Close air support
History: 

World War I: 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. The aircraft was a visible and personal enemy – unlike artillery – presenting a personal threat to enemy troops, while providing friendly forces assurance that their superiors were concerned about their situation.
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.
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.
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.
At this time, British doctrine came to recognize two forms of air support; trench strafing (the modern-day doctrine of CAS), and ground strafing (the modern-day doctrine of air interdiction) – attacking tactical ground targets away from the land battle. As well as strafing with machine-guns, planes engaged in such operations were commonly modified with bomb racks; the plane would fly in very low to the ground and release the bombs just above the trenches.

The Germans were also quick to adopt this new form of warfare and were able to deploy aircraft in a similar capacity at Cambrai. While the British used single-seater planes, the Germans preferred the use of heavier two-seaters with an additional machine gunner in the aft cockpit. The Germans adopted the powerful Hannover CL.II and built the first purpose-built ground attack aircraft, the Junkers J.I. During the 1918 German spring offensive, the Germans employed 30 squadrons, or Schlasta, of ground attack fighters and were able to achieve some initial tactical success. The British later deployed the Sopwith Salamander as a specialized ground attack aircraft, although it was too late to see much action.
During the Sinai and Palestine Campaign of 1918, CAS aircraft functioned as an important factor in ultimate victory. After the British achieved air superiority over the German aircraft sent to aid the Ottoman Turks, squadrons of S.E 5a's and D.H. 4s were sent on wide-ranging attacks against German and Turkish positions near the Jordan river. Combined with a ground assault led by General Edmund Allenby, three Turkish armies soon collapsed into a full rout. In the words of the attacking squadron's official report:

No 1 Squadron made six heavy raids during the day, dropped three tons of bombs and fired nearly 24,000 machine gun rounds.

Inter-war period: The close air support doctrine was further developed in the interwar period. Most theorists advocated the adaptation of fighters or light bombers into the role. During this period, airpower advocates crystallized their views on the role of air-power in warfare. Aviators and ground officers developed largely opposing views on the importance of CAS, views that would frame institutional battles for CAS in the 20th century.
The inter-war period saw the use of CAS in a number of conflicts, including the Polish–Soviet War, the Spanish Civil War, the Iraqi revolt of 1920 and the Gran Chaco War.
The British used air power to great effect in various colonial hotspots in the Middle East and North Africa during the immediate postwar period. The newly formed Royal Air force (RAF) contributed to the defeat of the Afghan military during the Third Anglo-Afghan War by harassing Afghani troops and breaking up their formations. Z Force, an RAF air squadron, was also used to support ground operations during the Somaliland campaign, in which the Darawiish king  Diiriye Guure's insurgency was defeated. Following from these successes, the decision was made to create a unified RAF Iraq Command to use air power as a more cost-effective way of controlling large areas than the use of conventional land forces. It was effectively used to suppress the Great Iraqi Revolution of 1920 and various other tribal revolts.
During the Spanish Civil War German volunteer aviators of the Condor Legion on the Nationalist side, despite little official support from their government, developed close air support tactics that proved highly influential for subsequent Luftwaffe doctrine.
U.S. Marine Corps Aviation was used as an intervention force in support of U.S. Marine Corps ground forces during the Banana Wars, in places such as Haiti, the Dominican Republic and Nicaragua. Marine Aviators experimented with air-ground tactics and in Haiti and Nicaragua they adopted the tactic of dive bombing.
The observers and participants of these wars would base their CAS strategies on their experience of the conflict. Aviators, who wanted institutional independence from the Army, pushed for a view of air-power centered around interdiction, which would relieve them of the necessity of integrating with ground forces and allow them to operate as an independent military arm. They saw close air support as both the most difficult and most inefficient use of aerial assets.
Close air support was the most difficult mission, requiring identifying and distinguishing between friendly and hostile units. At the same time, targets engaged in combat are dispersed and concealed, reducing the effectiveness of air attacks. They also argued that the CAS mission merely duplicated the abilities of artillery, whereas interdiction provided a unique capability. Ground officers contended there was rarely sufficient artillery available, and the flexibility of aircraft would be ideal for massing firepower at critical points, while producing a greater psychological effect on friendly and hostile forces alike. Moreover, unlike massive, indiscriminate artillery strikes, small aerial bombs would not render ground untrafficable, slowing attacking friendly forces.
Although the prevailing view in official circles was largely indifferent to CAS during the interwar period, its importance was expounded upon by military theorists, such as J. F. C. Fuller and Basil Liddell Hart. Hart, who was an advocate of what later came to be known as 'Blitzkrieg' tactics, thought that the speed of armoured tanks would render conventional artillery incapable of providing support fire. Instead he proposed that:

actual 'offensive' support must come from an even more mobile artillery moving alongside. For this purpose the close co-operation of low-flying aircraft ... is essential

World War II: 

Luftwaffe: As a continental power intent on offensive operations, Germany could not ignore the need for aerial support of ground operations. Though the Luftwaffe, like its counterparts, tended to focus on strategic bombing, it was unique in its willingness to commit forces to CAS. Unlike the Allies, the Germans were not able to develop powerful strategic bombing capabilities, which implied industrial developments they were forbidden to take according to the Treaty of Versailles. In joint exercises with Sweden in 1934, the Germans were first exposed to dive-bombing, which permitted greater accuracy while making attack aircraft more difficult to track by antiaircraft gunners. As a result, Ernst Udet, chief of the Luftwaffe's development, initiated procurement of close support dive bombers on the model of the U.S. 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.