But it is. International law is basically an illusion. It only works while participants agree - there's no way to enforce it because countries have sovereignty

But it is. International law is basically an illusion. It only works while participants agree - there's no way to enforce it because countries have sovereignty and as such they can do as they please if they really want to. Treaties are glorified pinky promises, and the only real recourse is threat military action, which can't be used against nuclear-capable states. There's only one country able to colonize Mars in the foreseeable future, and within the US there are only two entities possibly capable of such a feat : SpaceX and NASA. So at a minimum, there will be US law there initially, and if colony is viable the US will claim Mars. "International lawyers" will be told to pound sand shortly thereafter. Then I suppose people there will show us here on Earth their long martian middle finger and go their own way, much like the US itself did hundreds of years ago, and we won't be able to do jack shit about it.