Enclave

The Enclave  is a remnant of the United States pre-War government.

Background
The Enclave was comprised of members of the government, Joint Chiefs, and successful scientists. Research initiatives of the entity included those centered on power armor, rocketry, and energy weapon prototypes. During the Resource Wars, members of the Enclave made contingency plans in case of nuclear war, included securing installations such as the presidential oil rig and congressional bunker. In March 2077, the president of the United States retreated to the oil rig, resulting in news outlets such as the Boston Bugle reporting the action "giving credence to the long-running rumors of a secret, militarized shadow government, known as the Enclave." 

Appearances
The Enclave is mentioned in all games throughout the Fallout series, with the exception of Fallout and Fallout: Brotherhood of Steel.

Behind the scenes

 * The Enclave was planned for inclusion in Van Buren and mentioned in the Fallout Bible.
 * The faction is mentioned in the Fallout 3 Official Game Guide and the Fallout: Wasteland Warfare Roleplaying Guide.