International cooperation can still work even though there is very rarely a realistic prospect of enforcement. No one is going to war to enforce WTO rulings or nuclear non-proliferation treaties, for example.

It's true that when things get really hairy international law tends to fall by the wayside, eg countries leaving the Land Mine Ban Treaty now that it seems possible they may actually have to deal with a foreign invader on their soil. But they can still be effective at regulating states' behaviour in more peaceful times, which is still useful.

But it does require the major powers to be willing (i) to talk to each other, and (ii) to think about the world beyond their own borders, which means it's unlikely to happen given the current leadership in certain of the big space-going nations.