Lasted from 1914 to 1918

AKA The Great War and The European War.
When Archduke Ferdinad was shot (June 4, 1914), Austria sent Serbia an ultimatum demanding concessions that they didn't expect to get. They were trying to provoke a war. Serbia sent back their answer a mere two minutes before the deadline. It did not make clear what their answer was, but it was not unfriendly. Austria attacked anyway. Russia had promised Serbia support in case of an attack. France had assured Russia of its support. Great Britain was hoping to reach a peaceful solution, and Germany had told everyone to stay out of the whole thing.

It ended up with Germany declaring war on Russia (August 1), and then on France (August 3). En route to France, Germany attacked Belgium. Belgium was on Great Britain's side, so Great Britain declared war on Germany (August 4).

It was big at this point, but it got bigger. All I'll add is that the U.S. was trying to get everyone to stop fighting, but the Germans kept sinking their boats. Then the U.S. found out that the Germans were trying to get Mexico to attack them. The U.S. joined the Allies.

10 million died, 20 million wounded. Estimated $200 billion spent trying to kill people.

Please add. This war had much more to it.