Tank (military)
Introduced in the First World War, a tank is a land combat vehicle that, in its basic form, can travel over a wide range of terrain because it uses tracks rather than wheels, has at least one main main high-velocity gun and one or more machine guns, and some level of armor and other protection. The design of each tank type is a balance among mobility, firepower and protection. A physically heavy tank can carry large guns and thick armor, but may be slow, and civilian bridges may not support its weight. If weight is saved in the gun, the tank may be able to support infantry but not fight other armored vehicles. A lightly armored and maneuverable version may not be able to take hits from even light antitank weapons.
"Tank" was actually a British code word for their first vehicles, intended to break the stalemate by being able to cross barbed wire and trenches, leading the way for infantry. They were first used, in force, at the First Battle of Cambrai (20 November - 3 December 1917), by the British, and, while primitive by modern standards, were so effective that the British were unprepared to exploit the massive hole they opened in the German lines.
There is little physical resemblance between a modern tank and those of the First World War, and there have been many changes in doctrine. Originally, their role was seen as infantry support, but one of the key aspects of the World War II blitzkrieg was the use of teams of tanks, vehicle-borne infantry, mobile artillery, and close air support; the role of the traditional foot-bound infantry was to seal the flanks of the holes the combined arms, tank-led force had opened in enemy lines.
During the Second World War, there was much tank development, and a number of variants proved impractical or usable only in special situations. The British used the terminology of "infantry" tanks and "cruiser" tanks; the more modern terminology would be a heavy tank in direct support of infantry and a medium tank operating in all-motorized formations. Light tanks and even lighter "tankettes" were given scouting roles, but often proved impractical because they were too vulnerable. There are armored scouting vehicles in effective use today, but one of the first things one will encounter in their builders' description is "this is not a light tank". One general principle of a viable tank is that it can fight other tanks, and, the design of many medium or, in modern terms, main battle tanks is that they are optimized for antitank roles and may not be as useful in other roles.
In recent years, with the advent of antitank guided missiles and effective tank-killer aircraft, there have been predictions that the tank will go the way of the armored knight, the heavy gun naval battleship, and other obsolete technologies. Tanks keep evolving, however, as well as variant armored vehicles that complement the true tanks.