The M551 "Sheridan" AR/AAV (Armored Reconnaissance/Airborne Assault Vehicle) was a light tank developed by the United States and named after General of the Army Philip Sheridan, from 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 152mm 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 of Military Forces in South Vietnam at the time, the M551 was rushed into combat service in Vietnam in January 1969. In April and August 1969, M551s were deployed to units in Europe and South Korea, respectively. Now retired from service, it saw extensive combat in the Vietnam War, and limited service in Operation Just Cause in Panama, and the Persian Gulf War in Kuwait. The Australian Army also trialled two Sheridans during 1967 and 1968, but judged that the type did not meet its requirements.
The Sheridan was retired without replacement officially in 1996. A large bulk of Sheridans were retained into 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.Read more on Wikipedia