https://www.reuters.com/world/europe/us-seizes-iranian-oil-c…
LONDON/ATHENS, May 26 (Reuters) - The United States has confiscated Iranian oil held on a Russian-operated ship near Greece and will send the cargo to the United States aboard another vessel, three sources familiar with the matter said.
It is probably not widely known, but the US Constitution gives Congress the power to grant letters of marque and reprisal. Basically, the government can sanction piracy on the high seas, if it is sufficiently motivated to do so.
https://www.law.cornell.edu/constitution/articlei
Article I, Section 8
The Congress shall have power...
...To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water;
Letter of marque and reprisal
https://en.wikipedia.org/wiki/Letter_of_marque
A letter of marque and reprisal (French: lettre de marque; lettre de course) was a government license in the Age of Sail that authorized a private person, known as a privateer or corsair, to attack and capture vessels of a nation at war with the issuer. After capturing, the privateers could bring the case of that prize before their own admiralty court for condemnation and transfer of ownership to the privateer. A letter of marque and reprisal would include permission to cross an international border to conduct a reprisal (take some action against an attack or injury) and was authorized by an issuing jurisdiction to conduct reprisal operations outside its borders.
I doubt they went through such formalities in this Iranian case. Rather, they just cited some existing official sanctions against Iran or Russia or both. Also, it was apparently the Greek authorities that did the initial confiscation of the ship and cargo.
- Pete