otherwise it’ll completely legitimise nations to start wars to take land for themselves

Have you been sleeping for the last decades? There were plenty of wars. The US didn't stop them, in fact, the US started a bunch of them, and more were started by US allies, or waged with US help. The US supports illegal occupation ("taking land") all over, too.

So is there actually any norm anyone adheres to? Seems to me the actual norm is "don't do anything the US doesn't like". It's got nothing to do with starting wars or taking land.