Franz Halder
Senior Allied officers who assisted the author included General Maxwell D. Taylor, Lieutenant General James M. Gavin, Lieutenant General Sir Frederick E. Morgan and General Sir Richard Nelson Gale. German officers who assisted with the book included Generaloberst Franz Halder, Hauptmann Hellmuth Lang and General der Infanterie Günther Blumentritt. The author also used Allied and German post action reports, War diaries, histories and official records.