Right, USSR would have to declare war on Japan to move into USA controlled Korea.
If this is the case, and Japan attacks USA controlled Korea with USSR units in it, nothing happens with the pact or Mongolia.
The Mongolians only join Russia if Japan attacks a USSR controlled territory bordering Mongolia.
The Mongolia, Neutral, and Dutch questions just keep pouring in. I made a thread trying to lay out these rules in a different way, in an attempt to make them more understandable. You can read them here. Maybe I should ask a Mod to sticky this thread. Go to the first couple posts of this thread. I believe the correct answers to both Boldfresh and Karl’s questions can be found in them.
http://www.axisandallies.org/forums/index.php?topic=30776.15