Archduke Ferdinand of Austria-Hungry was traveling with his wife when he was gunned down by a Bosnian terrorist on June 28, 1914.  This terrorirst wanted Bosnia to break away from Austria-Hungry and join Serbia.  The government of Austria-Hungry accused the Serbian government of the assasination and would declare war on Serbia one month later.

The assassination of Archduke Ferdinand was the immpediate cause of World War I.  Once war was declared on Serbia each side's alliances became involved in the war.  The rise of the militaries made it easier to transport the troops, supplies and machines to the enemies land and the nationalism in each nation made both sides determined to win the war at all costs.