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 (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 Austro-Hungarian Compromise of 1867 and came into existence on 30 March 1867. Austria-Hungary consisted of two monarchies (Austria and Hungary), and one autonomous region: the Kingdom of Croatia-Slavonia under the Hungarian crown, which negotiated the Croatian–Hungarian Settlement ("Nagodba") 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 , 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 the United Kingdom. Austria-Hungary also became the world's third largest manufacturer and exporter of electric home appliances, electric industrial appliances and power generation apparatus for power plants, after the United States and the German Empire.
What caused its downfall?
when it collapsed as a result of defeat in World War I.