Germany (Hitler) was trying to conquer land and countries for his own personal agenda. The European Nations used appeasement to try to make Hitler stop, by giving him some land, but dictators do not stop. So Europe went to war, and then Japan bombed the U.S. through Pearl Harbor in 1941, so the U.S. joined the fight.
The first world war. Germany lost so much during it, especially their pride when they had to do what was stated in the Treaty of Versaille, so when Hitler said he could fix everything, they were more than happy to let him (although they didn't know he was going to attempt a genocide)