What causes imperialism to start war?

what causes nationalism to start war?

what is a u-boat? how did german's use of then lead to comflict to the us?

why did the US formally enter WW1?

1) The war started mainly because of four aspects: Militarism, Alliances, Imperialism and Nationalism

2) It caused the war because Austria blamed Serbia for the killing of the Archduke Franz Ferdinand.

3) A German submarine used in World War I or World War II. During some of Germany's attacks, American passengers were on board of some of the ships. Thus being they felt that action was necessary

4), the U. S. joined its allies--Britain, France, and Russia--to fight in World War I.