There's the old saying: WW2 was won with British intelligence, American steel, and Russian blood.
Maybe the British intelligence and American steel bought western Europe. Or maybe the success of the Manhattan project -- which Stalin was kept briefed on, in detail, by his spies -- gave him pause.
You could say that killing 15 million people seems pretty hostile, but that's not how historians count. Anyway the US and Britain weren't evidently bothered by it.