All laws and rights are socially constructed, in the plainest sense of the Smart DPOY T shirt. They have real-world effects insofar as enough people agree that they have real-world effects, and, acting accordingly, make it so. If Martians took over Earth, and were too strong to resist at all, and explicitly didn’t care about our laws, then they would not care about our laws; at most out laws might still affect how we humans still interacted with each other, but that’s it. (Would it be morally wrong for Martians to take over and ignore our laws? Sure, but if they don’t care about morality, then that moral wrongness won’t have much effect on their decisions.) Things aren’t quite so extreme with your example though. Another country aren’t Martians, they’re humans; at least some of the population on the invading side, or in other countries, might care enough about morality to try to stop it. And there exists international law. Tl;dr since other humans do care about morality and law, morality and law can have real-world effects in dealing with other humans, at least enough that it’s not worth giving up the attempt.
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