How did it become ‘normal’ to bomb civilians? Focusing on the aerial bombardment of China, Germany, Britain, and Japan in 1937-45, this essay spotlights the role of transnational learning in the construction and destruction of ‘home fronts’. Belligerents vigorously studied each other's strategies to destroy the enemy's cities and ‘morale’, while investigating efforts to defend one's own home front by means of ‘civilian defence’. The inclusion of Japan, as bomber and bombed, contributes to a more global, connected history of the Second World War. Japan's sustained bombardment of Chinese cities not only reflected emerging transnational ideas of strategic bombing and total war, but also imparted new ‘lessons’ to Western air forces. Moreover, the devastating US firebombing of Japanese cities in 1945 challenges widely accepted judgments that bombing was generally ineffective, serving only to stiffen civilian morale. Why Japanese cities were bombed, and how they were bombed, was not an exceptional story, but was intimately connected to what the Allies had learned from bombing European urban areas.