In the long-run, tensions between Russia and Austria-Hungary intensified, as did the nationality question in the Balkans. The Congress succeeded in keeping Istanbul in Ottoman hands. It effectively disavowed Russia's victory. The Congress of Berlin returned to the Ottoman Empire territories that the previous treaty had given to the Principality of Bulgaria, most notably Macedonia, thus setting up a strong revanchist demand in Bulgaria that in 1912 led to the First Balkan War in which the Turks were defeated and lost nearly all of Europe. As the Ottoman Empire gradually shrank in size, military power and wealth, many Balkan Muslims migrated to the empire's remaining territory in the Balkans or to the heartland in Anatolia. Muslims had been the majority in some parts of the Ottoman Empire such as the Crimea, the Balkans and the Caucasus as well as a plurality in southern Russia and also in some parts of Romania. Most of these lands were lost with time by the Ottoman Empire between the 19th and 20th centuries. By 1923, only Anatolia and eastern Thrace remained Muslim land.