The return of other nation's treasure, like the Parthenon Marbles (a.k.a. Elgin Marbles) is controversial.
It was the subject of a good TV debate recently. There were persuasive arguments on both sides, centering around the questions "were the artefacts in question legally obtained?" and "where should they ultimately be located?". There was also the global issue of where would the greatest number of people benefit from them.
The best suggestion to me was that exact copies of the artefacts should be made, so that the originals could be returned to their place of origin, with the proviso that all reasonable steps to protect them be taken.
It is not only the legality of one country holding another's treasures that is the problem. As with so many things, it is the perception. Should the UK be seen as a country holding on to spoils from an imperial past? I suggest not.