Advances in tank design, armour, and engine technology allowed tank designers to increase the capabilities of tanks significantly without always resorting to heavier designs, although weights did gradually increase. However, HEAT ammunition was a huge threat to tanks and could penetrate steel armour thicker than was practical to put on a tank. Advances such as the British-designed Chobham armour did much to limit the effectiveness of weaker HEAT rounds, but the vulnerability has since remained. The demise of the heavy tank meant that what had been medium-sized vehicles were now the heaviest. What remained were developments of the more heavy-set cruiser tanks of Britain, and medium tanks intended for anti-tank work of other nations, but with a focus on weapon power and mobility greater than ever before. The name "Main Battle Tank" (MBT) gained widespread use.
Many Cold War MBTs evolved more or less directly from late WWII medium tank designs: the US Patton series of tanks was a series of successive evolutions of the M26 Pershing, for example, and the Russian T54/55 MBT was a direct descendant of the T-44, itself an evolution of the T-34. This means than many MBTs retained something of their "medium tank" origins in terms of their balance size, weight, mobility, and protection. However, in the 1960s and 1970s, a generation of purpose-designed Main Battle Tanks appeared, starting with the British Chieftain tank. These vehicles are less obviously influenced by wartime templates (the Chieftain, for example), weighing as much as a WWII Tiger tank and possessing far greater firepower and armour, whilst retaining the mobility of the previous Centurion design. Similarly, the US M1 Abrams series, the German Leopard 2, and British Challenger tanks sit in a nebulous area between what was once considered the "medium" and "heavy" weight category. Perhaps the most defining feature of the Main Battle Tank type is neither its weight, mobility, nor firepower, but instead the idea that only one type of heavily armoured vehicle is required to carry out the roles of breakthrough, exploitation and infantry support.
The term "Main Battle Tank" is applied to tanks designed to function as the backbone of modern ground forces. It is all-around armed and armoured to face as many kinds of threats as possible, but especially direct hits from other tanks and lighter infantry anti-tank weapons. However, the threats to MBTs on a modern battlefield are numerous.
Even heavily armoured MBTs are vulnerable to all manner of anti-tank weapons, often designed to attack the most vulnerable locations: the top, the bottom, and the tracks. Tanks also retain much of their vulnerability to artillery fire and mines. While a tank can afford to have half a metre of armour on the front, it can't have such a thick slab of metal guarding all of its sides without losing major maneuvering ability.
The solution was to focus on the traits that allowed the tank to survive: mobility and firepower. The amount of armour added was usually sufficient to stop at least previous-generation projectiles from penetrating. Armour on more advanced MBTs has been shown to deflect older-generation projectiles, but there is little public information on the armour levels of the latest MBTs, as such information is generally kept secret. Some of the known examples are from friendly fire. For example, in the 1991 Gulf War it was shown that a U.S. Hellfire anti-tank missile could destroy an M1 Abrams and during the 2003 invasion of Iraq, a Challenger II MBT was fired upon by another, destroying it and killing two of its crew.