The history of the tank begins with World War I, when armoured all-terrain fighting vehicles were introduced as a response to the problems of trench warfare, ushering in a new era of mechanized warfare. Though initially crude and unreliable, tanks eventually became a mainstay of ground armies. By World War II, tank design had advanced significantly, and tanks were used in quantity in all land theatres of the war. The Cold War saw the rise of modern tank doctrine and the rise of the general-purpose main battle tank. The tank still provides the backbone to land combat operations in the 21st century.