Almost immediately after France fell to the Nazis in 1940, the Allies planned a cross Channel assault on the German occupying forces. At the Quebec Conference in August 1943, Winston Churchill and Franklin Roosevelt reaffirmed the plan, which was code named Overlord. Although Churchill acceded begrudgingly to the operation, historians note that the British still harbored persistent doubts about whether Overlord would succeed. The decision to mount the invasion was cemented at the Tehran...