I guess the question you raise is, basically, should America have retained its isolationist stance, and should Britain have just let things be with Hitler?
First off, I'm going to be a realist here about the Allied nations rather than romanticizing them as heroic defenders of freedom, democracy, and equality. They were filled with corrupt individuals respecting profit and competitive business practices over any type of principle. They enabled the rise of Hitler on many levels, and corporations in America were more than eager to do business with his regime during most of the 1930s. They weren't "good guys." They weren't "evil" either, of course. They were profiteers with business as their main goal.
However, Hitler was among the worst of the worst. He always had conquest in mind, and was doing so in Europe before Britain declared war. He was a mess that the Allied nations created, and I'm glad he basically forced them to clean it up. He should have been looked at as an object lesson of what happens when a country full of people give in to desperation, emotion, and fear, and exactly what governments under the current world order can become if allowed to take the policies they live on to its logical extreme. Of course, he wasn't, but is simply dismissed as an "evil" individual who behaved as he did just because, I suppose. Let us keep in mind that one of the Allied heads of state was Stalin, who was no better than Hitler but found himself in an adversarial with the latter and thus took the side of the "good guys" until after the war when he found himself in a similar adversarial/competitive position with the USA and other plutocratic "democracies." Then he became a "bad guy" to much of the West. Remember this rule of thumb: a fascist dictator is only a "bad guy" to the USA and its allies when he doesn't play by the rules set forth by the American government and corporations. So long as they do, they are considered on the side of the angels, like Saudi Arabia, Kuwait, etc.
Any one of those nations were a potential fascist state, and they still are. Note again to my recent exchange with Tom on the war in Ukraine as to how even the smartest and the bravest of us can give in to nationalist zeal and rationalize proto-fascistic behavior like censorship and the glamorization of heads of state as heroes looking out for the masses. Even the best of us is vulnerable to emotional manipulation into a tribalistic frenzy when the mainstream media only allows one side to be reported. We MAPs should know that better than anyone, right?
To respond to another of Tom's critiques that is relevant here: I am not supporting anarchy over a genuine Left government. I am supporting a government of a different sort than any nation currently has, which is one run from bottom to top by workers on the basis of cooperation, not by corporatists and bureaucrats on the basis of national and international competition over resources. A Left leaning government that falls short of that will ultimately carry the seeds of potentially becoming a system that justifies policies which are supposed to be anathema to what the classical Left was supposed to embrace: diplomacy, freedom of speech, and equal rights for all. You very clearly lose these things when you're based on a competitive system that naturally breeds war.
I just wanted to make that clear.