At risk of being wrong,
a) You cannot attack a land territory that has neutral units. You must declare war against them before attacking.
Combat: A power can’t attack a territory controlled
by or containing units belonging to a power with
which it is not at war.
(The next sentence deals with naval battles, which are very different–and may lead to some confusion regarding land battles)
If a power at war attacks a sea
zone containing units belonging to both a power with
which it’s already at war and a power with which it’s
not at war, the latter power’s units are ignored. Those
units won’t participate in the battle in any way, and a
state of war with that power will not result
BUT, --and this is where I might be wrong – the Russians cannot move to Korea without declaring war on Japanese. Even if the US owns it. The basic political rule for Pacific is:
At the beginning of the game, Japan is at war only with China. Japan considers movement of units into or
through China by any other Allied power as an act of war against it.
Now there is ambiguity on what is considered “China”. I would defer to the WW II definition that Korea was a province of China that Japan annexed early in the war. Thus, Russia cannot move towards Korea – even to “defend” the US territory without declaring war on the Japanese.