The radical libertarian city builders of the tech-bro set have an audacious new proposal: They want to convert Guantánamo Bay, host to the infamous prison, into the high-tech charter city of their wildest imaginations, which will double as a “proving ground” for migrants seeking to enter the United States. The Charter Cities Institute, or CCI, which has lobbied the Trump administration on setting up so-called freedom cities in the U.S, suggests the president take advantage of Guantánamo’s special legal status to convert the controversial detention camp into “a beacon of 21st-century prosperity.”
Cuba objects based on the Vienna Convention treaty on treaties from the Carter era, but that explicitly states it doesn’t apply retroactively. Cuba doesn’t like it, it’s likely unethical, and it would be illegal if it went into effect today, but as it stands it’s not illegal under international law.
The same would also apply to Puerto Rico and other islands won from Spain.
That original treaty with Cuba was renegotiated later (to be more in Cuba’s favor) and in any event requires that both sides agree or the US vacate the area in order to annul the treaty. Obama tried to close down the prison (one of the best things he tried to do as president), but Congress stopped him.