Carpet bombing
Early history: One of the first attempts at carpet bombing was at the Battle of El Mazuco during the Spanish Civil War in 1937, against widely-dispersed infantry on rocky slopes, and the attacking Condor Legion learned that carpet bombing was not very effective in such terrain.
In March 1938, the Bombing of Barcelona saw Italian and German airstrikes killing up to 1,300 people and wounding 2,000. It is considered the first carpet bombing of a city, and set a precedent for several such bombings in World War II.
The Japanese bombing of China's wartime capital Chongqing from 18 February 1938 to 23 August 1943 caused 23,600 deaths and over 30,000 wounded.

During World War II: 

Pre-war expectation: In the inter-war years, a growing expectation developed that, on the outbreak of war, cities would be rapidly destroyed by bombing raids. The use of poison gas was expected and a high level of devastation was anticipated from high explosive bombs. This originated, in part, from the views of military experts such as Douhet, and was taken up by politicians and journalists, with, for example, Stanley Baldwin coining the phrase "The bomber will always get through". The targeting of the civilian population would, some theories suggested, cause a breakdown in morale that would lead to civil unrest that would compel a government to surrender. The combatant powers could, according to Baldwin, be in a competition to break the morale of the other side's civilian population first.
There were two results from this. Firstly, civil defence programs were set up, with gas masks being issued, plans for air raid shelters were set up and organisations to manage civilians before a raid and deal with damage and casualties after one were put in place. Secondly, agreements were sought to make the targeting of civilians illegal under international law. At the time that Douhet and others were publishing their ideas, no air force had planned their capabilities with the intent of making a "knockout blow" against civilian targets. The Hague Rules of Air Warfare were developed in 1922/23 to prevent deliberately attacking civilians, yet it was not ratified by any country. At the start of World War II, the Royal Air Force had an initial instruction to abide by the Hague Rules for as long as the enemy did. This restraint was followed by both Britain and Germany until 11 May 1940, when, with Winston Churchill now in the role of Prime Minister and the war in France going badly, the RAF attacked industrial and transport infrastructure targets in Mönchengladbach. This raid caused civilian casualties.

Bombing by Germany: In the European theatre, the first city to suffer heavily from aerial bombardment was Warsaw, on 25 September 1939 following the start of the German invasion of Poland. Continuing this trend in warfare, the Rotterdam Blitz was an aerial bombardment of Rotterdam by 90 bombers of the German air force on 14 May 1940, during the German invasion of the Netherlands. The objective was to support the German assault on the city, break Dutch resistance, and force the Dutch to surrender. Despite a ceasefire, the bombing destroyed almost the entire historic city centre, killing nearly nine hundred civilians and leaving 30,000 people homeless. The destructive success of the bombing led the Oberkommando der Luftwaffe (OKL, the Luftwaffe high command) to threaten to destroy the city of Utrecht if the Dutch Government did not surrender. The Dutch capitulated early the next morning.
As the war progressed, the Battle of Britain developed from a fight for air supremacy into the strategic and aerial bombing of London, Liverpool, Coventry and other British cities.

Bombing by the Western Allies: At the beginning of war, RAF Bomber Command lacked both the navigation systems for finding a target and the numbers of bombers that were needed to make attacks of any scale in Germany. 
As heavy bombers were brought into service and technology and tactics were improved, the selection of targets was changed. The intention of avoiding civilian casualties as collateral damage disappeared. Instead, the civilian population which worked in war-related industries – and their housing – became the target. 
Some of this change came from a wish to retaliate for the German attack on Coventry. 
It was also based on what was learnt from being the target in the Blitz. It had been found that factory buildings were more resistant to critical damage than the homes of their workers. Absenteeism of the workforce rose significantly if their housing was uninhabitable, so affecting industrial production. Whilst morale was still discussed, the meaning of the word changed from its pre-war usage. Now a reduction in morale was intended to reduce industrial production that supported the war effort. The area bombing directive was issued to RAF Bomber Command  in 1942 
The  Eighth Air Force of the USAAF arrived in Britain over the summer of 1942. Despite Roosevelt's pleas to Hitler to avoid bombing civilians prior to the US joining the war, he was a supporter of bombing Germany. Both Churchill and Roosevelt were in the position that Stalin was pressing for the Western Allies to open a new front in Europe – something which they were not ready to do. Therefore a bombing campaign - the Combined Bomber Offensive - following the Casablanca directive to the Allied air forces was all they could offer to support the Soviet Union. 
Operation Gomorrah, carried out by Bomber Command  against Hamburg, targeted a city with both high susceptibility to fire and a large number of factories making products for the German war effort. The raid caused substantial damage to the city, especially the housing of industrial workers. A carefully developed mix of high explosive bombs and incendiaries was used. High explosives broke windows and made fire-fighting dangerous, whilst the incendiaries set the buildings on fire. This methodology was used for further attacks on urban areas (though not with such major effect) throughout the war, with Dresden being one of the final targets.
Carpet bombing was also used as close air support (as "flying artillery") for ground operations. The massive bombing was concentrated in a narrow and shallow area of the front (a few kilometers by a few hundred meters deep), closely coordinated with the advance of friendly troops. The first successful use of the technique was on 6 May 1943, at the end of the Tunisia Campaign. Carried out under Sir Arthur Tedder, it was hailed by the press as Tedder's bomb-carpet (or Tedder's carpet). The bombing was concentrated in a four by three-mile area, preparing the way for the First Army. This tactic was later used in many cases in the Normandy Campaign; for example, in the Battle for Caen.

Pacific War: In the Pacific War, carpet bombing was used extensively against Japanese cities such as Tokyo. On the night of 9–10 March 1945, 334 B-29 Superfortress heavy bombers were directed to attack the most heavily populated civilian sectors of Tokyo. In just one night, over 100,000 people burned to death from a heavy bombardment of incendiary bombs, comparable to the wartime number of U.S. casualties in the entire Pacific theater. Another 100,000 to one million Japanese were left homeless. These attacks were followed by similar ones against Kobe, Osaka, and Nagoya, as well as other sectors of Tokyo, where over 9,373 tons of incendiary bombs were dropped on civilian and military targets. By the time of the dropping of the atomic bombs on Hiroshima and Nagasaki, light and medium bombers were being directed to bomb targets of convenience, as most urban areas had already been destroyed. In the 9-month long bombing campaign, over 300,000 Japanese civilians died and 400,000 were wounded.

During the final months of the war in the Philippines, the United States military used carpet bombing against the Japanese forces in Manila and Baguio, reducing much of the cities to rubble. Manila became the second-most-destroyed city of World War II.

Vietnam War: During the Vietnam War, with the escalating situation in Southeast Asia, twenty-eight B-52Fs were fitted with external racks for twenty-four 750-pound (340 kg) bombs under project South Bay in June 1964; an additional forty-six aircraft received similar modifications under project Sun Bath. In March 1965, the United States commenced Operation Rolling Thunder. The first combat mission, Operation Arc Light, was flown by B-52Fs on 18 June 1965, when 30 bombers of the 9th and 441st Bombardment Squadrons struck a communist stronghold near the Bến Cát District in South Vietnam. The first wave of bombers arrived too early at a designated rendezvous point, and while maneuvering to maintain station, two B-52s collided, which resulted in the loss of both bombers and eight crewmen. The remaining bombers, minus one more that turned back for mechanical problems, continued toward the target. Twenty-seven Stratofortresses dropped on a one-mile by two-mile target box from between 19,000 and 22,000 feet (5,800 and 6,700 m), a little more than 50% of the bombs falling within the target zone. The force returned to Andersen AFB except for one bomber with electrical problems that recovered to Clark AFB, the mission having lasted 13 hours. Post-strike assessment by teams of South Vietnamese troops with American advisors found evidence that the VC had departed the area before the raid, and it was suspected that infiltration of the south's forces may have tipped off the north because of the ARVN troops involved in the post-strike inspection.