Churchill said that the Battle Of The Atlantic was the theatre that worried him the most, even allowing for crash programmes in increased food cultivation at all levels.
But there was also fuel, here the Mediterranean and North African campaigns that Britain fought were vital, if now somewhat unsung.
Both in that theatre, as well as the Atlantic and Battle Of Britain, Hitler failed to get a knock out victory.
Britain being able to roll back Hitler was an impossibility by 1941, but it was by no means certain that Hitler would ever be in any position to visit London to see the Swastika raised over that city.
In late 1941, the US was a superpower in only the economic sphere, not militarily, the USN
, while big and impressive, was still not as large as the RN
, (who would in 1940, even without air cover and heavy losses, have massacred any German attempt to cross the Channel in a bunch of converted river barges and only a quite small German fleet to screen them, having taking a beating in the Norway campaign, despite winning there eventually).
But being an economic superpower, untroubled by air raids, made becoming a military superpower in short order much easier!
(In this respect, Lend Lease was a useful precursor to US mobilisation).
Despite the trouble we were in in 1941, after the invasion of Russia, the UK sent supplies to Russia, via the dangerous Arctic Convoys, another now sadly unsung effort.
It probably made little actual military difference, as least at first, politically it was vital however.
Then there is the Manhattan Project, a non US commitment to Europe would likely have made that effort if not delayed, maybe not even happening at all.
As those who created the whole enterprise, were motivated by fears of a Nazi bomb, including scientists who had fled from Hitler as well as British scientists too, working on this massive US project.
Now we know that Hitler was some way off getting one, not least due to efforts by the SOE, Commandos and Norwegian Resistance, but unencumbered by US entry into the war, he'd have got there eventually, along with an 'Amerika Bomber' or rocket, to carry it to NY and Washington.
Maybe after 'testing' it on London and/or Moscow.
What would be the point of eradicating European Jews and leaving a bunch in the US, which as a democracy and potential superpower, was always a medium to long term threat to a 1000 Year Reich.'