Speidel was a lieutenant colonel and chief of staff of the Eighteenth Army during the invasion of France, and he was the officer who accepted the surrender of Paris on 14 June 1940. He was subsequently appointed chief of staff to General der Infanterie (U.S. equiv. lieutenant general) Otto von Stülpnagel, German military governor of France. In March 1942, Speidel was assigned to the Eastern Front, where he served for two years during the battles of Vyasma and Kharkov and the assault on Stalingrad. Although he distinguished himself and was promoted to Generalleutnant (U.S. equiv. major general) during his service in Russia, Speidel became convinced that the war could not be won and that Adolf Hitler's regime had to be overthrown.
In April 1944, Speidel was appointed chief of staff to Field Marshal Erwin Rommel's Army Group B in France. Speidel's opposition to Hitler led him to endeavor to convince Rommel to back the coup against Hitler. However, Speidel was not part of the inner circle of officers who carried out the 20 July 1944 assassination attempt against Hitler, which undoubtedly saved Spiedel's life. Speidel helped ensure that Paris was not demolished as Hitler had ordered, and he was arrested by the Gestapo on 7 September. Imprisoned in Berlin, he was acquitted by a court-martial headed by Field Marshal Karl R. G. von Rundstedt. His expulsion from the army would have brought him to trial before the People's Court. Interrogated about his possible role in the plot against Hitler, he outwitted his questioners and insisted that he and Rommel were not involved. He was held prisoner for seven months and then managed to escape and go into hiding. French troops liberated him on 29 April 1945.
After the war, Speidel became a professor of modern history at the University of Tübingen, but he remained a consultant on defense matters. In 1954, he was appointed chief negotiator for the Federal Republic of Germany with France regarding German entry into the North Atlantic Treaty Organization (NATO). He was also appointed a lieutenant general in the new German army, the Bundeswehr. In April 1957, Speidel became commander in chief of NATO Land Forces in central Europe and was promoted to full general. He held that post until September 1963. In March 1964, Speidel retired from the army and resumed academic work and writing. He died at Bad Honnef, Germany, on 28 November 1984.
Irving, David. The Trail of the Fox: The Search for the True Field Marshal Rommel. New York: E. P. Dutton, 1977.; Müller, Klaus-Jürgen. "Witzleben, Stülpnagel and Speidel." In Correlli Barnett, ed., Hitler's Generals, 43–72. New York: George Weidenfeld, 1989.