One of the factions that appears in Fallout 3 is The Enclave, a group comprised of the last remaining members of the U.S government. They maintain a chain of command mirroring that of pre-war administrations and are led by a president. They also possess a powerful military branch and have some of the most technologically advanced weapons at their disposal, even more so than the Brotherhood of Steel. In Fallout 2, it was discovered that they played a major hand the Vault Experiment Program, the purpose of which was to test the effects of different living conditions on human behavior.
The Enclave serves as a major antagonistic force in the game. They aim to take back the continental United States by force, eradicating all beings that have been exposed to radiation regardless of whether or not they have been physically mutated.
