This essay reveals the vital role of transnational learning in structuring air defense in Japan, Germany, and Britain in World War II, while comparing how each regime shaped the contours of home-front mobilization. In the decades between the two world wars, states increasingly recognized the new threat of aerial bombardment of cities, and they actively investigated other nations' efforts at “civilian defense” and “total war.” Learning continued during World War II. In countries experiencing bombing, civil defense programs did more to mobilize daily life than any other wartime imperative. Remarkably, civil defense operations in Imperial Japan, Nazi Germany, and democratic Britain resembled each other—recruiting or conscripting millions of men, women, and youths to serve as neighborhood-based air raid wardens, “fire watchers,” first-aid workers, and members of civil defense associations. At the same time, differences in regimes and circumstances affected the degree of compulsion in each case.