Close air support
Navy's Curtiss Helldiver, resulting in the Henschel Hs 123, which was later replaced by the famous Junkers Ju 87 Stuka. Experience in the Spanish Civil War lead to the creation of five ground-attack groups in 1938, four of which would be equipped with Stukas. The Luftwaffe matched its material acquisitions with advances in the air-ground coordination. General Wolfram von Richthofen organized a limited number of air liaison detachments that were attached to ground units of the main effort. These detachments existed to pass requests from the ground to the air, and receive reconnaissance reports, but they were not trained to guide aircraft onto targets.
These preparations did not prove fruitful in the invasion of Poland, where the Luftwaffe focused on interdiction and dedicated few assets to close air support. But the value of CAS was demonstrated at the crossing of the Meuse River during the Invasion of France in 1940. General Heinz Guderian, one of the creators of the combined-arms tactical doctrine commonly known as "blitzkrieg", believed the best way to provide cover for the crossing would be a continuous stream of ground attack aircraft on French defenders. Though few guns were hit, the attacks kept the French under cover and prevented them from manning their guns. Aided by the sirens attached to Stukas, the psychological impact was disproportional to the destructive power of close air support (although as often as not, the Stukas were used as tactical bombers instead of close air support, leaving much of the actual work to the older Hs 123 units for the first years of the war). In addition, the reliance on air support over artillery reduced the demand for logistical support through the Ardennes. Though there were difficulties in coordinating air support with the rapid advance, the Germans demonstrated consistently superior CAS tactics to those of the British and French defenders. Later, on the Eastern front, the Germans would devise visual ground signals to mark friendly units and to indicate direction and distance to enemy emplacements.
Despite these accomplishments, German CAS was not perfect and suffered from the same misunderstanding and interservice rivalry that plagued other nations' air arms, and friendly fire was not uncommon. For example, on the eve of the Meuse offensive, Guderian's superior cancelled his CAS plans and called for high-altitude strikes from medium bombers, which would have required halting the offensive until the air strikes were complete. Fortunately for the Germans, his order was issued too late to be implemented, and the Luftwaffe commander followed the schedule he had previously worked out with Guderian. As late as November 1941, the Luftwaffe refused to provide Erwin Rommel with an air liaison officer for the Afrika Korps, because it "would be against the best use of the air force as a whole."
German CAS was also extensively used on the Eastern Front during the period 1941–1943. Their decline was caused by the growing strength of the Red Air Force and the redeployment of assets to defend against American and British strategic bombardment. Luftwaffe's loss of air superiority, combined with a declining supply of aircraft and fuel, crippled their ability to provide effective CAS on the western front after 1943.

RAF and USAAF: The Royal Air Force (RAF) entered the war woefully unprepared to provide CAS. In 1940 during the Battle of France, the Royal Air Force and Army headquarters in France were located at separate positions, resulting in unreliable communications. After the RAF was withdrawn in May, Army officers had to telephone the War Office in London to arrange for air support. The stunning effectiveness of German air-ground coordination spurred change. On the basis of tests in Northern Ireland in August 1940, Group Captain A. H. Wann RAF and Colonel J.D. Woodall (British Army) issued the Wann-Woodall Report, recommending the creation of a distinct tactical air force liaison officer (known colloquially as "tentacles") to accompany Army divisions and brigades. Their report spurred the RAF to create an RAF Army Cooperation Command and to develop tentacle equipment and procedures placing an Air Liaison Officer with each brigade.
Although the RAF was working on its CAS doctrine in London, officers in North Africa improvised their own coordination techniques. In October 1941, Sir Arthur Tedder and Arthur Coningham, senior RAF commanders in North Africa, created joint RAF-Army Air Support Control staffs at each corps and armored division headquarters, and placed a Forward Air Support Link at each brigade to forward air support requests. When trained tentacle teams arrived in 1942, they cut response time on support requests to thirty minutes. It was also in the North Africa desert that the cab rank strategy was developed. It used a series of three aircraft, each in turn directed by the pertinent ground control by radio. One aircraft would be attacking, another in flight to the battle area, while a third was being refuelled and rearmed at its base. If the first attack failed to destroy the tactical target, the aircraft in flight would be directed to continue the attack. The first aircraft would land for its own refuelling and rearming once the third had taken off. The CAS tactics developed and refined by the British during the campaign in North Africa served as the basis for the Allied system used to subsequently gain victory in the air over Germany in 1944 and devastate its cities and industries.
The use of forward air control to guide close air support (CAS) aircraft, so as to ensure that their attack hits the intended target and not friendly troops, was first used by the British Desert Air Force in North Africa, but not by the USAAF until operations in Salerno. During the North African Campaign in 1941 the British Army and the Royal Air Force established Forward Air Support Links (FASL), a mobile air support system using ground vehicles. Light reconnaissance aircraft would observe enemy activity and report it by radio to the FASL which was attached at brigade level. The FASL was in communication (a two-way radio link known as a "tentacle") with the Air Support Control (ASC) Headquarters attached to the corps or armoured division which could summon support through a Rear Air Support Link with the airfields. They also introduced the system of ground direction of air strikes by what was originally termed a "Mobile Fighter Controller" traveling with the forward troops. The controller rode in the "leading tank or armoured car" and directed a "cab rank" of aircraft above the battlefield. This system of close co-operation first used by the Desert Air Force, was steadily refined and perfected, during the campaigns in Italy, Normandy and Germany.

By the time the Italian Campaign had reached Rome, the Allies had established air superiority. They were then able to pre-schedule strikes by fighter-bomber squadrons; however, by the time the aircraft arrived in the strike area, oftentimes the targets, which were usually trucks, had fled. The initial solution to fleeing targets was the British "Rover" system. These were pairings of air controllers and army liaison officers at the front but able to switch communications seamlessly from one brigade to another – hence Rover. Incoming strike aircraft arrived with pre-briefed targets, which they would strike 20 minutes after arriving on station only if the Rovers had not directed them to another more pressing target. Rovers might call on artillery to mark targets with smoke shells, or they might direct the fighters to map grid coordinates, or they might resort to a description of prominent terrain features as guidance. However, one drawback for the Rovers was the constant rotation of pilots, who were there for fortnightly stints, leading to a lack of institutional memory. US commanders, impressed by the British tactics at the Salerno landings, adapted their own doctrine to include many features of the British system.
At the start of the War, the United States Army Air Forces (USAAF) had, as its principal mission, the doctrine of strategic bombing. This incorporated the unerring belief that unescorted bombers could win the war without the advent of ground troops. This doctrine proved to be fundamentally flawed. However, during the entire course of the war the USAAF top brass clung to this doctrine, and hence operated independently of the rest of the Army. Thus it was initially unprepared to provide CAS, and in fact, had to be dragged "kicking and screaming" into the CAS function with the ground troops. USAAF doctrinal priorities for tactical aviation were, in order, air superiority, isolation of the battlefield via supply interdiction, and thirdly, close air support. Hence, during the North African Campaign, CAS was poorly executed, if at all. So few aerial assets were assigned to U.S. troops that they fired on anything in the air. And in 1943, the USAAF changed their radios to a frequency incompatible with ground radios.
The situation improved during the Italian Campaign, where American and British forces, working in close cooperation, exchanged CAS techniques and ideas. There, the AAF's XII Air Support Command and the Fifth U.S. Army shared headquarters, meeting every evening to plan strikes and devising a network of liaisons and radios for communications. However, friendly fire continued to be a concern – pilots did not know recognition signals and regularly bombed friendly units, until an A-36 was shot down in self-defense by Allied tanks. The expectation of losses to friendly fire from the ground during the planned invasion of France prompted the black and white invasion stripes painted on all Allied aircraft from 1944.
In 1944, USAAF commander Lt. Gen.