The M900 was the next logical step in the progression of APC's, after the M59, and M79, it was however most likely a variant of the M113. It differed from the M113 in that it had machine guns at the front, both sides, and the rear. (Fury's Pilgrims)
The M113 was a fully tracked armoured personnel carrier that was developed by Food Machinery Corp (FMC). The vehicle was first fielded by the United States Army's mechanized infantry units in Vietnam in April 1962. The M113 was the most widely used armored vehicle of the U.S. Army in the Vietnam War, earning the nickname 'Green Dragon' by the Viet Cong as it was used to break through heavy thickets in the midst of the jungle to attack and overrun enemy positions, but largely known as an APC and ACAV (armoured cavalry assault vehicle) by the allied forces.
The M113 introduced new aluminum armour that made the vehicle much lighter than earlier vehicles; it was thick enough to protect the crew and passengers against small arms fire but light enough that the vehicle was air transportable and moderately amphibious. In the U.S. Army, the M113 series have long been replaced as front-line combat vehicles by the M2 and M3 Bradley, but large numbers are still used in support roles such as armoured ambulance, mortar carrier, engineer vehicle, command vehicle, etc.