Very interesting question. The use of aircraft in the tactical role is essentially as old as the use of combat aircraft themselves. I'd very much like to look into this in some detail. so we'll start at the genesis of aircraft as a weapon of war during the First World War.

WW1: Birth of ground attack aircraft WW1 saw aircraft mature into an integral part of modern warfare and find their role on the battlefield. When people think of aircraft in the Great War, they are probably thinking of the aerial battles between wheeling flocks of fighter aircraft and little beyond this. What most will fail to infer is that these pilots weren't hurling fabric and balsa wood flying machines at one another in the hope that a good reason for doing so would come about. The whole point of these battles was to fight for control of the skies over their own and the enemies front-line, what we would now refer to as air superiority.

Air superiority was desired for good reason: Unimpeded by marauding enemy fighters, observation aircraft and balloons could do there work spotting for artillery targets. What it also allowed was for aircraft to operate over enemy territory to attack ground targets of opportunity, close air support in modern parlance. Against the front line positions and trenches, these attacks could be of variable effectiveness. Although such attacks were impeding and demoralizing, they were not particularly potent due to the limitations of aircraft weaponry and communications. With no radios and other signalling methods like lamps and flares being unsuitable for proper air to ground interaction, pilots had to pick out and engage targets of their own choosing, so air support directed by ground observers in an organic sense would be only found later in the war. Beyond the machine guns which could be employed for strafing, air dropped munitions were a long way from maturity. Flechettes, grenades and small early bombs were too light in explosive payload and inaccurate (being dropped by hand) to be of any use outside of producing casualties. It would be a while before aircraft could match artillery in its destructive might and ability to demolish field fortifications. Also, these sort of attacks were risky. Troops quickly became fond of replying against aerial marauders with fusillades of rifle and machine gun fire. Flying at low speed and in a canvas cockpit and fragile plane, no pilot was invulnerable to these barrages, and the problem would only become greater as both makeshift and purpose built anti aircraft weapons came about. This didn't stop the large scale use of aircraft in conjunction with infantry attack on enemy positions, such as the massed used of British aircraft against the German front lines during the Battle of Messines and the Third Battle of Ypres, but this particular use resulted in such prohibitive losses in aircraft it was ended quickly. An interesting aspect of this use was the use of aircraft best suited to the ground attack role: Early use showed the single seat fighter a poorer choice for the role due to the vulnerability to attack from enemy aircraft while the pilot was distracted. Two seat aircraft were quickly developed into the niche as the rear seater could both defend the aircraft from attack from enemy fighter and drop bombs, giving the pilot the task of avoiding ground fire. The Germans made use of some excellent ground attack platforms in the war, in some cases developed from fighters originally designed for escort observer aircraft and formed into groups called Schlachtstaffeln (battle flights) for dedicated ground attack operations. Some of these aircraft would incorporate radios for air to ground coordination and racks for bombs or grenades, and proved impressively effective in some offensives when used in conjunction with ground forces. Arguably the most impressive part of the development of ground attack in WW1 was the production the worlds first dedicated all-metal ground attack aircraft, the Junkers J.I. Whereas many ground attack aircraft relied on a speed and maneuverability along with tough construction to protect them, the J.I was actually designed from the outset to soak up punishment, with armored "bathtubs" for protecting the engine, fuel tanks and crew from ground fire, along with control surfaces which eschewed easily severed steel cables for cranks and push rods, and even more incredibly, redundant design features to keep the plane operating in the event of battle damage, such as a two cell fuel tank. What makes this so amazing is that modern attack aircraft lime the legendary A-10 would be built with exactly these features: armour protection, toughness and redundancy.

A very broad question!

This area of air fighting doctine, in particular, was implicitly affected by the nation under consideration. To ignore the development of ground attack in WW1- these lessons being largely forgotten after the war anyway- I will start in the 1930s.

In Britain and the US, military fliers were busy defending their turf from the other services (the army and navy) as budget-conscious politicians wondered if their could afford a separate air force (Britain) or quasi-separate Air Corps (US). Navy men denied the threat from aeroplanes, and the ability of planes to defend the coastline. But the spread of a book by an Italian named Douhet meant that in many countries there was fear that the next war would involve the use of poison gas against civilians. He advoctaed the use of bombers to make ground fighting redundant. This gave rise to both a fear of attack on a nation's civilians (and the implication that the government and military should stop it), and presented the opportunity to avoid the horror of years of trench warfare- a fresh and immediate memory in Europe in the 192s and 30s, and one which tended to blur the distinction between 'civilian' and 'soldier' anyway since those dead of 1914-18 were mostly conscripted, civilian men in uniform.

All this meant that 'army co-operation' was very much the poor, cousin in the aerial forces in many nations. Air Force commanders were fighter and bomber pilots who wanted fast fighters and bombers: decisive, and preferably strategic weapons. To an extent this is still so today.

Germany was denied an air arm in 1919 so her army and air force re-emerged together, being built up in a more co-ordinated effort. Lacking naval distractions as a continental power with land to re-acquire, and with almost unlimited budgets and militaristic governments, Germany put far more effort into supporting the ground forces and the turf war was far less of an issue.

Thus, in 1940 the Royal Air Force had a few meagre 'Army Co-operation' squadrons equipped with then-new Lysanders, which were too slow and vulnerable to be useful in combat. Germany had fleets of Stukas (a generic name for dive-bombers), well-trained as an elite force. The USA was still mostly sitting things out, and the USSR, many of whose aircraft designers were working from prison at this time, was still trying to catch up with the new generation of fighters. Her light bombers were quite capable of tactical attacks.

The British took years to really develop a good ground attack doctrine: the Battle of Britain and growth of Bomber Command as a strategic force, and absence of army operations in Europe, made Co-operation more of an orphan than ever. It was in the desert that Tedder, who later commanded all Allied tactical air forces in Europe, built up units of Hurricanes and Kittyhawks with the skill and weapons to attack German units in the open. But before this could happen, those same aircraft were needed just to secure the air as fighters, and prevent attacks on Allied troops. Meanwhile, Beaufort and Beaufighter planes roamed the Mediterranean Sea attacking shipping with bombs and torpedos.

It wasn't until D-Day loomed as a real possibility- from late 1943- that the Allies had to think about direct support of armies in the field. By now Mosquitoes and Hurricanes (it was by now well established that a ground- attack aircraft had to be almost as good as a fighter) had often been called to attack precison targets like radar sites, bridges and cross roads, so there was knowledge beyond level bombing and a cadre within the air force who took pride in such work. At the same time, the vaunted Hawker Typhoon had proven disappointing as a fighter because it did not have good high-altitude performance: a fatal flaw in European air combat. But this large aircraft was ideal for ground attack: large, steady, and devastatingly fast at low level. The USAAF passed the job increasingly to P-47s as the superior P-51s took over as the best air superiority fighter (to use a later term).

As well as the planes themselves, controlling and co-ordinating the attacks was an issue. The Aliies developed a similar approach to that the Germans had used since 1939, of having air force men in special armoured vehicles, right up on the front line. Of course they had to work this out for themselves, a key part of the Tedder reforms. There were various instances of ground support falling apart at crucial moments when the Air Force liaison man's tank was hit, but that's battlefield chaos for youand usually the system worked very, very well.

If the war in Western Europe was mostly about air superiority and strategic bombing, on the Eastern Front the ground fighting, endless as it was, as all-important. Every weapons, every operation, was to get the soldier on the ground further west, or east. The Stuka, scarcely seen by RAF men, soldiered on in the East until late 1944, covered where possible by fighters (of which there were very few in the East after mid-1943 when the vast majority of Luftwaffe fighters were pulled back for 'defence of the Reich' duties, opposing British and US bombers). Focke-Wulf 190 fighters, converted for ground attack with armour and extra bomb carrying capacity, took over from the Stukas in units whose skills and training were centred on hitting ground targets.

The Soviets, thinking independently, developed the IL-2 Sturmovik, which was just entering service as the war began. Taking a different attack to the 'Stuka' approach, the Red Air Force (VVS) eschewed diving and instead built a heavily-armoured aircraft, using unique technology, specifically to attack enemy army units on the battlefield. Although many were lost, once the Luftwaffe pulled its fighters out of Russia they reigned supreme and became the most-produced combat aircraft of all time.

By the closing stages of World War II most Allied single-engined aircraft were turned to ground attack as enemy fighter opposition disappeared. A key point is that there was little sharing of information and experiences about ground attack between nations and even forces of a given nation in different theatres.

After the war, there was a reversion to the pre-war situation: Air Forces, now armed with the ultimate weapon, ceased taking an interest in their naval and army cousins and in Korea and Vietnam fighters built for air-to-air combat were pressed into service as ground attackers. Air forces that had pursued fast-climbing, supersonic jets suddenly found they wanted slower, well-protected planes that could loiter over the battlefield. The budgets and haste of development of planes like the A-7 and Jaguar showed that air forces' hearts were not in ground attack. Vietnam saw a repetition, with Thunderchiefs being too heavy and expensive to be optimal, A-10s being the planes the air forces called for after re-discovering the need for army support.

I'd recommend 'Attack Aircraft or the West' by Gunston for a discussion of these issues in postwar Western air forces.

Tactical aircraft (i.e. planes that directly attacked troops on the ground as opposed to bombing civilian or production areas in the rear) made their first appearance in WWI, but in the first years of that war such attacks were very rare. Germany had a general policy of not allowing aircraft to venture over enemy lines (a sensible policy given that they were outnumbered in aircraft), so they had few opportunities to attack enemy soldiers. Pilots on both sides wanted no part of enemy ground fire (the Red Baron famously was probably killed by a machine gun on the ground), and the leaders on both sides were not willing to risk expensive and rare airplanes and expensively-trained pilots in ground attacks that would have achieved virtually nothing during the time of static trench warfare.

During the Spring Offensives by Germany in 1918, however, the Allies temporarily found themselves in dire straights, and an attitude of "throw anything available at the enemy" prevailed. Scouts (fighters) were sent to strafe marching columns of troops and to attack supply lines and transport behind enemy lines. Whereas troops dug into trenches and bunkers were almost entirely immune to aircraft, troops on the march were tempting and vulnerable targets. Allied aircraft did substantial damage in this role and played a major part in halting the German attack (some German memoirs refer to this period as "the time of airplane aggressiveness"). Pilot casualties were much higher than normal thanks to ground fire, but the Allied air forces were more than large enough to sustain these losses at this point.

The devastating effect of aircraft used in the ground attack role became apparent to both sides at this point, and various attempts to produce dedicated ground-attack aircraft were made before the end of the war (like the British Sopwith Salamander). After the war, as the speed of aircraft increased and they became increasingly difficult to hit from the ground, their utility only became more obvious. So I think it's fair to say that the Spring Offensives were the seminal moment in tactical aircraft development.

Source: Hundred Days: The Campaign That Ended World War I, Nick Lloyd