Persians from their places, and the couch of Rostom, that 
	had been set up for him, was overturned. He hastened to ride his mule and 
	flee, but the Muslims overtook him and killed him. They also killed al-Jalinus, 
	the commander of the Persian vanguard, and the Persians were defeated – 
	praise be to Allah – to the last man and the Muslims pursued them. On that 
	day those who were chained were killed in their entirety, of whom there were 
	thirty thousand. Ten thousand were killed in the battle, and before that a 
	similar number had also been killed. On that day and on the previous days, 
	two thousand five hundred Muslims were killed, may Allah have mercy on 
	them. 

	The Muslims pursued those who fled until they entered the 
	capital city behind them, namely al-Madaa’in (Ctesiphon), which was the seat 
	of the court of Chosroes. 

	From the battle of al-Qaadisiyyah the Muslims seized an 
	indescribably huge amount of wealth and weapons. The booty was gathered 
	after giving the troops their shares, and the khums and glad tidings of the 
	victory were sent to Ameer al-Mu’mineen ‘Umar ibn al-Khattaab (may Allah be 
	pleased with him). 

	Al-Bidaayah wa’n-Nihaayah 
	(9/630) 

	Then the Muslims continued their advance into Persian 
	territory, and were enabled to conquer the southern part of Iran, whilst the 
	ruler Yazdegerd III retreated to the region of Isfahan in central Iran, 
	where he began to assemble troops in an attempt to retake what he had lost. 
	The Muslims and the troops of Yazdegerd met in the Battle of Jalula in 18 
	AH, which ended with the defeat of Yazdegerd and his army, and his retreat 
	towards Isfahan. 

	At-Tabari (may Allah have mercy on him) said: 

	On that day Allah killed one hundred thousand of them, and 
	the slain covered [jallalat] the ground in all directions. It was called 
	Jalula because it was covered with the bodies of their slain. 

	Tareekh at-Tabari 
	(4/26) 

	Then Yazdegerd met the Muslims in the decisive Battle of 
	Nahawand in 21 AH, where the Muslims won a clear victory, after which the 
	Sasanids no longer had any power. The Muslims seized a great deal of booty, 
	which led to them dubbing this battle “the conquest of conquests”. 

	Ibn Katheer (may Allah have mercy on him) said: It was a 
	mighty battle of great significance and wondrous events, and the Muslims 
	used to call it the conquest of conquests. End quote. 

	Al-Bidaayah wa’n-Bihaayah 
	(10/111) 

	It took the Muslims ten years to gain control over all the 
	Iranian lands, because of their huge size and difficult terrain. 

	One of the things that played a role in the spread of Islam 
	in Iran was the migration of many Arab tribes to Iranians land in which they 
	settled and mixed with the Iranians. 

	For nearly nine centuries, Iran followed the madhhab of Ahl 
	as-Sunnah wa’l-Jamaa‘ah, until the Shi‘i Safavid state seized control of it 
	in 906 AH. 

	See: 

	Tareekh at-Tabari 
	by Ibn Jareer at-Tabari

	Siyar A‘laam an-Nubala’ 
	by al-Haafiz Shams ad-Deen adh-Dhahabi

	Al-Bidaayah wa’n-Nihaayah 
	by al-Haafiz Ibn Katheer ad-Dimashqi

	Al-Mufassal fi Tareekh al-‘Arab qabl al-Islam 
	by Dr Jawaad ‘Ali

	Iran fi Zill al-Islam fi’l-‘Usooi as-Sunniyyah 
	wa’sh-Shi‘iyyah by Dr 
	‘Abd al-Mun‘im Hasanayn

	Qaadat Fath Bilaad Faaris 
	– Mahmoud Sheet Khattaab

	Iran wa Afghanistan 
	– Mahmoud Shaakir

	Iran mundhu Fajr at-Tareekh hatta al-Fath al-Islami 
	by Dr Muhammad ‘Abd al-Qaadir Muhammad

	And Allah knows best.