Peter I (Serbian: Petar I Karađorđević, Петар I Карађорђевић) (29 June 1844 -- 16 August 1921), was the King of Serbia from 1903 to 1918, and subsequently the ruler of the Kingdom of Serbs, Croats and Slovenes (officially renamed Kingdom of Yugoslavia in 1929). He was a member of the Royal House of Karađorđević. As the leader of the victorious Serbian army in World War I, he also received the nickname "Liberator" (Oslobodilac) after the war.