The Second Balkan War was a conflict which 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 June 1913. Serbian and Greek armies repulsed the Bulgarian offensive and counter-attacked, penetrating into Bulgaria. Having previously engaged in territorial disputes with Romania, the new war caused a 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 give up much of its First Balkan War gains to Serbia, Greece, Romania and the Ottomans.

The war caused a permanent break up of the Russo-Bulgarian alliance, and thus left Serbia as the only ally of Russia in this critical region. For this reason Serbia had Russia's full support. This allowed some Serbian extremist to ignite the July crisis of 1914, and after that to keep the uncompromising position against Austria-Hungary that led (through the existing European chains of alliances) to World War I. Some sources however say, that the real reason for war was that Germany and Austro-Hungary saw Serbia as on obstacle to expansion to the east and building of Berlin-Baghdad railway, which prompted Austro-Hungary to push Bulgaria into the war. This also led to first world war as Austro-Hungary wasn't satisfied with Serbian gains in Balkan wars and saw Serbia as a an obstacle. Therefore Austro-Hungary was looking for a provocation which would trigger the war..

The Balkan Wars were two conflicts that took place in the Balkan Peninsula in south-eastern Europe in 1912 and 1913.

By the early 20th century, Montenegro, Bulgaria, Greece and Serbia had achieved independence from the Ottoman Empire of Turkey. But large parts of their ethnic populations remained under Ottoman rule. In 1912, these countries formed the Balkan League. The First Balkan War broke out when the League attacked Turkey on 8 October 1912 and was ended seven months later by the Treaty of London. After five centuries, Turkey lost virtually all of its possessions in the Balkans.

The Second Balkan War broke out on 16 June 1913. Bulgaria was dissatisfied over the division of the spoils in Macedonia, made in secret by its former allies, Serbia and Greece, and attacked them. The Serbian and Greek armies repulsed the Bulgarian offensive and counter-attacked into Bulgaria, while Romania and Turkey also attacked Bulgaria and gained (or regained) territory. In the resulting Treaty of Bucharest, Bulgaria lost most of the territories it had gained in the First Balkan War.