The M551 Sheridan was a light tank

Light tank

A light tank is a tank variant initially designed for rapid movement, and now primarily employed in low-intensity conflict. Early light tanks were generally armed and armored similar to an armored car, but used tracks in order to provide better cross-country mobility.The light tank was a major...

 developed by the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and named after Civil War

American Civil War

The American Civil War was a civil war fought in the United States of America. In response to the election of Abraham Lincoln as President of the United States, 11 southern slave states declared their secession from the United States and formed the Confederate States of America ; the other 25...

 General Philip Sheridan

Philip Sheridan

Philip Henry Sheridan was a career United States Army officer and a Union general in the American Civil War. His career was noted for his rapid rise to major general and his close association with Lt. Gen. Ulysses S...

. 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

MGM-51 Shillelagh

The Ford MGM-51 Shillelagh was an American anti-tank guided missile designed to be launched from a conventional gun . It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an...

 guided anti-tank missile.

It entered U.S. Army service in 1967. Under the urging of General Creighton Abrams

Creighton Abrams

Creighton Williams Abrams Jr. was a general in the United States Army who commanded military operations in the Vietnam War from 1968–72 which saw U.S. troop strength in Vietnam fall from a peak of 543,000 to 49,000. He served as Chief of Staff of the United States Army from 1972 until shortly...

, the US Commander of Military Forces in Vietnam

Vietnam War

The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

 at the time, the M551 was rushed into combat service in Vietnam in January 1969.