Balkan nationalism refers to the desire for the various ethnic groups living on the Balkan Peninsula to have countries of their own, a spirit that led directly to World War I. This region, south of the Danube, Sava and Kupa Rivers, is a hodgepodge of over a dozen ethnic groups, many of whom have clashed throughout history.
In the early 1900s, the Ottoman Empire, which had controlled the Balkans since at least the 1600s, was weakening. It pulled much of its influence out of the Balkan area after Greece, Serbia, Montenegro and Bulgaria were successful in fighting for their independence. Russia, which had imperialistic designs on the region, inspired several of the new states to form the Balkan League in order to drive the Turks completely out of the area. When they were successful, Serbia in particular was able to add large regions to its territory.
The great success of the Balkan countries led to a rising sense of nationalism, especially in Serbia. After their success in defeating their Ottoman overlords, the Serbs turned to the other great power that had controlled them for centuries: the Austro-Hungarian Empire, another great nation that was showing signs of weakening. After multiple attempts, a Serbian nationalist assassinated the Archduke Franz Ferdinand, nephew to the Austrian emperor Franz Joseph, in Sarajevo, precipitating the First World War.