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.

A development of the Centurion, the Chieftain introduced the supine (reclining) driver position to British design allowing a heavily sloped hull with reduced height.

The multi-fuel engine proved to be the design's primary drawback;[5] 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.

in 1954 the decision was taken that "the next UK tank should be an evolutionary one designed to carry an HV gun with a bagged charge ammunition system".

However, it was eventually decided not to sell the marque to the Israelis (since, at that period of time in the late 1960s, the UK was more friendly towards the Arab states and Jordan than to Israel),[15] which prompted them to follow their own development programme.

The early Leyland engine delivered around 450 bhp (340 kW) to the sprocket, which meant a top road speed of around 25 mph (40 km/h) and cross-country performance was limited.

For security reasons early prototypes had a canvas screen covering the mantlet and a sheet metal box mounted over the sloping glacis plate to disguise the configuration of the vehicle.

This differed from most contemporary main tank armament as it used projectiles and charges that were loaded separately, as opposed to a single fixed round.

The design of the Bagged Charge System was initiated after the earlier Liquid Propellant Gun project had failed due to the injection pump being so large that "no tank could accommodate it".

[12] This led to design studies to find "means of reducing the ammunition size and from this the bagged charge concept emerged".

In the event of a hit penetrating the fighting compartment, the jacket would rupture soaking the charges and preventing a catastrophic propellant explosion.

The tank commander had a rotating cupola with nine vision blocks -giving all round view, plus the 7.62 mm machine-gun and an infrared (IR) capable projector coaxial with the weapon.

The commander could rotate his cupola to bring his sight onto a target and then engage the mechanism that brought the turret round on to the correct bearing so that the gunner could complete the aiming.

[19] The left side of the turret had a large searchlight with an electrically controlled infrared filter inside an armoured box, with a relatively long range – up to 1–1.5 kilometres (0.62–0.93 mi).

Many later examples had Stillbrew armour, intended to defeat Soviet 125 mm tank guns and heavy anti-tank missiles.

Like its European competitors, the Chieftain found a large export market in the Middle East, but unlike the Centurion, it was not adopted by any other NATO or Commonwealth country.

The final Chieftain version, which was used by the British Army until 1995, incorporated "Stillbrew" armour named after Colonel Still and John Brewer from the Military Vehicles and Engineering Establishment (MVEE), the Improved Fire Control System (IFCS) and the Thermal Observation Gunnery Sight (TOGS).

This experience spurred the creation of the indigenous Israeli Merkava, the development programme was led by General Israel Tal, who had worked closely with the British in the Anglo-Israeli Chieftain project.

The largest foreign sale was to Iran, who, at the recommendation of General Tal, took delivery of 707 Mk-3P and Mk-5P (the letter P standing for Persia), as well as 187 FV4030-1, 41 ARV and 14 AVLB before the 1979 revolution.

It was first used extensively by Iran, during the Iran–Iraq War of 1980–88, including the largest tank battle of the war, with mixed results as the Chieftain Mk 3/5 suffered from chronic engine problems and low power-to-weight ratio, making it unreliable and slow when manoeuvering over harsh terrain, which in turn made it prone to breakdowns in the midst of battle or a sluggish target and thus vulnerable to enemy tank fire.

According to Makki, when the British telephoned the Iraqi director of Armor, Salah Askar, he responded with "We don't want your stupid tanks!"

Thirty-seven Chieftains of the Kuwaiti 35th Armored Brigade fought at the Battle of the Bridges against elements of the Iraqi Hammurabi and Medina divisions before withdrawing over the Saudi border.

[34] Kuwaiti 35th Armored Brigade failed to block the Mutla Pass and were ineffective at delaying the Iraqis, however they withdrew into the desert and became part of Joint Command Forces East during the 1991 Gulf War.

"Exercise Totem Pole" was carried out in six-to-nine phases depending on the Mark of vehicle being modified (Chieftain Mk.5's already had some of the required changes incorporated at the factory) between 1975 and 1979 and included fitment of the Marconi Improved Fire Control System (IFCS), replacement of the searchlight with the Barr & Stroud Thermal Observation Gunnery System (TOGS), along with modifications for using FSAPDS ammunition.

These, in themselves, had been preceded by "Dark Morn", "High Noon", and the initial "Fleetfoot" engine development programme which had completed in October 1971.