In the 1930s, a dramatic debate developed on the future scenarios of air warfare: apocalyptic pictures were drawn, anticipating a nuclear conflict. When civil war broke out in Spain, the German and Italian air forces experimented with aircraft and bombing techniques. While the Luftwaffe concentrated on tactical dive-bombing, the Regia Aeronautica carried out strategic attacks against urban centres (the most important case being Barcelona). The effects of the air weapon were also studied by military experts from various countries, who came to Spain to report to their governments in view of a future general conflict. This report aims to reconstruct aspects of this study, reporting on the confirmations that these experts found in the Spanish events, but above all on the surprises: the predicted fall in the morale of the urban population under the bombs did not occur; the gases were not used; the aeroplane did not seem able to paralyse the enemy’s industrial production as had been predicted. Nonetheless, what came from Spain was a terrible lesson: civilians were considered an integral part of military operations, no humanitarian illusions could be nurtured any longer.