During World War II, there was a concerted effort to develop heavier and heavier tanks, often stretching past the limits of practicality and even credulity. Some of the larger examples were well over 100 tons, huge by today's standards. Almost none were ever deployed in battle. But they displayed a school of thought similar to that of battleships, where sheer armor and weaponry took precedence over anything else.
The Japanese developed several prototypes for massive tanks to be used in the Pacific Theater. The O-I superheavy tank was conceived due to the profound inferiority of Japanese Army armor facing off against Soviet armor in a series of severe border clashes at Khalkin Gol on the Manchurian border. A single functional model was built by 1945, weighing in at a gigantic 120 tons and armed with a 105mm gun and two rocket launchers. Under murky circumstances it was shipped to Manchuria and it is unknown whether it ever saw combat. It was scrapped after the war. Only its tracks remain in an Japanese museum.
The German Panzer VIII, jokingly named the Maus, or Mouse, was the largest and heaviest tank design that was ever actually built, though it never saw any frontline service. It weighed in at 188 tons, over six times as heavy as a U.S. M4 Sherman, and was conceived as a way to break through heavy field fortifications in frontal assaults. It was armed with a 128mm gun that could easily destroy any Allied tank out to very long ranges. It also carried a 75 mm gun as a secondary armament that was equal to the main gun on the M4. Several prototypes were constructed, but were captured by the Soviets in 1945. Only one, assembled from the prototypes, remains in the Kubinka tank museum in Russia.
The United States and the British also worked on vehicles in the 70-100 ton range, but they were conceived more as large armored self-propelled artillery, such as the American T28 and the British Tortoise. Neither entered production before the end of the war.
Despite their awesome appearance, superheavy tank designs were almost uniformly a failure. The size and weight of the tanks made traversing rough terrain difficult if not impossible, and they were often far too heavy for most bridges, restricting them to fording the rivers using snorkels. But river fords shallow enough for passage were not always available, a severe restriction on the tank's tactical flexibility. Also, tanks were generally transported long distances by rail, and the extreme difficulty of doing so with 100-plus ton tanks was a serious disadvantage.
Heavy armor alone was not enough to make up for low speed and presenting a large target. Tanks in the open are extremely vulnerable to air attack, and a slow, large target was even more so. A 250-pound bomb from above would kill a superheavy tank as quickly as a light one.
Even light artillery could at the least knock off one of the tracks, leaving the tank immobilized and helpless. Low maneuverability and speed meant lighter enemy tanks could outflank them and hit them from the sides and rear, where the armor was weakest. Far greater numbers of regular tanks like the American M4 and the famed Soviet T-34 could be built, and it was these that overcame the often superior German tanks through tactics and numbers.
But the single biggest problem facing superheavy tank designs was one that plagued many of their smaller cousins: mechanical reliability. The engines available were uniformly underpowered, and the huge weight of armor and weapons took a terrible toll on transmissions, suspensions, and turret mechanisms. A broken-down tank was just as useless as one destroyed by the enemy. Even the German King Tiger II, still large at 68 tons, lost more tanks to mechanical breakdown than to the enemy.
Following the war, improvements in armor and gun technology made superheavy tanks unnecessary. Advances like composite armor and better engines made tanks more survivable while faster and more maneuverable, and ever more effective airpower made monster tanks more of a target than a weapon. The M1 Abrams, the mainstay tank of the United States for over 30 years, weighs in at less than a third of the Panzer VIII.
Like so many "miracle weapons," the superheavy tanks never panned out. It proved more effective to have larger numbers of smaller, economical, and more reliable tanks, rather than a small number of large ones. Modern tank design in particular has concluded that bigger is not always better.