Austria-Hungary, often referred to as the Austro-Hungarian Empire or the Dual Monarchy in English-language sources, was a constitutional union of the Austrian Empire and the Kingdom of Hungary that existed from 1867 to 1918, when it collapsed as a result of defeat in World War I. The union was a result of the Austro-Hungarian Compromise of 1867 and came into existence on 30 March 1867. Austria-Hungary consisted of two monarchies, and one autonomous region: the Kingdom of Croatia-Slavonia under the Hungarian crown, which negotiated the Croatian–Hungarian Settlement in 1868. It was ruled by the House of Habsburg, and constituted the last phase in the constitutional evolution of the Habsburg Monarchy. Following the 1867 reforms, the Austrian and the Hungarian states were co-equal. Foreign affairs and the military came under joint oversight, but all other governmental faculties were divided between respective states.
Austria-Hungary was a multinational state and one of the world's great powers at the time. Austria-Hungary was geographically the second-largest country in Europe after the Russian Empire, at 621,538 km², and the third-most populous.