In the years following independence, Bulgaria became increasingly militarized and was often referred to as "the Balkan Prussia", with regard to its desire to revise the Treaty of Berlin through warfare.^[40]The partition of territories in the Balkans by the Great Powers without regard to ethnic composition led to a wave of discontent not only in Bulgaria, but also in its neighbouring countries. In 1911, Nationalist Prime Minister Ivan Geshov formed an alliance with Greece and Serbia to jointly attack the Ottomans and revise the existing agreements around ethnic lines.^[41]

In February 1912 a secret treaty was signed between Bulgaria and Serbia and in May 1912 a similar agreement was sealed with Greece. Montenegro was also brought into the pact. The treaties provided for the partition of the regions of Macedonia and Thrace between the allies, although the lines of partition were left dangerously vague. After the Ottoman Empire refused to implement reforms in the disputed areas, the First Balkan War broke out in October 1912 at a time when the Ottomans were tied down in a major war with Italy in Libya. The allies easily defeated the Ottomans and seized most of its European territory.^[41]

Bulgaria sustained the heaviest casualties of any of the allies while also making the largest territorial claims. The Serbs in particular did not agree 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), saying that the Bulgarian army had failed to accomplish its pre-war goals at Adrianople (to capture it without Serbian help) and that the pre-war agreement 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 promised Greece Thrace to Greece if it helped Serbia defend the territory it had captured in Macedonia; the Greek Prime Minister Eleftherios Venizelos agreed. Seeing this as a violation of the pre-war agreements, and privately encouraged by Germany and Austria-Hungary, Tsar Ferdinand declared war on Serbia and Greece on June 29.

The Serbian and Greek forces were initially beaten back from Bulgaria's western border, but they quickly gained the advantage and forced Bulgaria to retreat. The fighting was very harsh, with many casualties, especially during the key Battle of Bregalnitsa. Soon afterward, the Romania entered the war on the side of Greece and Serbia, attacking Bulgaria from the north. The Ottoman Empire saw this as an opportunity to regain its lost territories and also attacked from the south-east.

Facing war on three different fronts, Bulgaria sued for peace. It was forced to relinquish most of its territorial acquisitions in Macedonia to Serbia and Greece, Adrianapole to the Ottoman Empire, and the region of Southern Dobruja to Romania. The two Balkan wars greatly destabilized Bulgaria, stopping its hitherto steady economic growth, and leaving 58,000 dead and over 100,000 wounded. The bitterness at the perceived betrayal of its former allies empowered political movements who demanded the restoration of Macedonia to Bulgaria.^[42]