in tank design was achieved in the Soviet Union when the T-64 was produced which for the first time used an automatic loader, reducing the crew of the tank to three crewmen. Subsequently, this model, and the later T-72 and T-80 tanks introduced further innovations that influenced armoured warfare by introducing guided missiles into the tank ammunition mix, allowing ATGW fire from standard tank guns. The Soviet Union was also one of the countries that used two Main Battle Tanks: The high-quality T-80s and lower quality T-72s. Modern Soviet tanks, like the ones mentioned, are typically armed with smooth bore guns. Advancements in Soviet tanks include improved Fire Control Systems, strong armour protected by ERA, and defensive countermeasures (such as Shtora-1 and Arena). The most advanced Soviet tank, up until the end of the Cold War, was the T-80U, which shared similar characteristics with the M1A1(Turbine engine, advanced Fire Control Systems, strong armour, and firepower) Infantry fighting vehicles were first developed in the 1960s with the Soviet Union's BMP-1, for the first time allowing supporting infantry to accompany tanks on a battlefield when nuclear weapon use was expected. The T-64s and BMP-1s were also joined by the self-propelled guns and more importantly Mi-24 Rotary-wing aircraft capable of firing anti-tank missiles entering production in 1970 which were built and theorised as "flying tanks". The Soviet tank troops, as they were known in the USSR, included armoured units, armoured training regiments and other formations and units. Vietnam War M113 armoured personnel carriers proved effective in the terrain of Vietnam against enemy forces which, until 1968, rarely deployed their armour. Though they were soon countered with mines and RPGs, M-113's continued service during the war, primarily evolving into infantry fighting vehicles, known as the ACAV (Armoured Cavalry Assault Vehicle); and functioning as a "light tank." More heavily armed infantry fighting vehicles such as the M2/M3 Bradley Fighting Vehicle would be based on experience with the M113. Gun trucks were also introduced as M35 trucks fitted with armour and guns to protect convoys. In 1968, Communist forces primarily deployed the Soviet built PT-76 light tank. By 1971, the larger T-54 medium tanks were fielded, proving themselves susceptible to the M-72 LAW rocket, ARVN M41 Walker Bulldog light tanks, as well as the larger M48A3 Pattons. In January 1969, U.S. armoured cavalry units began exchanging their M48A3 Patton tanks for the M551 Sheridan Armoured Airborne