They started out as enemies of the Allies, then they became enemies of the Axis powers when they changed their allegiance. And when they were pretending to be friends of the Polish people under Nazi rule, they intentionally failed to show up as promised to help out with the Warsaw uprising so that the Polish uprising leaders would die and not get in the way when they instituted communism in Poland (I saw the new "Warsaw Rising Museum" this past summer). Brings to mind the phrase "with friends like these, who needs enemies?" But to set clearly set the record straight, the Russians were indeed the enemy at some point no matter which side you were on.
The Soviet Union was always a US ally during the period of the war in which the US was a combatant. With 26,000,000 or so dead, [1] it is hard argue that they put any lower value on lives in Warsaw than its own citizens.[2]
Russia definitely did not promise to help out with Warsaw uprising and actively discouraged it. Uprising was a coup by Polish leaders to liberate Warsaw before Russian army arrives to be in a better negotiating position after the end of war. Soviet Union was under no obligation to help anti-soviet elements to fight Germans - enemy of my enemy is my friend is true only up to the point.
I don't know that you can back up your first sentence. I'm pretty sure the museum in Warsaw claims that they did plan to work together. Also, the museum shows part of a TV show from after the war which served as Soviet propaganda. The TV show rewrote history to say the Soviets fought alongside them. My Polish friend is in his mid-30s like me and explained how they were still showing this old TV program when he was a child in the 80s. Now he knows it was a lie.
Tell that to the Poles, the Finns and other nations that were invaded by the Russians.
Only after Germany invaded Russia, Stalin decided that they are an enemy that should be dealt with.
