The region referred to as "the powder keg of Europe" was the Balkans in the early 20th century before World War I. Along with growing nationalistic movements declaring themselves independent countries, a number of European nations claimed influence and territories in the Balkans.
Much of the area in dispute was former territory of the declining Ottoman Empire. Nations that declared independence just before the turn of the 20th century included Greece, Serbia, Romania and Montenegro. In 1878, Austria-Hungary occupied Bosnia, and in 1908 it annexed Bosnia, provoking outrage in Russia and Serbia. The Ottomans complicated the situation by recognizing Austria-Hungarian control after getting paid for the territory. In October 1912, Greece, Serbia, Bulgaria and Montenegro started the First Balkan War to exploit the Ottoman Empire's weakness and gain territory. Argument over the spoils ignited the Second Balkan War. During these conflicts, Serbia strengthened itself as a national power.
The spark that ignited the powder keg was the assassination of Archduke Franz Ferdinand of Austria. A Serbian assassin shot him as he toured the city of Sarajevo, Bosnia, in an open car. Confident of German support, Austria-Hungary submitted a list of retaliatory demands on Serbia, and when all the demands were not met, they declared war. Due to a complex web of alliances such as the Triple Alliance of Germany, Austria-Hungary and Italy, and the Triple Entente of Great Britain, France and Russia, all of these countries and eventually the United States were drawn into the bloody conflict that became World War I.