Operation Garden Plot is a general U.S. Army and National Guard plan to respond to major domestic civil disturbances within the United States. The plan was developed in response to the civil disorders of the 1960s and is now under the control of the U.S. Northern Command (NORTHCOM). It provides Federal military and law enforcement assistance to local governments during times of major civil disturbances.
Garden Plot was last activated (as Noble Eagle) to provide military assistance to civil authorities following the September 11, 2001 attacks on the United States. The Pentagon also activated it to restore order during the 1992 Los Angeles Riots.
Under Homeland Security restructuring, it has been suggested that similar models be followed.
"Oversight of these homeland security missions should be provided by the National Guard Bureau based on the long-standing Garden Plot model in which National Guard units are trained and equipped to support civil authorities in crowd control and civil disturbance missions." Testimony of Major General Richard C. Alexander, ANGUS (Ret.), Executive Director, National Guard Association of the United States, Senate Appropriations Committee Hearing on Homeland Defense, April 11, 2002
Rex 84, short for Readiness Exercise 1984, is a plan by the United States federal government to test their ability to detain large numbers of American citizens in case of civil unrest or national emergency.
According to scholar Diana Reynolds:
Existence of a master military contingency plan, "Garden Plot" and a similar earlier exercise, "Lantern Spike" were originally revealed by journalist Ron Ridenhour, who summarized his findings in "Garden Plot and the New Action Army."
Rex 84 was mentioned during the Iran-Contra Hearings in 1987, and subsequently reported on by the Miami Herald on July 5, 1987.  Nonetheless, the basic facts about Rex 84 and other contingency planning readiness exercises--and the potential threat they pose to civil liberties if fully implemented in a real operation--are taken seriously by scholars and civil libertarians.
Exercises similar to Rex 84 happen regularly. Plans for roundups of large numbers of persons in the United States in times of crisis are constructed during periods of increased political repression such as the Palmer Raids and the McCarthy Era.
For example, from 1967 to 1971 the FBI kept a list of over 100,000 persons to be rounded up as subversive, dubbed the "ADEX" list. This list contained many labor leaders, scholars, and public figures of the time.
In 2008, for the first time an active military unit has been given a dedicated assignment stateside for civil unrest containment. It is assigned to Northcom, a joint command established in 2002 to provide command and control for federal homeland defense efforts and coordinate defense support of civil authorities.
^ Reynolds, 
^ Ridenhour, Ron (1975). "Garden Plot and the New Action Army". CounterSpy.
^ Chip Berlet, Right Woos Left, 
^ Berlet and Reynolds
^ Diana Reynolds, "The Rise of the National Security State: FEMA and the NSC," CovertAction Information Bulletin, issue #33 (Winter 1990).
^ Donner, Frank (1980). The Age of Surveillance: The Aims & Methods of America's Political Intelligence System. New York: Alfred Knopf. pp. 166.
^ Cavallaro, Gina; "Brigade homeland tours start Oct. 1"; Army Times, online, Monday Sep 8, 2008 
Operation Urban Warrior - Marine Corps assistance to civil authorities.
"Reagan Aides and the 'Secret' Government," Alfonso Chardy, Miami Herald, July 5, 1987. Also available as photoscans in PDF format here.