In a hypothetical scenario that Russia, China or USA decide to "occupy" the Moon, they can not legally do that now due to having signed the Outer Space Treaty.
Can one of the signing countries exit the treaty?
If so, what are the practical consequences for that country? Is there some sort of penalty/collateral etc... (aside from "goodwill"/"trust levels")?
Please note that the generic game theoretic "Nobody will trust that country to enter treaties in the future" and "Everyone else will exit the treaty and space arms race will commense, possibly followed by space war" are both plausible but not what I consider "practical" for the purposes of this answer.