Three bipedal Metal Gear designs: TX-55 Metal Gear, Metal Gear D, and Metal Gear REX.

Metal Gear is the codename for a series of powerful bipedal tanks, usually armed with nuclear weapons. The general definition of a Metal Gear prior to the collapse of the war economy is a mobile nuclear launch platform similar in purpose to ballistic missile submarines. However, its precise definition is debatable, as many other weapons have been designed to fill a similar role, or have used the same technology. The individual units have varied greatly in appearance and size, but their purpose has always been the same—to destabilize the global balance of power via the threat of covert long-range nuclear attack. The term "Metal Gear" was coined by Soviet weapons scientist Aleksandr Leonovitch Granin to describe in 1964 (1616 N.E) the "missing link" between infantry and artillery: infantry could roam the battlefield at will, but were both vulnerable and lacking in firepower; artillery could inflict immense damage upon an opponent while resisting similar damage, but was at the mercy of the terrain. Granin envisioned a tank with legs, enabling it to quickly traverse the battlefield, dealing and repelling force on a scale never before seen. As of now, Metal Gears are very rarely used by the Alliance of Nations and Axis of Empires, however, it is commonly used by a variety of factions on Dol Earth.

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