The war started with assasination of Archduke Franz Ferdinand of Austria by Gavrilo Princip who was the member of the so-called organization "Black Hand".
According to some historians the lack of power in the Balkans especially after the weakining of Ottoman Empire had laid foundations for the WWI. The famous Eastern Question between the Great Powers begun actually following the Russo-Turkish War in 1774. The scenery after the war made the Great Powers to think on the future. What will happen after the possible dissolution of Ottoman Empire in the European territories (mostly in Balkans)?.