 It became a potent political force during World War I with the assassination of Archduke Franz Ferdinand of Austria by the Yugoslavist militant Gavrilo Princip and the subsequent invasion of Serbia by Austria-Hungary.During the war the Yugoslav Committee composed of South Slav emigres from Austria-Hungary (including twelve Serbs, three Croats, and one Slovene), supported Serbia and vouched for the creation of a Yugoslav state.On 1 December 1918, King Peter of Serbia proclaimed the Kingdom of Serbs, Croats, and Slovenes, commonly known as "Yugoslavia".