The Second Balkan War was fought in 1913 between Bulgaria on one side and its First Balkan War allies Serbia, Greece, and Montenegro on the other side, with Romania and the Ottoman Empire intervening against Bulgaria. The outcome turned Serbia, an ally of the Russian Empire, into an important regional power, alarming Austria-Hungary and with that, unknowingly providing an important cause for World War I.
The Second Balkan war broke due to when The Treaty of London ended the First Balkan war by signing on the 30th of May 1913 , acknowledging that the Balkan states gained west of the Enos-Medea line and had created an independent Albania, meaning that the Balkan League (consisting of Serbia, Montenegro, Greece, and Bulgaria) succeeded in conquering the European provinces of the Ottoman Empire (Albania, Macedonia and Thrace), leaving the Ottomans with only the Chatalja and Gallipoli peninsulas. This did not satisfy anyone.
The Balkan states had made no agreement on the partition of the conquered territories, especially with Macedonia. Bulgaria felt that their territorial rewards from the war, particularly in Macedonia, were too little, and laid claim to the important city of Salonika, where a Bulgarian regiment was already stationed. Greece and Serbia were already unhappy with being forced to evacuate Albania, and responded to the perceived Bulgarian threat by entering into negotiations, hoping to prevent Bulgarian expansion. They settled their mutual differences and signed a military alliance on the 1st of May 1913, followed by a treaty of “mutual friendship and protection” on May 19th 1913. A further dispute of Bulgaria was with Romania, over the latter’s claim on the Bulgarian fortress of Silistra on the Danube river, as the price for their neutrality in the First Balkan war.
Russia didn’t wish to lose either of its Slavic allies in the Balkans. During the negotiations, skirmishing continued in Macedonia, mainly between