Following the Russo-Turkish War and the 1878 Treaty of Berlin, the Danube Vilayet of the Ottoman Empire was transformed into the autonomous Principality of Bulgaria. The Muslim population of the Danube Vilayet prior to the war numbered 1,120,000. This principality was expanded after the First Balkan War when largely-Muslim Rhodope and Western Thrace regions were incorporated into the country's territory. This process was accompanied by forced Christianization of Muslim Pomak settlements. These events changed the ethnic and religious makeup of Bulgaria.