The M45 Quadmount was a towed anti-aircraft gun consisting of four .50 caliber M2 Browning machine guns mounted in pairs on either side of an armored open-top gunner's compartment with electrical laying. It was developed by the W. L. Maxson Corporation to replace the earlier M33 twin mount. Although designed as an anti-aircraft weapon, it was also used against ground targets, where it earned the nicknames "meat chopper" and "Krautmower" Introduced in 1944, it saw service as late as the Vietnam War.

Anti-aircraft warfare is the counter to aerial warfare and it includes "all measures designed to nullify or reduce the effectiveness of hostile air action". It includes surface based, subsurface, and air-based weapon systems, associated sensor systems, command and control arrangements, and passive measures. It may be used to protect naval, ground, and air forces in any location. However, for most countries, the main effort has tended to be homeland defence. Missile defence is an extension of air defence, as are initiatives to adapt air defence to the task of intercepting any projectile in flight.