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The M551 "Sheridan" AR/AAV (Armored Reconnaissance/Airborne Assault Vehicle) was a light tank developed by the United States and named after General Philip Sheridan, of American Civil War fame. It was designed to be landed by parachute and to swim across rivers. It was armed with the technically advanced but troublesome M81/M81 Modified/M81E1 152 mm gun/launcher, which fired both conventional ammunition and the MGM-51 Shillelagh guided anti-tank missile.
The M551 Sheridan entered service with the United States Army in 1967. At the urging of General Creighton Abrams, the U.S. Commander, Military Assistance Command Vietnam, at the time, the M551 was rushed into combat service to South Vietnam in January 1969. Later that year, M551s were deployed to units in Europe and South Korea. The Sheridan saw extensive combat in the Vietnam War, where problems with the platform became evident, particularly its poor survivability and reliability.
Based on its experiences in Vietnam, the Army realized the shortcomings of the Sheridan, and after the war in 1975 began to eliminate the vehicle from its units in 1979. A modest fleet of vehicles remained in the 82nd Airborne Division and the National Guard. Various improvement programs were successfully undertaken to improve the Sheridan's reliability. Problems persisted with the 152 mm gun/launcher, and various efforts explored to replace it with a more conventional model. The Sheridan went on to serve in the invasion of Panama and the Gulf War. The Army sought to replace the Sheridan with the M8 Armored Gun System, but this was canceled in 1996, late in its development. The Sheridan was retired without a designated replacement in 1996. The Army acquired the M1128 Mobile Gun System to fulfill a similar requirement, but this is being retired in 2022. The Army current light tank acquisition program is Mobile Protected Firepower.
A large bulk of Sheridans were retained in service at the National Training Center (NTC) at Fort Irwin, California and as Armor Officer Basic training at Armor Training Center, then located at Fort Knox, Kentucky. They worked as simulated Soviet armored opposition force (OPFOR) to train U.S. military units on simulated tank on tank armored combat to test on combat effectiveness in a desert environment. They were finally retired from the NTC in 2003.

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