Replying to HMWMH

(Help me with my homework)
Ive got a test on this tommorow so tell me what you know. Google is not giving me answers on this.

I understand that in the 1940s the USSR was controlled by Stalin who was a communist. And the Nazi party was socialist. So if this is true why did they hate eachother enough for the US to support communism in Stalin and the USSR? I mean isn''t socialism basically one step below communism (I may be totally wrong on this)? I feel like it would have made more sense for the Nazis not to have hated the USSR so much and for them to have kept their original treaty together throughout WWII.

Anyways, I guess my question here is why did Hitler hate Stalin and how the hell did the US end up supporting Russia's communism over the Nazi's socialism.