Gavrilo princip shoots archduke franz ferdinand in sarajevo and starts

Explanation:

Austrian Archduke Franz Ferdinand was assassinated in Sarajevo in 1914 by Gavrilo Princip, a member of a Serbian radical group called The Black Hand.  The reactions by the Austrian and Serbian governments kicked into gear various alliances that had been made in the years prior to the start of the Great War.  Austria threatened Serbia, holding the Serbian government responsible for actions of  the Black Hand group.  Russia stood ready to protect Serbia.  Germany sided with Austria over against Russia.  When Russian troops mobilized, Germany declared war on Russia ... and went and attacked France (which was an ally of Russia).  Before long the whole affair plunged Europe into war, pulling many nations into the conflict in support of prewar alliances they had made.