Answer: an executive agreement made by the previous president.
Executive agreements are agreements made between the President of the United States and the head of government of another country, but that have not been ratified by the legislature in the way treaties are. These agreements are binding, but can be dissolved if a new president enters office. Moreover, a president cannot unilaterally enter into executive agreements on matters beyond his constitutional authority.
After JFK set up the quarentine to prevent supplies to be sent to Cuba, Fidel Castro asked asked Nikita Khrushchev the leader of the CCCP (Soviet Union) for advice or help
