In 1911 the Nationalist Prime Minister, Ivan Geshov, set about forming an alliance with Greece and Serbia, and the three allied agreed to put aside their rivalries to plan a joint attack on the Ottomans. In February 1912 a secret treaty was signed between Bulgaria and Serbia, and in May 1912 a similar treaty was signed with Greece. Montenegro was also brought into the pact. The treaties provided for the partition of Macedonia and Thrace between the allies, although the lines of partition were left dangerously vague. After the Ottomans refused to implement reforms in the disputed areas the 1st Balkan War broke out in October 1912. (See Balkan Wars for details.)

Boundaries on the Balkans after the First and the Second Balkan War (1912-1913)The allies were swiftly successful, and the Ottomans sued for peace in December. Negotiations broke down, and fighting resumed in February 1913. A second armistice followed in March, with the Ottomans losing all their European possessions west of the Midia-Enos line. Bulgaria gained possession of most of Thrace, including Adrianople and the Aegean port of Dedeagach (today Alexandroupoli). Bulgaria also gained a slice of Macedonia, north and east of Thessaloniki (which went to Greece), but only some small areas along her western borders.

Bulgaria sustained the heaviest casualties of any of the allies, and felt entitled to the largest share of the spoils. The Serbs in particular did not see things this way, and refused to vacate any of the territory they had seized in northern Macedonia (that is, the territory roughly corresponding to the modern Republic of Macedonia). In June 1913 Serbia persuaded Greece to form a new alliance, against Bulgaria. The Serbian Prime Minister, Nikola Pasic, told Greece she could have Thrace if Greece helped Serbia evict Bulgaria from Macedonia, and the Greek Prime Minister Eleftherios Venizelos agreed. Bulgaria got wind of this and struck first, on June 29. But the war soon turned against Bulgaria. She lost her share of Macedonia to Serbia and western Thrace to Greece, while the revived Ottomans retook Adrianople. Romania also intervened against Bulgaria, and was awarded the southern-Dobruja region (also known as Cadrilater).