Hague Convention of 1907
1970 UNESCO
1995 UNIDROIT
US:
https://en.m.wikipedia.org/wiki/Lieber_Code
2004 HR1047 section 204.
(Those are more about stopping theft / destruction- not return.)
UK:
Except for human tissue, it’s basically not legally regulated.
Australia:
No laws. There is a government program for aboriginal art and remains.
But as an example, Egypt frequently requests the return of the Rosetta Stone from the British Museum.
I can see a compelling argument someone might claim ownership of the stone, I'm curious where in the process they were shut down, if they didn't try, or they did and their suit was dismissed.
The closest “easy” thing would be a statutory claim I think.
But you can’t make it if the debt is over 6 years old.
https://www.gov.uk/statutory-demands
It appears you can sue for the return of property in civil court.
https://www.askthe.police.uk/view-category/?id=2ec7e112-6ad2...
I’m Sure though there’s complexity that make all of that not relevant if it’s a country suing.
Usually international treaty comes into place when things are international.
However, some interesting cases against the British museum.
https://www.artnews.com/art-news/news/a-uk-heritage-group-wi...
https://www.nytimes.com/2015/05/15/world/europe/greece-briti...
There’s a whole book on this dispute from a litigator.
https://www.elginism.com/elgin-marbles/why-dont-we-just-sue-...
And the British museum policy on giving back objects. https://www.britishmuseum.org/sites/default/files/2019-10/De...
Finally, it looks like The government may be preventing this. They may have been acted upon in 2005?
And also https://www.museumsassociation.org/museums-journal/news/2022...
However, this may answer all your questions.
https://davidallengreen.com/2021/05/the-british-museum-loote...