The FV4201 Chieftain was the main battle tank (MBT) of the United Kingdom from the 1960s into 1990s. When introduced, it was among the most heavily armed MBTs of the era, mounting a 120 mm Royal Ordnance L11 gun, the equal of the much larger specialist heavy tanks then in service. It also was among the most heavily armoured, with up to 195 mm (7.7 in) that was highly sloped to offer 388 mm (15.3 in) thickness along the line of sight.
A development of the Centurion, the Chieftain introduced the supine (reclining) driver position to British design allowing a heavily sloped hull with reduced height. A new powerpack and improved transmission gave it higher speed than the Centurion despite being heavier due to major upgrades to armour protection and the armament; this allowed it to replace both the Centurion MBT and Conqueror heavy tank while performing their roles effectively.
The engine and especially suspension system were of older design and proved to be the design's primary drawback; it was said that the Chieftain was extremely effective if it broke down in a useful location. This led to a series of improved models with new armor, sensors, engines and suspension systems, and these saw sales export success. Among these was the Shir 2 version for Iran, which added Chobham armor, one of first British uses. This order was cancelled due to the Iranian Revolution in 1979.
It was intended that the Chieftain would be replaced by a new design, the MBT-80. When tensions with the Warsaw Pact rose in 1980, Vickers offered a further updated version of the Shir 2 which became Challenger 1. This could be available years earlier and still met many of the MBT-80 design goals. Challenger deliveries began in 1983, but initially proved problematic and the Chieftain remained in front-line service until 1996 with the introduction of the Challenger 2.
