Close air support (CAS) is defined as aerial warfare actions—often air-to-ground actions such as strafes or airstrikes—by military aircraft against hostile targets in close proximity to friendly forces. A form of fire support, CAS requires detailed integration of each air mission with fire and movement of all forces involved. CAS may be conducted using aerial bombs, glide bombs, missiles, rockets, autocannons, machine guns, and even directed-energy weapons such as lasers.

The requirement for detailed integration because of proximity, fires or movement is the determining factor. CAS may need to be conducted during shaping operations with special forces if the mission requires detailed integration with the fire and movement of those forces. A closely related subset of air interdiction, battlefield air interdiction, denotes interdiction against units with near-term effects on friendly units, but which does not require integration with friendly troop movements. CAS requires excellent coordination with ground forces, typically handled by specialists such as artillery observers, joint terminal attack controllers, and forward air controllers.

World War I was the first conflict to make extensive use of CAS, albeit using relatively primitive methods in contrast to later military tactics, though it was made evident that proper coordination between aerial and ground forces via radio made attacks more effective. Several conflicts during the interwar period—including the Polish–Soviet War, the Spanish Civil War, the Iraqi Revolt, and the Chaco War—made notable use of CAS. World War II marked the universal acceptance of the integration of air power into combined arms warfare, with all of the war's major combatants having developed effective air-ground coordination techniques by the conflict's end. New techniques, such as the use of forward air control to guide CAS aircraft and identifying invasion stripes, also emerged at this time, being heavily shaped by the Italian Campaign and the invasion of Normandy. CAS continued to advance during the conflicts of the Cold War, especially the Korean War and the Vietnam War; major milestones included the introduction of attack helicopters, gunships, and dedicated CAS attack jets.

The use of aircraft in the close air support of ground forces dates back to World War I, the first conflict to make significant military use of aerial forces. Air warfare, and indeed aviation itself, was still in its infancy – and the direct effect of rifle caliber machine guns and light bombs of World War I aircraft was very limited compared with the power of (for instance) an average fighter bomber of World War II, but CAS aircraft were still able to achieve a powerful psychological impact. Unlike artillery, the aircraft was a visible and personal enemy presenting a direct threat to enemy troops, while at the same time providing friendly forces proof of support from their superiors.

The most successful attacks of 1917–1918 had included planning for co-ordination between aerial and ground units, although it was relatively difficult at this early date to co-ordinate these attacks due to the primitive nature of air-to-ground radio communication. Though most air-power proponents sought independence from ground commanders and hence pushed the importance of interdiction and strategic bombing, they nonetheless recognized the need for close air support.^{[page needed]}

From the commencement of hostilities in 1914, aviators engaged in sporadic and spontaneous attacks on ground forces, but it was not until 1916 that an air support doctrine was elaborated and dedicated fighters for the job were put into service. By that point, the startling and demoralizing effect that attack from the air could have on the troops in the trenches had been made clear.^{[citation needed]}

At the Battle of the Somme, 18 British armed reconnaissance planes strafed the enemy trenches after conducting surveillance operations. The success of this improvised assault spurred innovation on both sides. In 1917, following the Second Battle of the Aisne, the British debuted the first ground-attack aircraft, a modified F.E 2b fighter carrying 20 lb (9.1 kg) bombs and mounted machine-guns. After exhausting their ammunition, the planes returned to base for refueling and rearming before returning to the battle-zone. Other modified planes used in this role were the Airco DH.5 and Sopwith Camel – the latter was particularly successful in this role.

Aircraft support was first integrated into a battle plan on a large scale at the 1917 Battle of Cambrai, where a significantly larger number of tanks were deployed than previously. By that time, effective anti-aircraft tactics were being used by the enemy infantry and pilot casualties were high, although air support was later judged as having been of a critical importance in places where the infantry had got pinned down.