Take an extreme hypothetical. Suppose in the 1500s the government of Spain had taken some artifacts relating to the indigenous peoples of New Mexico. Would the government of the United States really be the rightful owner of these artifacts?

For other countries it's not quite as extreme, but in general the link between ancient culture in place X and modern country in place X is less strong than people try to make out.

It is an interesting theory, if I understand it. Is the Idea that controlling a geographic region makes one the rightful heir to any artifacts created by people and cultures previously in the region?

Does the USA have a claim to all indigenous Artifacts created in the US? It doesn't seem that different than Egypt laying claim to Egyptian artifacts.

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).