There was no such country as "Greece" in 1801. Athens was a city in the Ottoman Empire and "Greece" referred to a country that hadn't existed for nearly 2000 ye

There was no such country as "Greece" in 1801. Athens was a city in the Ottoman Empire and "Greece" referred to a country that hadn't existed for nearly 2000 years. "Greece" stopped existing as a country in 146 BC, when the Romans defeated its last city state, Corinth. The former country was then merely a region of the Roman Empire, followed by the Byzantine Empire, followed by the Ottoman Empire. If you want to give the marbles back, which is a popular sentiment these days, you should give them back to the legal successor of the Ottoman Empire, which is Turkey. The marbles have never belonged to the modern Greek state established in 1821. Once Turkey gets their marbles back, they could put them in the Acropolis museum if they think it appropriate.