Peter I (Serbian: Petar I Karađorđević, Петар I Карађорђевић) (29 June 1844 – 16 August 1921), was King of Serbia from 1903 to 1918, and subsequently the ruler of Kingdom of the Serbs, Croats and Slovenes (later 1929 Kingdom of Yugoslavia). He was member of the Royal House of Karađorđević. As the leader of victorious Serbian army in World War I, he also received the nickname "Liberator" (Oslobodilac) after the war. The Western-educated King attempted to liberalise Serbia with the goal of creating a Western-style constitutional monarchy, even translating John Stuart Mill's "On Liberty" into Serbian. Peter chose to "retire" due to ill health following the Balkan Wars which, from a Serb perspective, were a great success. Executive power passed to his son Alexander. The King was relatively inactive during the First World War, although he did occasionally visit trenches to check up on his troops. One memorable visit in 1915 involved Peter, by then 71, picking up a rifle and shooting at enemy soldiers. Following Serbia's military defeat to the forces of Austro-Hungary Peter led the army and civilian refugees through the mountains to the sea on a 'Calvary known to few peoples'. The King had on 24 June 1914 reassigned his royal prerogatives to the Heir apparent Crown Prince Alexander. His last public appearance was on 1 December 1918, when he was proclaimed King of the Serbs, Croats and Slovenes. King Peter I died in Belgrade in 1921 at the age of 77.