The attack power of 18 men is so weak, you don’t need much to defend, or deter it.
Japan; minimum 6 men plus 4 planes plus 1 AAA–may still an invite an attack, but he’ll lose to your ripping fighter fire.
If he flew in Air, the calculus changes but Moscow is weakened so that’s good.
More and he wont do it, because even if he takes Manchuria or Korea, he gets $3 and loses 10-15 guys, so his only power over there is lost, which is a win for japan
Just don’t defend both, only 1 of the two.
If you bait an attack, make him go all in 18 guys, that’s usually in Japans favor because Russia will be stuck on the Coast in win/lose/retreat allowing you to wipe that out.