The M551 Sheridan was a light tank developed by the United States, named after Civil War General Philip Sheridan. It was designed to have both an air drop and swimming capability. It was armed with the MGM-51 Shillelagh gun-launched missile system. Production began in 1966, entering the US Army inventory 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. Now retired from service, it saw extensive combat in Vietnam, and limited service in Operation Just Cause (Panama), and Operation Desert Shield/Desert Storm (Iraq).DADĒt the time of the M551's entry into service in the 1960s, the United States Army no longer used the heavy, medium, and light tank classifications. The US Army had adopted the new doctrine of Main Battle Tank (MBT), a single tank filling all combat roles. The US Army still retained the M41 Walker Bulldog light tank in the Army National Guard, but other than the units under going the transitional process, the regular army only consisted of MBTs. Partly because of this policy, the new M551 could not be classified as a light tank, and was referred to as a "armored reconnaissance/airborne assault vehicle".