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Rex 84, short for Readiness Exercise 1984, was a plan by the United States federal government to accommodate the detention of large numbers of American citizens in case of massive civil unrest or national emergency.
According to scholar Diana Reynolds:
- The Rex-84 Alpha Explan (Readiness Exercise 1984, Exercise Plan), indicates that FEMA in association with 34 other federal civil departments and agencies conducted a civil readiness exercise during April 5-13, 1984. It was conducted in coordination and simultaneously with a Joint Chiefs exercise, Night Train 84, a worldwide military command post exercise (including Continental U.S. Forces or CONUS) based on multi-emergency scenarios operating both abroad and at home. In the combined exercise, Rex-84 Bravo, FEMA and DOD led the other federal agencies and departments, including the Central Intelligence Agency, the Secret Service, the Treasury, the Federal Bureau of Investigation, and the Veterans Administration through a gaming exercise to test military assistance in civil defense.
- The exercise anticipated civil disturbances, major demonstrations and strikes that would affect continuity of government and/or resource mobilization. To fight subversive activities, there was authorization for the military to implement government ordered movements of civilian populations at state and regional levels, the arrest of certain unidentified segments of the population, and the imposition of martial rule. 
Existence of the Rex 84 plan was first revealed during the Iran-Contra Hearings in 1987, and subsequently reported by the Miami Herald on July 5, 1987. As news of the exercise spread, it was amplified by networks of people pursuing more elaborate conspiracy theories. 
Diana Reynolds, "The Rise of the National Security State: FEMA and the NSC," CovertAction Information Bulletin, issue #33 (Winter 1990). (see external links below)