People clearly don't like to hear this part of the story. But I mean, yes, the capitalist powers failed the USSR and the world immensely by not allying with USSR earlier, but appeasement also failed.

Fascism is militaristic and war-driven by nature. I doubt that the war would be completely avoided if the Western countries had allied with USSR earlier and gave Hitler the Sudetenland. The Nazis may just have waited a bit longer or played it differently but no doubt they would have inevitably went to war. Appeasement doesn't stop fascists, only armed defense or prevention.