Why would be angry at Poland defeating the Bolsheviks, if I thought that the Russian invasion of Poland was an act of imperialism?
Because you would support the Bolsheviks over some nation's right to self-determination. Communists don't believe in national self-determination, remember? Maybe you would blame lack of revolution in Germany on Polish independence because Russian troops couldn't march into Germany to aid it.
edit: Maybe you would adopt the definition of imperialism offered above, where only capitalist nations can be imperialist, by definition?
"This is my test of character. There you have the despotic instinct of men. They do not like the cat because the cat is free, and will never consent to become a slave. He will do nothing to your order, as the other animals do." — Jean-Jacques Rousseau.
"The intellectual and emotional refusal 'to go along' appears neurotic and impotent." — Herbert Marcuse.
"Our blight is ideologies — they are the long-expected Antichrist!" — Carl Gustav Jung