World War I started on July 28th, 1914. Tensions between the Serbians and the Austrians caused the war to start.
Franz Ferdinand and his wife were assassinated on June 28th, 1914. Having the Archduke of Austria assassinated was a big deal and later was known as one of the main reasons the war started.
With the enemies alliances growing so was the allied force's. The main allies in the war consists of Great Britain, France, Russia, and the United States.
Tanks were introduced during world war I. They were used as transportation to get across "no mans land." They provided protection from bullets and barbed wire.
The Lusitania was sunk by a German U-boat. The Germans at that time were using a policy called the unrestricted submarine warfare. 120 United states citizens died on that boat.
World War I was the first war that used aircraft. The aircraft however only served as reconnaissance until later when they could hold machine guns. Sometimes as the pilots flew over the enemy trenches they would drop darts which could pierce the helmets.