The M551 Sheridan was a light tank developed by the United States and named after Civil War General Philip Sheridan. It was designed to be landed by parachute and to swim across rivers. It was armed with the technically advanced but troublesome M81/M81E1 152mm gun/launcher which fired conventional ammunition and the MGM-51 Shillelagh guided anti-tank missile.

It entered U.S. Army service in 1967. Under the urging of General Creighton Abrams, the US Commander of Military Forces in 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 Korea, respectively. Now retired from service, it saw extensive combat in Vietnam, and limited service in Operation Just Cause (Panama), and the Gulf War (Kuwait).

At the time of the M551's acceptance into service production in 1966, the United States Army no longer used the heavy, medium, and light tank classifications. In 1960, with the deactivation of its last (M103) heavy tank battalion, and the fielding of the new M60 series tank, the U.S. Army had adopted a main battle tank (MBT) doctrine; a single tank filling all combat roles. The U.S. Army still retained the M41 Walker Bulldog light tank in the Army National Guard, but other than the units undergoing the transitional process, the regular army consisted of MBTs. Partly because of this policy, the new M551 could not be classified as a light tank, and was officially classified as an "Armored Reconnaissance/Airborne Assault Vehicle".