That was certainly the case on The Walking Dead with the various surviving communities. But we should hope the actual world would operate a little more lawfully than a post-apocalyptic free for all.

The law is only lawful because the guys with guns say it is

So Russia's invasion of Ukraine will be legal if Russia wins? I doubt most people in the West will see it that way. Might makes right has never been a good basis for law.

I would argue that the concept of "legal" has no meaning in this setting. But if Russia wins in Ukraine, everyone will call it illegal, and nobody will do a damn thing to push them out. Eventually the world will recognize it as Russian territory just like they recognized it as Soviet territory and part of the Russian Empire before that. So yeah, it will be legal.

Keep hoping!