I wouldn't call them supporters, they just had interest in poland and both sides never intended it to be even an middle term agreement.
No, they literally signed an agreement between each other which they kept secret and was only revealed during the Nuremberg trials. Soviets only started fighting "the nazis" when they themselves were attacked by Germany in 1941. What's hilarious is Russians are still in denial about their role in the war and teach that the war started in 1941, not 1939. Also to this day people can be fined for correctly pointing out details of the invasion.
https://en.wikipedia.org/wiki/Molotov%E2%80%93Ribbentrop_Pac...
Oh, they can win or at least achieve something they could sell as a victory to their domestic audience. Russia's biggest strength is that they have endless hoards of people who are for some reason more than happy to go die for the dreams of their leader. No amount of US support is going to change that.