can be used in a direct-fire role against surface targets to great effect. In the early 21st century, missiles (generally mounted on similar turrets) largely supplanted anti-aircraft guns, though guns have recently shown revived utility against slow, low-flying drones. Artillery tractor An artillery tractor, also referred to as a gun tractor, is a specialized heavy-duty form of tractor unit used to tow artillery pieces of varying weights and calibres. It may be wheeled, tracked, or half-tracked. There are two main types of artillery tractors, depending on the type of traction: wheeled and tracked. Wheeled tractors are usually variations of lorries adapted for military service. Tracked tractors run on continuous track; in some cases are built on a modified tank chassis with the superstructure replaced with a compartment for the gun crew or ammunition. In addition, half-track tractors were used in the interwar period and in World War II, especially by the Wehrmacht. This type of tractor was mostly discontinued postwar. Self-propelled artillery Self-propelled artillery vehicles give mobility to artillery. Within the term are covered self-propelled guns (or howitzers) and rocket artillery. They are highly mobile, usually based on tracked chassis carrying either a large howitzer or other field gun or alternatively a mortar or some form of rocket or missile launcher. They are usually used for long-range indirect bombardment support on the battlefield. In the past, self-propelled artillery has included direct-fire "Gun Motor Carriage" vehicles, such as assault guns and tank destroyers (also known as self-propelled anti-tank guns). These have been heavily armoured vehicles, the former providing danger-close fire-support for infantry and the latter acting as specialized anti-tank vehicles. Modern self-propelled artillery vehicles may superficially resemble tanks, but they are generally lightly armoured, too lightly to survive in direct-fire combat. However, they protect their crews against shrapnel and small arms and are therefore usually included as armoured fighting vehicles. Many are equipped with machine guns for defence against enemy infantry. The key advantage of self-propelled over towed artillery is that it can be brought into action much faster. Before towed artillery can be used, it has to stop, unlimber and the guns set up. To move position, the guns must be limbered up again and brought – usually towed – to the new location. By comparison, self-propelled artillery in combination with modern communications, can stop at a chosen location and begin firing almost immediately, then quickly move on to a