Imagine if the US told Britain, YOU ARE ON YOUR OWN, when they began to battle Germany in WW 1?
Answer is, an armistice, a retrenching of all parties and NO HITLER.
We should have told Britain that unless they allowed our ships past their blockade of Germany, we wouldn't risk our ships to supply them.
Except European nations have been around for a very long time and fought countless brutal wars and had dictators long, long before America was even founded, and they still managed to develop the most advanced civilization on the planet and give us the basis of most of our technologies and norms today.
Europe has managed to do quite well without American intervention.
Hitler is not the first evil director Europe has had through history and they managed to deal with the others without US intervention.