In 1939, the German army shocked and terrorized the world with Blitzkrieg, its form of mobilized warfare. How the Germans rebuilt their army after defeat in World War I - circumventing the prohibition of the treaty at Versailles - is one of the major questions in military history. Citino shows that German officers of the army of the Weimer Republic (the Reichswehr), men like General Hans von Seeckt, General Wilhelm Groener, and Colonel Oswald Lutz, initiated and carried out a thorough reform of the army's warfighting doctrine and capability that laid the groundwork for Hitler's seemingly effortless rearmament of Germany. Using largely unpublished materials from US and German archives, the author grounds his book in a study of key autumn manoeuvres of the German army in the thirties. His analysis traces the smooth and inexorable development of the Reichswehr into the Wehrmacht, quite likely the finest military machine in history.