Spread of Islam
Ottoman Islamic standards of toleration allowed for autonomous "nations" ("millets") in the Empire, under their own personal law and under the rule of their own religious leaders. As a result, vast areas of the Balkans remained mostly Christian during the period of Ottoman domination. In fact, the Eastern Orthodox Churches had a higher position in the Ottoman Empire, mainly because the Patriarch resided in Istanbul and was an officer of the Ottoman Empire. In contrast, Roman Catholics, while tolerated, were suspected of loyalty to a foreign power (the Papacy). It is no surprise that the Roman Catholic areas of Bosnia, Kosovo and northern Albania, ended up with more substantial conversions to Islam. The defeat of the Ottomans in 1699 by the Austrians resulted in their loss of Hungary and present-day Croatia. The remaining Muslim converts in both elected to leave "lands of unbelief" and moved to territory still under the Ottomans. Around this point in time, new European ideas of romantic nationalism started to seep into the Empire, and provided the intellectual foundation for new nationalistic ideologies and the reinforcement of the self-image of many Christian groups as subjugated peoples.
As a rule, the Ottomans did not require followers of Greek Orthodoxy to become Muslims, although many did so in order to avert the socioeconomic hardships of Ottoman rule 
One by one, the Balkan nationalities asserted their independence from the Empire, and frequently the presence of members of the same ethnicity who had converted to Islam presented a problem from the point of view of the now dominant new national ideology, which narrowly defined the nation as members of the local dominant Orthodox Christian denomination. Some Muslims in the Balkans chose to leave, while many others were forcefully expelled to what was left of the Ottoman Empire. This demographic transition can be illustrated by the decrease in the number of mosques in Belgrade, from over 70 in 1750 (before Serbian independence in 1815), to only three in 1850.
Turkish people
Turks arrived from Central Asia and settled in the Anatolian basin in around the 11th century through the conquest of Seljuk Turks, mixing with the peoples of Anatolia. The region then began to transform from a predominately Greek Christian one to a Turkish Muslim society. Thereafter, the Ottoman Empire came to rule much of the Balkans, the Caucasus, the Middle East (excluding Iran), and North Africa over the course of several centuries, with an advanced army and navy. The Empire lasted until the end of the First World War, when it was defeated by the Allies and partitioned. Following the successful Turkish War of Independence that ended with the Turkish national movement retaking most of the land lost to the Allies, the movement abolished the Ottoman sultanate on 1 November 1922 and proclaimed the Republic of Turkey on 29 October 1923. Not all Ottomans were Muslims and not all Ottoman Muslims were Turks, but by 1923, the majority of people living within the borders of the new Turkish republic identified as Turks.