alternatehistory.com

It's been a while, but as I recall this was after the USSR's victory over Japan, after which China didn't seem particularly dangerous. Plus there were (at least theoretically) communist allies on the ground and no one else was in a position to intervene and help the Chinese.

Stalin says "the rotten edifice that is their empire will collapse and China and Korea will fall into the hands of their respective proletariat" right before the full attack on Japan.
The same paragraph says later "At the same time Red Army troops who had been stationed in Soviet occupied Xinjiang began an offensive against the Nationalist aligned warlord Ma Bufang."

So Stalin seems to attack both once. Was he expecting a groundswell of support from the Chinese? Why attack Ma Bufang at all? Why not wait until Japan's mainland forces have been defeated?

You know, you're right. It's been so long since I wrote that TL that I've forgotten Stalin's original motivation.