To meet the challenge, the United States, Great Britain, and Canada all turned the greater part of their energies to the task of launching an assault and establishing a beach-head. Their venture was code-named Operation Overlord; nearly every citizen of the three nations involved made a direct personal contribution to launching it.
As a consequence, Elsenhower’s problems did not include a shortage of material. He had an abundant supply of tanks, guns, trucks. His problem was how to get them across the Channel and into battle. The tanks and heavy artillery could only be brought ashore gradually, especially on D-Day itself and for a few days after that. Thus, the Allied forces would be at their most vulnerable after the first wave had landed and before the follow-up waves got ashore with their tanks and guns. The troops themselves would be heavily outnumbered (by as much as ten to one) in the first days of the invasion, and as late as D-Day plus one month the ratio would be five to one. But many of the German divisions, fifty-five in all, were scattered all across France; many were immobile, and many were of low quality. Furthermore, Elsenhower could count on the Allied air forces to keep German movement to a minimum, at least in daylight. And he had chosen as the invasion site the area west of the Orne River: this avoided the bulk of German strength in France, which was north and east of the mouth of the Seine. In that area, and most of all around the Pas de Calais, German defences were strongest. In addition, the Germans had most of their panzer strength in the Pas de Calais.
Because the panzers were to the east, the most dangerous flank of the invasion for the Allies was the left flank. It was closest to the major German counter -attack formations and therefore the place where Eisenhower expected the most determined – and most dangerous – counter-attacks.
For immediate counter-attack purposes, Rommel had two armoured divisions, the 12th SS Panzer and the 21st Panzer, stationed in and to the east ofCaen. Elsenhower’s greatest fear was that Rommel would send those divisions, operating as a coordinated unit, on a counter-attack against his left flank, code-named Sword Beach, just west of the mouth of the Orne River. It was possible that those two panzer divisions would drive the British 3rd Infantry Division on Sword back into the sea. It was also possible that, on D-Day plus one or two, additional panzer divisions would come into Normandy to participate in flank attacks along the beaches. They would strike first against Juno, then Gold, and finally the American beaches at Omaha and Utah. With fighting going on along the beaches, all Elsenhower’s loading schedules would be disrupted.
To prevent such a catastrophe, Eisenhower expected to delay and harass the German tanks moving into Normandy by using the Allied air force, which had complete command of the air. The trouble was that the air forces could not operate either at night or in bad weather. By themselves, they would not be able to isolate the battlefield. Eisenhower needed some additional way to protect Sword Beach and his critical left flank.
To solve his problem, Eisenhower turned to another of the assets that Allied control of the air made available to him -airborne forces, extraordinarily mobile and elite units. German success with paratroopers and gliderborne troops in the first years of World War II had convinced the British and American armies of the need to create their own airborne divisions. Now Eisenhower had four such divisions available to him, the US 82nd and 101st Airborne and the British 1st and 6th Airborne. He decided to use them on his flanks: offensively to provide immediate tactical assistance by seizing bridges, road junctions, and the like; defensively to keep the Germans occupied and confused. The British 6th Airborne, dropping east of Sword Beach, had another critical task: setting up a blocking force to keep the German panzers away from the left flank.
Critical though those tasks were, they did not seem critical enough to George C. Marshall, the US Army Chief of Staff. Marshall was so strongly opposed to Elsenhower’s plan that he sent Eisenhower what amounted to a reprimand – and was certainly the most critical letter he ever wrote to his protege. Marshall’s criticism, and Elsenhower’s response, bring out very clearly the advantages and disadvantages of airborne troops.