During the First Balkan War, which began in October 1912, the Balkan League (Bulgaria, Serbia, Montenegro and Greece) succeeded in driving out the Ottoman Empire

An Ottoman prisoner doing wudhu for his final prayer before being hanged by the Bulgarians during The Balkan War 1913.

The Second Balkan War was a conflict that broke out when Bulgaria, dissatisfied with its share of the spoils of the First Balkan War, attacked its former allies, Serbia and Greece, on 16 (O.S.) / 29 (N.S.) June 1913. Serbian and Greek armies repulsed the Bulgarian offensive and counterattacked, entering Bulgaria. With Bulgaria's also having previously engaged in territorial disputes with Romania and the bulk of Bulgarian forces engaged in the south, the prospect of an easy victory incited Romanian intervention against Bulgaria. The Ottoman Empire also took advantage of the situation to regain some lost territories from the previous war.

When Romanian troops approached the capital Sofia, Bulgaria asked for an armistice, resulting in the Treaty of Bucharest, in which Bulgaria had to cede portions of its First Balkan War gains to Serbia, Greece and Romania. In the Treaty of Constantinople, it lost Adrianople to the Ottomans.

The political developments and military preparations for the Second Balkan War attracted an estimated 200 to 300 war correspondents from around the world.

During the First Balkan War, which began in October 1912, the Balkan League (Bulgaria, Serbia, Montenegro and Greece) succeeded in driving out the Ottoman Empire from its European provinces (Albania, Macedonia, Sandžak and Thrace), leaving the Ottomans with only East Thrace. The Treaty of London, signed on 30 May 1913, which ended the war, acknowledged the Balkan states' gains west of the Enos–Midia line, drawn from Midia (Kıyıköy) on the Black Sea coast to Enos (Enez) on the Aegean Sea coast, on an uti possidetis basis, and created an independent Albania.

However, the relations between the victorious Balkan allies quickly soured over the division of the spoils, specifically in Macedonia. During the prewar negotiations that resulted in the Balkan League's establishment, a secret agreement on 13 March 1912 was signed by Serbia and Bulgaria, which determined their future boundaries, effectively sharing northern Macedonia. In case of a postwar disagreement, the area to the north of the Kriva Palanka–Ohrid line (with both cities going to the Bulgarians) had been designated as a "disputed zone" under Russian arbitration, with the area to the south of this line assigned to Bulgaria. During the war, the Serbs succeeded in capturing an area far south of the agreed border, down to the Bitola–Gevgelija line (both in Serbian hands). At the same time, the Greeks advanced north, occupying Thessaloniki shortly before the Bulgarians arrived and establishing a common Greek border with Serbia.

When Bulgarian delegates in London bluntly warned the Serbs that they must not expect Bulgarian support on their Adriatic claims, the Serbs angrily replied that that was a blatant withdrawal from the prewar agreement of mutual understanding according to the Kriva Palanka-Adriatic line of expansion. The Bulgarians insisted that the Vardar Macedonian part of the agreement remained active, and the Serbs were still obliged to surrender the area as agreed. The Serbs responded by accusing the Bulgarians of maximalism, pointing out that if they lost both northern Albania and Vardar Macedonia, their participation in the common war would have been virtually for nothing.

When Bulgaria called upon Serbia to honour the prewar agreement over northern Macedonia, the Serbs, displeased at the Great Powers' requiring them to give up their gains in north Albania, adamantly refused to evacuate any more territory. The developments ended the Serbo-Bulgarian alliance and made a future war between the two countries inevitable. Soon, minor clashes broke out along the borders of the occupation zones with the Bulgarians against the Serbs and the Greeks. Responding to the perceived Bulgarian threat, Serbia started negotiations with Greece, which also had reasons to be concerned about Bulgarian intentions.

On 19 May/1 June 1913, two days after the signing of the Treaty of London and just 28 days before the Bulgarian attack, Greece and Serbia signed a secret defensive alliance, confirming the current demarcation line between the two occupation zones as their mutual border and concluding an alliance in case of an attack from Bulgaria or from Austria-Hungary. With this agreement, Serbia succeeded in making Greece a part of its dispute over northern Macedonia, since Greece had guaranteed Serbia's current (and disputed) occupation zone in Macedonia.

In an attempt to halt the Serbo-Greek rapprochement, Bulgarian Prime Minister Geshov signed a protocol with Greece on 21 May, agreeing on a permanent boundary between their respective forces, effectively accepting Greek control over southern Macedonia. However, his later dismissal ended the diplomatic targeting of Serbia.

Another point of friction arose: Bulgaria's refusal to cede the fortress of Silistra to Romania. When Romania demanded its cession after the First Balkan War, Bulgaria's foreign minister offered instead some minor border changes, which excluded Silistra, and assurances for the rights of the Kutzovlachs in Macedonia. Romania threatened to occupy Bulgarian territory by force, but a Russian proposal for arbitration prevented hostilities. In the resulting Protocol of St. Petersburg of 9 May 1913, Bulgaria agreed to give up Silistra. The resulting agreement was a compromise between the Romanian demands for the city, two triangles at the Bulgaria–Romania border and Balchik and the land between it and Romania and the Bulgarian refusal to accept any cession of its territory. However, the fact that Russia failed to protect the territorial integrity of Bulgaria made the Bulgarians uncertain of the reliability of the expected Russian arbitration of the dispute with Serbia.

The Bulgarian behaviour also had a long-term impact on Russo-Bulgarian relations. The uncompromising Bulgarian position to review the prewar agreement with Serbia during a second Russian initiative for arbitration finally led Russia to cancel its alliance with Bulgaria. Both acts made conflict with Romania and Serbia inevitable.

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