An influential Italian military writer, General Giulio Douhet, actually argued for the sustained bombing of civilians. He predicted that they would become quickly demoralized by such bombing and would force their leaders to surrender. Despite the theories of Douhet, most at this time felt that bombing civilians was uncivilized and should be prohibited. In 1923, Britain, France, Italy, Japan, and the United States agreed to a set of rules for air warfare. One article prohibited “bombing from the air for the purpose of terrorizing the civilian population . . . or of injuring noncombatants. . . ." The participating governments, however, never ratified these rules, so they were not legally binding. At the Geneva Disarmament Conference of 1932, most of the world's powers agreed that air attacks on civilians violated the laws of war. But the conference broke up before approving a final agreement.
Zeppelins were used to drop explosive ordinance on cities in Belgium and England during the First World War.
As airplanes in the interwar period became faster, larger, and more powerful, Douhet's vision of the aircraft as the decisive force in war became a possibility. In the years leading up to World War II, Japan became the first power to attack civilians from the air. In 1932, Japanese warplanes bombed a worker district in Shanghai, China, an incident that produced worldwide outrage. The outrage did not stop Japan from bombing civilian areas of other Chinese cities. In 1936, Italian dictator Mussolini ordered an attack on the largely defenseless east African country of Ethiopia. When Mussolini's warplanes struck the capital city, causing many civilian casualties, the world again condemned the slaughter of innocent people.
The following year, the Germans bombed the Basque town of Guernica, an event made famous by Pablo Picasso's famed antiwar painting of the tragic attack on the town's populated marketplace during the Spanish Civil War (1936-1939). The town had no tactical military value and the indiscriminate bombing was an attempt to break the spirit of the civilians resisting Fascist rule. Guernica can be seen as Picasso's plea against the senseless violence and barbarity of war and a stark warning of the World War to come.
While Picasso was living in Nazi-occupied Paris during World War II, one German officer allegedly asked him, upon seeing a photo of Guernica in his apartment, “Did you do that?" Picasso responded, “No, you did."
Picasso's Guernica, 1937