For example, if China were to, say, start rolling across Siberia, besides moving the mother of all bowel movements, there's absolutely no way in my mind Russia could stop a complete Chinese defeat.
In fact, if China did invade Russia, they could do so to "stop a raving mad man" and I dare say be cheered on by many in the west. China could...almost redeem a piece of itself. That would start a nuclear war, however, because Putin would simply press the button..but it's fodder for thought how China could play a role.