Peter I (Serbian: Петар I Карађорђевић, Petar I Karadjordjević) (June 29, 1844 – August 16, 1921), was the King of Serbia from 1903 until his abdication in 1918. Peter Karadjordjević was a grandson of Karageorge, the founder of modern Serbia and a leader of the First Serbian Insurrection (1804-1813). Prince Peter was a third son of prince Aleksandar Karadjordjević, who ruled Serbia from 1842 to 1858, during the period of "Constitutionalist" After the death of his older brother, Peter became the head of the House of Karadjordjević.