Why did the great war start?

The causes of the Great War (or, World War I) were several in number. Principally, the assassination of a highly placed leader in the Austro-Hungarian Empire, the Archduke Franz Ferdinand, led to hostilities in the Balkans. As a result, the infamous 'alliance-system' of Europe led to the addition of other belligerents. Throughout, the rampant nationalist that dominated Europe at the time led to eager involvements from still other nations.