It’s a bit tricky, I had some trouble coming to grips with it as well.
Here’s the rules, as best I understand them:
Aggression-qualifying attacks consist of:
- Russia attacking Japan in Korea or any Japanese-controlled Chinese territory that borders Mongolia
- Japan attacking any Russian territory that borders Mongolia
Whoever does one of these first is the aggressor from the Mongolians’ point of view.
If Russia is the aggressor, Mongolia stays forever neutral.
If Japan is the aggressor, they become Russian at the END of Japan’s turn in which the aggression occurs. So Japan cannot in the same combat move attack both (without declaring war on all neutrals). Since China can’t move into Mongolian spaces, the earliest anyone can interact with these newly Russian units and territories is the UK turn after Japan’s aggression.
Hope that helps! Best to watch it unfold in others’ games to get a more concrete sense of what’s going on there.