Pretty much all international law is mostly a fictional construct, especially for major powers. Legality is dictated from the pulpit, and there is no overruling globalized government.

However, imo pretty much anything that a secret court dictates is a legal fiction dreamed up to legitimize illegal/morally decrepit actions.

How would that be handled?

The same way the current predicament will be… There may be some geopolitical blowback, but no one is going to hold anyone accountable.