A heavy tank was a subset of tank that provided equal or greater firepower as well as armor than tanks of lighter classes, at the cost of mobility and maneuverability.
Heavy tanks have usually been deployed to breakthrough enemy lines, though in practice have been more useful in the defensive role than in the attack. Design goals have included attacking obstacles, creating breakthroughs, and engaging enemy armoured formations.
There was not a consistent line distinguishing heavy and medium tanks. The line was drawn by each country's army according to its needs; very frequently the threshold was based on the weight sustainable by the country's most common bridges or by railroad loading limits.[note 1]
Heavy tanks achieved their greatest successes both fighting other, lighter tanks, and destroying fortifications with their very large guns. Although it is often assumed that heavy tanks suffered inferior mobility to mediums, this was not always the case, as many of the more sophisticated heavy tank designs featured advanced suspension and transmission precisely to counteract this drawback. But the greatest drawback is cost which translates into production, resulting in short supplies. The German Tiger I, for example, had similar speed and better terrain-handling characteristics when compared to the significantly lighter Panzer IV medium tank, albeit at the cost of low reliability and only 1,355 were produced compared to 8,800 Panzer IV and 58,000 Soviet T-34 and 40,000 American M4 Sherman medium tanks.
Heavy tanks feature very heavy armour and weapons relative to lighter tanks, though they tend to push onboard power generators to the limits. As a result they tend to be either underpowered and comparatively slow, or have engine and drive train problems from overworking their engines. Heavy tanks tend to have excellent protection compared to their lighter cousins.
The first British tank, the Mark I of World War I, was introduced to break through the German defensive lines of trenches and barbed wire. When a lighter faster tank was introduced this was designated "Medium Mark A" (and known as Whippet) and the larger tanks known as "heavies".
The Char 2C was one of the largest tanks ever produced. At the start of World War II, the French and the Soviets were the only countries to have inventories of heavy tanks, such as the Char B1, T-35, and KV-1. The Matilda II infantry tank, though not weighing as much as the others,[clarification needed] was designed to the British infantry tank concept and had thicker armour than most tanks in service at the time. Later war examples were the German Tiger I and II, as well as the Soviet IS series. Note that "heavy" versus "medium" is more a question of tactical roles than weight; the Panther, for example, was a "medium" tank that outweighed most Allied "heavy" tanks. American forces rarely fielded heavy tanks, preferring to use tank destroyers for anti-tank combat, and their Sherman medium tanks were outclassed in terms of armor and weapons by most heavy tanks.
The immediate post-war period saw the final fielding of heavy tanks, including the US M103 heavy tank, the British FV214 Conqueror, and the French ARL 44, all in response to the Soviet heavy tanks of the period. The largest tank guns were approaching maximum calibre whose shell could still be handled by the crew, even using awkward two-part ammunition (separate projectile and propellant case), which greatly slowed their rate of fire. Thanks to improved gun designs and fire control technology, postwar medium tanks were catching up to heavy tanks in firepower. The tactical value of heavy tanks thus declined to the point that no new designs were fielded; the heavily armed mediums came to be known as the main battle tank (MBT) - although some modern MBTs such as the M1 Abrams and Challenger 2 actually have weights similar to Cold War heavies. Less expensive self-propelled artillery could serve in the infantry support role.
Heavy tanks were finally rendered obsolete by anti-tank guided missiles and high explosive anti-tank (HEAT) ammunition. The much more flexible missiles are effective at ranges beyond a tank gun's range, and sheer armour mass was no longer a guarantee of survivability against the largest HEAT warheads of tank guns or missiles.