• Very heavy damage to infrastructure

Japan:

  • 330,000–500,000 civilians killed
  • 20,000 soldiers killed<br />(in Hiroshima)
  • Very heavy damage to industry

Italy:

  • 60,000–100,000 civilians killed
  • 5,000 soldiers killed

Bulgaria:

  • 1,374 dead and 1,743 injured<br />12,564 buildings damaged, of which 2,670 completely destroyed

Slovakia:

  • 300–770 civilians killed

World War II (1939–1945) involved sustained strategic bombing of railways, harbours, cities, workers' and civilian housing, and industrial districts in enemy territory. Strategic bombing as a military strategy is distinct both from close air support of ground forces and from tactical air power. During World War II, many military strategists of air power believed that air forces could win major victories by attacking industrial and political infrastructure, rather than purely military targets. Strategic bombing often involved bombing areas inhabited by civilians, and some campaigns were deliberately designed to target civilian populations in order to terrorize them or to weaken their morale. International law at the outset of World War II did not specifically forbid the aerial bombardment of cities – despite the prior occurrence of such bombing during World War I (1914–1918), the Spanish Civil War (1936–1939), and the Second Sino-Japanese War (1937–1945).

Strategic bombing during World War II in Europe began on 1 September 1939 when Germany invaded Poland and the Luftwaffe (German Air Force) began bombing Polish cities and the civilian population in an aerial bombardment campaign. As the war continued to expand, bombing by both the Axis and the Allies increased significantly. The Royal Air Force, in retaliation for Luftwaffe attacks on the UK which started on 16 October 1939, began bombing military targets in Germany, commencing with the Luftwaffe seaplane air base at Hörnum on the 19–20 March 1940. In September 1940 the Luftwaffe began targeting British civilians in the Blitz. When the United States began flying bombing missions against Germany, it reinforced British efforts. The Allies attacked oil installations, and controversial firebombings took place against Hamburg (1943), Dresden (1945), and other German cities.

In the Pacific War, the Japanese frequently bombed civilian populations as early as 1937–1938, such as in Shanghai and Chongqing. US air raids on Japan escalated from October 1944, culminating in widespread firebombing, and later in August 1945 with the atomic bombings of Hiroshima and Nagasaki. The effectiveness of the strategic bombing campaigns is controversial. Although they did not produce decisive military victories in themselves, some argue that strategic bombing of non-military targets significantly reduced enemy industrial capacity and production, This means that aerial bombardment of civilian areas in enemy territory by all major belligerents during World War II was not prohibited by positive or specific customary international humanitarian law.

Many reasons exist for the absence of international law regarding aerial bombing in World War II. Most nations had refused to ratify such laws or agreements because of the vague or impractical wording in treaties such as the 1923 Hague Rules of Air Warfare. Also, the major powers' possession of newly developed advanced bombers was a great military advantage; they would be hard pressed to accept any negotiated limitations regarding this new weapon. In the absence of specific laws relating to aerial warfare, the belligerents' aerial forces at the start of World War II used the 1907 Hague Conventions — signed and ratified by most major powers — as the customary standard to govern their conduct in warfare, and these conventions were interpreted by both sides to allow the indiscriminate bombing of enemy cities throughout the war.

General Telford Taylor, Chief Counsel for War Crimes at the Nuremberg Trials, wrote that: