Austria-Hungary (German: Österreich-Ungarn; Hungarian: Osztrák–Magyar Monarchia), also known by other names and often referred to as the Austro-Hungarian Empire in English-language sources, was a constitutional union of the Empire of Austria (The Kingdoms and Lands Represented in the Imperial Council or Cisleithania) and the Kingdom of Hungary (Lands of the Crown of Saint Stephen or Transleithania) 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 Compromise of 1867 and came into existence on 30 March 1867, when the compromise was ratified by the Hungarian parliament. Austria-Hungary consisted of two monarchies (Austria and Hungary), and one autonomous region: Croatia–Slavonia under the Hungarian crown, which negotiated its own compromise (Nagodba) with Hungary, 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. The Compromise required regular renewal, as did the customs union between the two components of the union. Foreign affairs and the military came under joint oversight, but all other governmental faculties were divided between respective states.
Austria-Hungary was a multinational realm 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 (239,977 sq mi), and the third-most populous (after Russia and the German Empire). The Empire built up the fourth-largest machine building industry of the world, after the United States, Germany, and Britain.
After 1878, Bosnia and Herzegovina was under Austro-Hungarian military and civilian rule until it was fully annexed in 1908, provoking a diplomatic crisis among the other powers. Sandžak/Raška, de jure northern part of the Ottoman Sanjak of Novi Pazar, was also under de facto joint occupation during that period but the Austro-Hungarian army withdrew as part of their annexation of Bosnia.
Austria-Hungary was one of the Central Powers in World War I. It was already effectively dissolved by the time the military authorities signed an armistice at Villa Giusti on 3 November 1918. The Hungarian Kingdom and the Austrian Republic were treated as its successors de jure, whereas the independence of the West Slavs and South Slavs of the Empire as the Czechoslovak Republic, the Republic of Poland and Kingdom of Serbs, Croats and Slovenes, respectively, and the Union of Transylvania and Bukovina with Romania were also recognized by the victorious powers in 1920.