World War I started in 1914 and involved more countries and had greater destruction than any other war except World War II. World War I began because a terrorist from Serbia (now part of Yugoslavia) assassinated the Archduke of Austria-Hungary.

France, Britain and Russia were the Allies who supported Serbia. They were against Austria-Hungary and Germany. The United States remained neutral at first. Then the Germans started sinking unarmed U.S. ships, so the United States joined up with the Allies. The U. S. helped the Allies win the war in the fall of 1918.

The war helped overthrow emperors of Austria-Hungary, Germany and Russia. New nations were developed because of the loss of power. These events created conditions that led to World War II.