Using existing Spanish and German documents and interviews with men who survived both the Spanish Civil War and World War II, Proctor details the origins of Germany's Condor Legion, sent by Hitler to assist Franco's forces during the Spanish Civil War. He investigates the problems encountered by the legion in Spain, including its organization, the extent of its training, the nature of its personnel, communications, and logistics, and the experience of operating in a foreign country as one of three allied forces in the civil war. The author provides detailed information about the German involvement in critical battles such as the Aragon Offensive, the Battle of Ebro, and the final assault on Catalonia. Proctor also assesses how effectively the Luftwaffe applied the lessons it learned in Spain to World War II and analyzes the lessons it missed.