Causes of war can be like a domino effect

The assassination of archduke triggered the starting of WW1

A conflict between Serbia and Austria-Hungary became a WW

There was a lot of nationalism before the WW.

Germany, Austria-Hungary, Bulgaria, Othman empire were enemies of their rivals.

Britain invented a powerful weapon called the machine gun.

Germany proved chemical weapons were effective but weren't used.

New war tactic that was common in the war.

For part of WW1 the U.S were considered isolationist.

British passenger ship torpedoed. It got the U.S mad.

Germany tried to form an alliance with Mexico

Great Britain, France, U.S, Russia, Romaina, Serbia, Greece won.