In the summer of 1912, Serbia, Greece, and Bulgaria formed a military alliance, the Balkan League, with the sole object of acquiring Turkey's provinces in southeastern Europe. On 17 October 1912, forces from the three nations moved into Turkish territory and, within six months, had defeated the Turks. Under the Treaty of London (30 May 1913), the majority of Turkey's possessions in southeastern Europe were awarded to Serbia, Bulgaria, and Greece.
The Bulgarian government, however, was dissatisfied with the division of the conquered lands. On the day the Treaty of London was signed, Bulgaria attacked both Serbia and Greece without a formal declaration of war. As Serbian and Greek resistance stiffened, both Rumania and Turkey declared war on Bulgaria on 15 July 1913. Almost immediately, Bulgaria sued for peace, and, with the Treaty of Bucharest (10 August 1913), ceded substantial territory. Serbia and Greece acquired Bulgarian Macedonia, while Turkey managed to reclaim some of the lands it had lost in the Treaty of London. Rumania was awarded the northern Bulgarian province of Dobrudja.
The Treaty of Bucharest did nothing to stabilize the Balkans. Bulgaria and Turkey were both determined to reacquire the territories lost to Serbia and Greece. The Serbs confidently emerged from the wars with renewed determination to incorporate the Austro-Hungarian provinces of Bosnia-Herzegovina, Croatia, and Slovenia into the Kingdom of Serbia. Russia actively encouraged Serbian ambitions for diplomatic reasons. Any Serbian expansion would weaken Austria-Hungary, Russia's traditional rival in eastern Europe.