They were on a quest for world domination, so naturally the US or the rest of the world wasnt going to let that happen...
In the late 30's Hitler rose to power and was considered to be (by Time Magazine) one of the most influential people of that era. The Germans adopted a battle tactic known a Blitzkrieg or lightning war, where they attacked several cities in Belgium and took control in a matter of a few hours (I believe). So in a nut shell the rise of Hitler to power is in fact the cause of WWII. Hitler had a dream of a powerful and dominating Germany. To do that required him to conquer the nations of Europe and ally with neighboring Italy. While the Nazi party had it's eye on Russia for some time the Marxists were allied with Nazi Germany for a few years before Hitler made the mistake of attacking an unaware Russia.
She ste nachoes man waz up with thaat blame it on hitler when it waz his goddy mama jank