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[63] 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.[64] Some Muslims in the Balkans chose to leave, while many others were forcefully expelled to what was left of the Ottoman Empire.[64] 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.