The AA gun is an artillery piece that is fired from the ground or shipboard in defense against aerial attack. Anti-aircraft weapons development began as early as 1910, when the airplane first became an effective weapon. In World War I, field artillery pieces up to about 90 mm (3.5 inches) in caliber were converted to anti-aircraft use by mountings that enabled them to fire nearly vertically.
In World War II, rapid-firing and automatic anti-aircraft guns were introduced, radar was applied to target tracking, and tiny radio-wave proximity fuses exploded the ammunition as it approached the target. Heavy anti-aircraft guns, up to 120 mm, were used against high-flying bombers. The most effective of these was the German 88mm Fliegerabwehrkanone; its abbreviated name, flak, became a universal term for anti-aircraft fire.
In 1953 the U.S. Army introduced the Skysweeper, a 75-millimetre automatic cannon firing 45 shells per minute, aimed and fired by its own radar-computer system. With the introduction of guided surface-to-air missiles in the 1950s and '60s, heavy anti-aircraft guns such as this were phased out, though radar-guided automatic guns of 20 to 40 mm continued to provide a defense against low-flying aircraft and helicopters.