Staying out of it was what the people in America were demanding, until we were attacked. We used to be an isolationist country for most of our history. Since WW2 though, our politicians seem determined to send our young men to die for causes which affect them only tangentially all over the world.
Germany would have developed the A bomb, the first intercontinental missiles and long range bombers. They probably would have failed at invasion but the third Reich and it's world influence would have left us as a primarily agrarian nation.