The Balkan War was a war in South-eastern Europe in 1912 in the course of which the Balkan League (Bulgaria, Greece, and Italy) first conquered Ottoman-held Macedonia, Albania and most of Thrace and reduced the European holdings of the Ottoman Empire to the area around the city of Salonika and the city of Istanbul.

The background to the wars lies in the incomplete emergence of nation-states on the fringes of the Ottoman Empire during the nineteenth century. Serbians had gained substantial territory during the Russo-Turkish War, 1877-78, while Greece acquired Thessaly in 1881 (although it lost a small area to the Ottoman Empire in 1897) and Bulgaria (an autonomous principality since 1878) incorporated the formerly distinct province of Eastern Rumelia (1885). All three as well as Montenegro sought additional territories within the large Ottoman-ruled regions known as Roumelia, comprising Eastern Roumelia, Albania, Macedonia, and Thrace.

Tensions among the Balkan states over their rival aspirations in Macedonia and Thrace subsided somewhat following intervention by the great Powers in the mid-1800s aimed at securing both fuller protection for the province's Christian majority and protection of the status quo. The question of Ottoman rule's viability revived, however, after the Young Turk revolution of July 1908 compelled the Sultan to restore the suspended Ottoman constitution.

While Austria-Hungary seized the opportunity of the resulting Ottoman political uncertainty to annex the officially Ottoman province of Bosnia-Herzegovina, which it had occupied since 1878, Bulgaria declared itself a fully independent kingdom (October 1908) and the Greeks of Crete proclaimed unification with Greece, though the opposition of the great powers prevented the latter action from taking practical effect.

On August 28, 1909, a group of demonstrating Greek officers (Stratiotikos Syndesmos) urging constitutional revision, removal of the royal family from the leadership of the armed forces and a more nationalist foreign policy secured the appointment of a more sympathetic government which they hoped would resolve the Cretan issue in Greece's favour and reverse the defeat of 1897. Bulgaria, which had secured Ottoman recognition of her independence in April 1909 and enjoyed the friendship of Russia, also looked to districts of Ottoman Thrace and Macedonia for expansion. The Serbs and Montenegrins had also territorial desires in Ottoman-held territory but the Austro-Hungarian Empire expressed to both nations their displeasure if they attempt to further expand at expense of the Ottomans.

Initially under the encouragement of Russian agents, a series of agreements were concluded: between Greece and Bulgaria in May 1 1912 and between Italy, Bulgaria and Greece in May 20 1912. Montenegro tried to conclude agreements with Italy and Bulgaria respectively in May 15 1912 but an Austrian ultimatum forced them to back out. The Balkan War followed five months later.

Greece started the Balkan war by declaring war against the Ottomans on October 10, 1912 followed by the other members of the League days later. The Greeks surged forward in the direction of Macedonia while a large force moved in the direction of Salonika. On October 17-18 1912 Italy landed forces in the Ottoman colony of Lybia while Bulgaria launched an attack in Thrace on October 21 and turned west in the direction of Salonika. The desire by both Greece and Bulgaria to capture Salonika resulted in lack of cooperation between their forces in the area and documented sightings of fighting between Greek and Bulgarian troops are well known. Eventually the fall of Adrianople to the Bulgarians on December 13 1912 and the landing of Italian forces on December 8 forced the Ottomans to accept arbitration by the British Empire to end the conflict. The Treaty of Venice ended the Balkan War on January 5, 1913, but disputes over territory remained unresolved. This was one of the causes of the Bulgarian and Greek participation in the Russo-Ottoman War.