The USA sold weapons to the allies largely because the president genuinely believe hitler would invade everything if not stopped. Congress wouldn't let him enter the war, because the US has a strong Isolationist vibe, but the President believed if we didn't go stop hitler in europe, hitler would eventually find his way to coming after the US and by that point we wouldn't have any allies left.

This is AFTER the allies attempted appeasement. If a foreign country wants to war, you can't not war. The only alternative is to roll over and accept new ownership, but usually new ownership disagrees with the people on how things should be done, so that "solution" largely isn't.

There was a period were Germany seemed unstoppable remember.