First Balkan War: King Peter I of Serbia issues a declaration "To the Serbian People", as his country joins the war.

The First Balkan War lasted from October 1912 to May 1913 and involved actions of the Balkan League against the Ottoman Empire. The Balkan states' combined armies overcame the initially numerically inferior and strategically disadvantaged Ottoman armies, achieving rapid success.

Peter I was King of Serbia from 15 June 1903 to 1 December 1918. On 1 December 1918, he became King of the Serbs, Croats and Slovenes, and he held that title until his death three years later. Since he was the king of Serbia during a period of great Serbian military success, he was remembered by Serbians as King Peter the Liberator and also as the Old King.