And the russians didn't turn on their ally?
Well in the pact with Hitler it was Hitler who attacked Stalin not the other way around. There is a big difference between nations attacking each other militarily and using diplomacy and clandestine operations. Both the USA and the USSR used the latter methods against each other.
But the real point is this fantasy of an attack on the USSR is only that. It was a political impossibility given the American people's mindset in 1945. Had we a system like the USSR or Nazi Germany where the People's opinion did not matter such a thing could have been done.