“Who was worse: Hitler or Stalin?” It isn’t a parlor game, but rather the historical equivalent of “ISIS or Iran (or proxy Syria)?” You can play it either way, but “a plague on both their houses” didn’t cut it in 1941, and it doesn’t cut it in 2015.
The Molotov-Ribbentrop Pact of 1939 gave Hitler room to conquer Western Europe without fear of an attack on his rear. Stalin was a committed member of the pact until Hitler broke it, at which point Stalin brazenly pivoted to the Allies and demanded rights as a) a full partner and b) an aid recipient. FDR might have said, “Boy, watching Adolph and Uncle Joe battle it out would be great – fascists vs. communists, and no American boots on the ground.” But even before Pearl Harbor and Germany’s declaration of war against the U.S., FDR and Churchill set aside their disgust for Soviet internal behavior. They knew full well about the millions dead in the Stalin-engineered Ukrainian famine, even if Walter Duranty was keeping it from NYT readers. But they determined that Hitler was the greater immediate threat.