Guantanamo Bay (shortened here to GB; nicknamed ‘Gitmo’), situated on the southeastern coast of Cuba, is an anomaly. At that site, the USA operates the only military base in a communist country.
A 1903 agreement permitted the unusual arrangement of GB, essentially in perpetuity. That agreement was renegotiated in 1934.
The USA does not recognize the legitimacy of Fidel Castro as Cuba’s leader.