There's something to be said for systems that, more-or-less, are practical and work without making perfect logical sense or being perfectly self-consistent.
Building a perfect legal regime has a lot in common with building a perfect software application.
I believe the parent's argument was that land seizure by any country which has signed the UN charter after they've signed the charter is not legitimate.
It may not have been legitimate before, but signing the charter saying you agree it's not legitimate does put a nice line in the sand.
Sibling poster is correct. International law is basically that you can do whatever you want if you have the strength to back it up without backing down. Meaning that conquest is still de facto okay, even if you signed the charter to make it de jure not-okay. And as long as it's still okay, all those conquered peoples that lost their land can go pound... well, I don't know what they can pound, since the sand doesn't belong to them any more.
Greece, Iran, Italy, Mongolia, Turkey, Russia, and Ukraine all have potential claims.
If annexation by force is no longer a legitimate means of acquiring land, then it is reasonable to say that it was never legitimate. If it is, then going to war to take territory is still possible. But there really is no fair way to declare a fixed epoch and say that forceful annexation was only legitimate in the prior era, and is now unacceptable in the following era.
Either conquest is still okay, or it never was.
If it never was, you must be prepared to support repatriation claims from aboriginal peoples until the end of time. Perhaps there is a case for establishing adverse possession for land sovereignty. If you can take it and keep control of it for 20 years, your claim is presumed valid.
And it could give national rivals a nice, limited territorial war every 18 years or so, to feed their military industrial complexes through hard times, when the world is just too peaceful to make a profit.~