The spark that started WW1 after the assassination of Franz Ferdinand, the Archduke of Austria: Austria tries to retaliate against Serbs and Russia comes to their aid as the Slavic "Big Brother". 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.