The participation of Greece in the Balkan Wars of 1912–1913 is one of the most important episodes in modern Greek history, as it allowed the Greek state to almost double its size and achieve most of its present territorial size. It also served as a catalyst of political developments, as it brought to prominence two personalities, whose relationship would dominate the next decade and have long-lasting repercussions for Greece: the Prime Minister Eleftherios Venizelos, and the Army’s commander-in-chief, the Crown Prince and later King, Constantine I.
In the First Balkan War, Greece was allied with Bulgaria, Serbia and Montenegro in the “Balkan League” against the Ottoman Empire. The war began on 8 October [O.S. 25 September] 1912 with the declaration of war by Montenegro, while Greece, Bulgaria and Serbia joined on 17 October [O.S. 4 October] 1912. During this war, Greece fought on two fronts on land, and also shouldered the main naval effort of the Balkan allies. The initial principal thrust on land was by the Army of Thessaly, which succeeded in occupying much of Macedonia, including the strategically important port of Thessaloniki, the latter just hours ahead of a Bulgarian division; this would result in increased tension between the two allies in the coming months, and would be one of the causes of the Second Balkan War. Following the successful conclusion of operations in Macedonia, the Greek Army shifted its weight to the Epirus front, where, after a prolonged siege, the city of Ioannina fell, and the Greeks advanced into “Northern Epirus” (modern southern Albania). In the Aegean Sea, the Greek Navy took possession of all the Aegean islands except for the Italian-occupied Dodecanese, and fought off two attempts by the Ottoman Navy to sally forth from the Dardanelles.
Although negotiations had started in London in December 1912, the war continued until 30 May [O.S. 17 May] 1913, when the Treaty of London was finally signed. The treaty failed to satisfy any party involved, with the chief point of friction being the partition of Macedonia. In the face of Bulgarian claims, Serbia and Greece formed an alliance, and on the evening of 29 June [O.S. 16 June] 1913, Bulgarian forces launched a surprise attack against their erstwhile allies. The Bulgarian attacks were soon contained, and pushed back. For Greece in particular, the battles of the Second Balkan War were very costly, as the Greek Army pushed its way into Bulgaria. Following the entry of Romania and the Ottoman Empire in the war, the Bulgarian position became hopeless, and an armistice was declared on 30 July. The war was concluded with the Treaty of Bucharest on 10 August 1913, which confirmed the Greek gains of Macedonia, Epirus (without Northern Epirus) and Crete.