Thats a bit dodgey...if you have the time and access one of those machines which shoots tennis balls or the cricket ones would do a good job...just adjust the angle...
Well my understanding of the US entry is (off the top of my head):
Entered due to German subs sinking a British ship that had Americans on it (not sole reason but sparked it)
US involvement meant the German Generals knew defeat was imminent.
So the German Generals decided to launch a spring...