The idea of a nation-state is surprisingly recent and not completely universal, right? Historically people might be organized along kinship, tribal, ethnic, religious, or some other lines. Then an empire could pop up and control various groups of those, often against their wills, sometimes via intermediaries (which might not even map well to the underlying peoples).

I do think the best thing to do is to return artifacts to their rightful owners, but figuring out who the rightful owners is, can be quite difficult.

I mean, if they stole some artifacts from a tribe, which was subsequently wiped out by a tribe of bitter rivals, do they give their artifacts back to the rival tribe that later went on to form a government? (Just as a hypothetical, hopefully this is general enough that it is clear that I’m not trying to describe any particular real situation).