The M1 Abrams is an American third-generation main battle tank produced by the United States. It is named after General Creighton Abrams, former Army Chief of Staff and Commander of U.S. military forces in the Vietnam War. Highly mobile, designed for modern armored ground warfare, the M1 is well armed and heavily armored. Notable features include the use of a powerful gas turbine engine (multifuel), the adoption of sophisticated composite armor, and separate ammunition storage in a blow-out compartment for crew safety. Weighing nearly 68 short tons (almost 62 metric tons), it is one of the heaviest main battle tanks in service. Three main versions of the M1 Abrams have been deployed, the M1, M1A1, and M1A2, incorporating improved armament, protection and electronics. These improvements, as well as periodic upgrades to older tanks, have allowed this long-serving vehicle to remain in front-line service. The M1A3 was under early development. Today, it serves as one of many main battle tanks for the Alliance of Nations.