During the First World War blood transfusion progressed from being a rarely used treatment to a major component of the resuscitation of exsanguinated casualties, relying on local donation and availability of medical expertise. In the 1920s and early 1930s, clinical use of transfusion gradually increased supported by growing civilian, often volunteer, blood donor systems. The Spanish Civil War (1936–1939) introduced the first systematic use of aerial bombardment to intimidate the civilian population and to destroy infrastructure; it also saw more mobile battlefronts, replacing the relatively static trench warfare of 1914-1918. New measures for the delivery of transfusion services emerged rapidly from primitive beginnings. These included large civilian blood donor organizations providing anti-coagulated “stored” blood for civilian and military medical use and land delivery services to civilian and military hospitals. Surgical units for the emergency management of casualties were required to be agile in moving as battlefronts evolved and carefully concealed to avoid air attack. Ideally the blood supply would follow. Under threat of a wider European conflict in 1938–1939, British authorities started developing plans for transfusion support in the management of civilian casualties of air attack and military casualties of armed conflict. The involvement, directly and indirectly, of British and other volunteer physicians returned from Spain, together with their Spanish colleagues, ensured that awareness of the Spanish experience was available to the British authorities. The system that was eventually put in place involved a civilian blood donor capability with one centre dedicated particularly to military supplies. Separate distribution systems were organized for civilian and military purposes, with the latter including distribution overseas by air. The military system delivered blood and components through a supply chain to mobile field transfusion units under command of a medical officer specially trained in transfusion and resuscitation, supporting mobile surgical units in the immediate rear of battlefronts. The broad principles developed in Spain (1936-39) for delivery of military blood transfusion practice still support current measures in battlefield casualty resuscitation.