June 28, 1914 Archduke Ferdinand of the Austro-Hungarian Empire was assassinated. The Duke and his wife Sophie were touring Sarajevo Bosnia, a part of Austro-Hungary at the time. Gavrilo Princip who was a member of a Serbian radical anarchy group, shot and killed both the Duke and his wife. Austro-Hungary blamed Serbia for the murder and ended up declaring war. This started a chain of events beginning with Russia backing Serbia, then Germany backed Austro-Hungary. So began the Great War.

More Info: en.wikipedia.org