Partitioning Poland isn't "playing along".You're missing the simple point that he was simply playing along with Hitler and feeling him out, seeing how beneficial an alliance with him would or wouldn't be for Russia. Hitler was probably doing essentially the same thing in that relationship, except Hitler was a bullheaded moron when it came to offensive strategy and he megalomaniacally turned on someone who would have made a much better ally then an enemy. He sure fucked that up and it was ultimately the most fatal mistake he ever made for his country.