The Balkan Wars consisted of two conflicts that took place in the Balkan Peninsula in 1912 and 1913. Four Balkan states defeated the Ottoman Empire in the First Balkan War. In the Second Balkan War, Bulgaria fought against all four original combatants of the first war. It also faced an attack from Romania from the north. The Ottoman Empire lost the bulk of its territory in Europe. Although not involved as a combatant, Austria-Hungary became relatively weaker as a much enlarged Serbia pushed for union of the South Slavic peoples. The war set the stage for the Balkan crisis of 1914 and thus served as a "prelude to the First World War".
By the early 20th century, Bulgaria, Greece, Montenegro and Serbia had achieved independence from the Ottoman Empire, but large elements of their ethnic populations remained under Ottoman rule. In 1912, these countries formed the Balkan League. The First Balkan War began on 8 October 1912, when the League member states attacked the Ottoman Empire, and ended eight months later with the signing of the Treaty of London on 30 May 1913.