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.
Hamidiye (cavalry)
Abdul Hamid II's reign has the reputation of being "the most despotic and centralized era in modern Ottoman History." Abdul Hamid is also considered the last sultan to have full control over Ottoman Empire. His reign struggled with the culmination of 75 years of change throughout the empire and an opposing reaction to that change. Abdul Hamid II was particularly concerned with the centralization of the empire. His efforts to centralize the Sublime Porte were not unheard of among other sultans. The Ottoman Empire's local provinces had more control over their areas than the central government. Abdul Hamid II's foreign relations came from a "policy of non-commitment." The sultan understood the fragility of the Ottoman military, and the Empire's weaknesses of its domestic control. Pan-Islamism became Abdülhamid's solution to the empire's loss of identity and power. His efforts to promote Pan-Islamism were for the most part unsuccessful because of the large non-Muslim population, and the European influence onto the empire. Abdul Hamid II's policies essentially isolated the Ottoman Empire, which further aided in its decline. Several of the elite who sought a new constitution and reform for the empire were forced to flee to Europe. After the Treaty of Berlin (1878), the Ottoman Empire began to contract and it lost certain territories. New groups of radicals began to threaten the power of the Ottoman Empire.