Ummm....
Communist presence as the "main force" opposition to fascism was a... sometime thing.
There was the <expletive deleted> Third Period of the Comintern, during which it was policy to turn all energies to fighting the social democrats--the "social fascists"--and leave fighting the real fascists until later. The hope was that the fall of political democracy would redound to the benefit of the Communists, rather than the fascists...
And then there was the period between August 23, 1939 and June 22, 1941... Which one of the Hollywood Ten was it who wrote _Johnny Got His Gun_ as part of a campaign organized from Moscow Center to try to keep the U.S. from aiding Britain against Hitler?
I think that the only three groups of people deserve credit for being the "main force against fascism": Winston Churchill and his circle, who pushed Britain into guaranteeing the integrity of Poland and then going to war against Hitler when he attacked Poland; Edouard Daladier and his circle, who went along with the British in drawing the line and then going to war. Only Britain and France went to war against Hitler. Everyone else stayed on the sidelines, waiting and hoping to avoid Hitler's notice, and was sucked into World War II in Europe only when Hitler went to war against them.
I said three groups of people, and I have only mentioned two. The third group: Franklin D. Roosevelt and his circle--Henry Stimson, Dean Acheson, and company--who did their damnedest to try to pull the U.S. into World War II even though isolationist sentiment was very strong...
Brad DeLong