A tank is a tracked, armoured fighting vehicle designed for front-line combat which combines operational mobility and tactical offensive and defensive capabilities. Firepower is normally provided by a large-calibre main gun in a rotating turret and secondary machine guns, while heavy armour and all-terrain mobility provide protection for the tank and its crew, allowing it to perform all primary tasks of the armoured troops on the battlefield.
Tanks in World War I were developed separately and simultaneously by Great Britain and France as a means to break the deadlock of trench warfare on the Western Front. Their first use in combat was by the British Army on September 15, 1916 at Flers-Courcelette, during the Battle of the Somme. The name "tank" was adopted by the British during the early stages of their development, as a security measure to conceal their purpose (see etymology). While the French and British built thousands of tanks between them, Germany developed and brought into service only a single design the A7V producing 20 vehicles due to lack of capacities or resources.
Tanks of the interwar period evolved into the designs of World War II. Important concepts of armoured warfare were developed; the Soviet Union launched the first mass tank/air attack at Khalkhin Gol (Nomonhan) in August 1939, which later resulted in the T-34, a predecessor of the main battle tank; this was quickly followed up by Germany on a larger scale when they introduced blitzkrieg ('lightning war') less than two weeks later; a technique which made use of massed concentrations of tanks supported by artillery and air power to break through the enemy front and cause a complete collapse in enemy resistance and morale.
Tanks in the Cold War advanced to counter greater battlefield threats. Tanks became larger and their armour became thicker and much more effective. Advances in manufacturing late in the war allowed the mass production of composite armor. Aspects of gun technology changed significantly as well, with advances in shell design.
During the 20th century, main battle tanks were considered a key component of modern armies. In the 21st century, with the increasing role of asymetrical warfare and the end of the Cold War, that also contributed to the increase of cost-effective Russian anti-tank weapons worldwide, the importance of tanks has waned. Modern tanks seldom operate alone, as they are organized into armoured units which involve the support of infantry, who may accompany the tanks in infantry fighting vehicles. They are also usually supported by reconnaissance or ground-attack aircraft.