In 1911, the Nationalist Prime Minister Ivan Geshov set about forming an alliance with Greece and Serbia, and the three allies 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 First Balkan War broke out in October 1912. (
See Balkan Wars for details.)

The allies had an astonishing success.
The Bulgarian army inflicted several crushing defeats on the Ottoman forces and advanced threateningly against Constantinople, while the Serbs and the Greeks took control of Macedonia.
The Ottomans sued for peace in December.
Negotiations broke down, and fighting resumed in February 1913.
The Ottomans lost Adrianople to a Bulgarian task force.
A second armistice followed in March, with the Ottomans losing all their European possessions west of the Midia-Enos line, not far from Istanbul.
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, but only some small areas along her western borders.

Bulgaria sustained the heaviest casualties of any of the allies, and on this basis 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 North Macedonia), stating that the Bulgarian army had failed to accomplish its pre-war goals at Adrianople (i.e., failing to capture it without Serbian help) and that the pre-war agreements on the division of Macedonia had to be revised.
Some circles in Bulgaria inclined toward going to war with Serbia and Greece on this issue.
In June 1913 Serbia and Greece formed a new alliance, against Bulgaria.
The Serbian Prime Minister, Nikola Pasic, told Greece it could have Thrace if Greece helped Serbia keep Bulgaria out of Serbian part of Macedonia, and the Greek Prime Minister Eleftherios Venizelos agreed.
Seeing this as a violation of the pre-war agreements, and discreetly encouraged by Germany and Austria–Hungary, Tsar Ferdinand declared war on Serbia and Greece and the Bulgarian army attacked on June 29.
The Serbian and the Greek forces were initially on the retreat on the western border, but they soon took the upper hand and forced Bulgaria into retreat.
The fighting was very harsh, with many casualties, especially during the key Battle of Bregalnica.
Soon Romania entered the war and attacked Bulgaria from the north.
The Ottoman Empire also attacked from the south-east.
The war was now definitely lost for Bulgaria, which had to abandon most of her claims of Macedonia to Serbia and Greece, while the revived Ottomans retook Adrianople.
Romania took possession of southern Dobruja.
In 1911, the Nationalist Prime Minister Ivan Geshov set about forming an alliance with Greece and Serbia, and the three allies 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 First Balkan War broke out in October 1912. (See Balkan Wars for details.)
The allies had an astonishing success. The Bulgarian army inflicted several crushing defeats on the Ottoman forces and advanced threateningly against Constantinople, while the Serbs and the Greeks took control of Macedonia. The Ottomans sued for peace in December. Negotiations broke down, and fighting resumed in February 1913. The Ottomans lost Adrianople to a Bulgarian task force. A second armistice followed in March, with the Ottomans losing all their European possessions west of the Midia-Enos line, not far from Istanbul. 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, but only some small areas along her western borders.
Bulgaria sustained the heaviest casualties of any of the allies, and on this basis 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 North Macedonia), stating that the Bulgarian army had failed to accomplish its pre-war goals at Adrianople (i.e., failing to capture it without Serbian help) and that the pre-war agreements on the division of Macedonia had to be revised. Some circles in Bulgaria inclined toward going to war with Serbia and Greece on this issue. In June 1913 Serbia and Greece formed a new alliance, against Bulgaria. The Serbian Prime Minister, Nikola Pasic, told Greece it could have Thrace if Greece helped Serbia keep Bulgaria out of Serbian part of Macedonia, and the Greek Prime Minister Eleftherios Venizelos agreed. Seeing this as a violation of the pre-war agreements, and discreetly encouraged by Germany and Austria–Hungary, Tsar Ferdinand declared war on Serbia and Greece and the Bulgarian army attacked on June 29. The Serbian and the Greek forces were initially on the retreat on the western border, but they soon took the upper hand and forced Bulgaria into retreat. The fighting was very harsh, with many casualties, especially during the key Battle of Bregalnica. Soon Romania entered the war and attacked Bulgaria from the north. The Ottoman Empire also attacked from the south-east. The war was now definitely lost for Bulgaria, which had to abandon most of her claims of Macedonia to Serbia and Greece, while the revived Ottomans retook Adrianople. Romania took possession of southern Dobruja.
Kingdom of Bulgaria