I know socialists see this as a critical component to their theories, and I understand why. But the truth is that the Soviet union, and China, both became very extreme totalitarian states ... not because of external aggression, but because they feared the civil war they themselves had used to get to power, before any capitalist aggression really began (long before WWII, and even during). They saw their own power (and with it, communism, at least in their minds) mortally threatened by the very mechanism that gave it birth, and therefore saw extreme measures as justified to prevent that.
I will agree that later on "capitalist" aggression made it worse. Although a significant (and the most complained about) part of capitalist "aggression" by far was accepting refugees from communism, and so perhaps aggression is not always the correct term. Other things weren't nearly so benign.
I would also like to see socialists explain why communist societies became so incredibly more aggressive after communes in the West (which were allowed to exist, in large numbers, where they weren't outright abusive) ... failed of their own accord. I mean, my idea is that they failed because they proved the truth of socialist fears : given free choice of it's members, every society will eliminate communism. Of course, I see history following the Soviet collapse as further reinforcing this idea.