By the late 1930s Communists were nationalists as well. The main difference between the Communists and the Nazis in the 1930s was the group each chose to demonize. By the end of the 40s, there was overlap even there.
Exactly. I've been making this point lately. In Europe the political axis defined as left/right ranks groups for their internationalism/nationism. In the US we use the same terms (left/right) to rank groups in terms of statism/individualism. These two axes don't measure the same things, but we use the same terms. That causes a lot of confusion. By our terms, fascists, nazis, and communists are just variants on leftism. By European terms, our conservatives don't fit on the axis at all. But since we tend to be for strong national defense and cultural values, they lump us with their "right." It's like asking, "How much is 2+2: blue or green?"
Fascism = National Socialism = Nazism
Communism = International Socialism
Liberalism = Progressivism = American Socialism
Conservatism = FREEDOM
True. Yet after the Cold War broke out, Communism's boasted internationalism proved to be more of a talking point than a reality. One of its most effective tools was to play upon the nationalist dreams of former colonies. Nationalist freedom fighters became common all over the world. This could be described as a "temporary" step along the way to the international utopia, but logical consistency has never been an attribute of the Communist mind, despite its claim to be scientific.