WWI began in Europe.
Austria-Hungary . Austria-Hungary was geographically the second-largest country in Europe after the Russian Empire , at 621538 km2 , and the third-most populous ( after Russia and the German Empire ) . World War I . The trigger for the war was the assassination of Archduke Franz Ferdinand of Austria , heir to the throne of Austria-Hungary , by Yugoslav nationalist Gavrilo Princip in Sarajevo on 28 June 1914 .