Question

Would the Allies have won WW2 without the USA's help?

Answer 1

It's very doubtful. When the Nazis marched into France, the French were just done. They didn't muster up a defense; they were literally handing baguettes and croissants to the Germans. Without American aid, it's likely that the Nazis would've been able to conquer all of Western Europe. However, Eastern Europe is more ambiguous given the power of the USSR. Stalin most likely would've prevented Hitler from taking over the East. Had the U.S. not fought against Japan, a divided, powerless China would've probably been unable to fight off Japan; China could've been a colony of Japan. There is also a bit more ambiguity here because the USSR may not have wanted a hostile Japanese neighbor in China, so it may have been compelled to fight Japan as well.

However, at the end of the day, it's impossible to deny the profound role the U.S. had in helping the allies.

Answer 2

Yes, @rogue put it very nicely. Also, there is the factor of Great Britain, which was the only real (Western) European power at the time that would and could fight the Germans off, to the last man standing. But they were growing increasingly weak with each raid and bomb, so the Americans had come at an opportune time. Before the bombing of Pearl Harbour, Britain was practically begging the US to join, but the US were laying off, even though everyone knew they were needed greatly. When they did join, things went much swifter.