Memes

I'm sure that's how you were taught in (presumably) Canada, but that's not what happened. The Soviets spent the previous decade trying to form an anti-Nazi alliance with Britain, France, etc, who had instead signed non-agression pacts with Nazi Germany. It wasn't until the eve of wartime that the Soviets agreed to a non-agression pact with the Nazis to buy time before the inevitable war.

The Soviets expected it. The Nazis attacked the Soviets just like everyone knew they would, because the Nazis wanted to commit genocide on the Slavs and because Nazism is inerently anti-communist, and communism inherently anti-fascist, as communism is proletarian and fascism is bourgeois.

Harry Truman, in 1941 in front of the Senate, stated:

If we see that Germany is winning we ought to help Russia, and if Russia is winning we ought to help Germany, and that way let them kill as many as possible, although I don’t want to see Hitler victorious under any circumstances.

The West wanted the Nazis to exterminate the communists.