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D-Day Planning & Preparation: The COSSAC Plan

D-Day Planning: The COSSAC Plan

In June 1942, I was ordered to England with instructions to begin preparation for United States participation in a cross-Channel attack against Fortress Europe which had been agreed upon by the American and British governments in April of that year as the Allied principal effort in the defeat of Germany. Some planning, in cooperation with Admiral Sir Bertram H. Ramsay, General Sir Bernard C. T. Paget, and Air Chief Marshal Sir Sholto Douglas, all of the British forces, was undertaken immediately following my arrival, but this had not proceeded beyond the informal, conversational stage when I received orders to take command of the Allied attack in Northwest Africa, decided upon on 26 July. As a result of these conversations it seemed clear that the Normandy region offered the greatest chance of success in an invasion of Europe, although some individuals favored a more direct attack against the Calais area.

The successful conclusion of the campaign in North Africa was necessary before the attention of the Allies could be devoted to a full-scale attack upon Europe, but at the Casablanca Conference, in January 1943, the Combined Chiefs of Staff felt that the time had come at least to evolve the outline tactical plans for cross Channel operations. They directed that preparations be undertaken for an emergency return to the Continent in tl,e event that Germany suddenly should weaken sufficiently to permit our landing in the face of light or negligible resistance. While preparations for such a hoped-for contingency were necessary, the chief problem was the planning for the full-scale assault to be launched against the Continent as early as possible in 1944. Some consideration was given to the possibility that a return to the Continent in force might take place late in 1943, but a review of the build-up figures of United States forces in the United Kingdom indicated that this would be impossible and that no large-scale attack could be undertaken until 1944.

Accordingly, in preparation for the day when a Supreme Commander should be appointed to command the Allied forces, the Combined Chiefs of Staff named Lieut. Gen. Sir F. E. Morgan to the post of Chief of Staff to the Supreme Allied Commander (designate). Using the initials of this title, the organization was called COSSAC, and, with a staff composed of both United States and British personnel, work began upon the creation of the assault plan against FortIess Europe. By July 1943 the Outline Plan for OVERLORD, as the operation was called, was ready for presentation to the Combined Chiefs of Staff. In August of the same year the Combined Chiefs, with the concurrence of President Roosevelt and Prime Minister Churchill, approved the plan at the Quebec Conference and ordered the completion of its details insofar as this was possible prior to the arrival of a Supreme Commander.

In the creation of the plan, COSSAC had been instructed on 25 May, through a supplementary directive from the Combined Chiefs of Staff, that the target date for the operation was to be 1 May 1944, and that simultaneous landing-eraft lift for the assau lt forces would be limited to craft sufficient to load five divisions. A total of 29 divisions in all was to be available for the assault and immediate build-up. Of these, five infantry divisions were to be simultaneously loaded in the landing craft, three to assault initially with two in the immediate follow-up. In addition, two airborne divisions were to be employed and two other infantry divisions were to follow as quickly as turn-around shipping became available. In all, then, nine divisions were to be initially employed in the assault and immediate followup period. The remaining 20 divisions were to be available for movement to the lodgement area as quickly as the build-up could be achieved.

The fact that simultaneous landing-craft lift for only five divisions had been allotted made mandatory a definite concentration of effort. The basic factor in determining where the initial assault was to be made lay in the requirement that the lodgement area should contain sufficient port facilities to maintain a force of some 26 to 30 divisions and enable that force to be augmented by follow-up shipments from the United States or elsewhere of additional divisions and supporting units at the rate of three to five divisions per month.

For such purposes, the Pas-de-Calais region offered advantages in that its proximity to England would facilitate air support and a quick turn-around for shipping. On the other hand, its beaches, while favorable to the actual landing, lacked good exits to the hinterland. Also the area was the most formidably defended on the whole French coast and was a focal point of the enemy fighter air forces disposed for defense. Moreover, the area did not offer good opportunities for expansion of the lodgement zone, and it would have been necessary to develop the beachhead to include either the Belgian ports as far as Antwerp or the Channel ports westward to include Le Havre and Rouen.

A second area considered was the Cotentin Peninsula where it was recognized that the assaulting forces would initially have a reasonable chance of success and would additionally gain the valuable port of Cherbourg. This area, however lacked suitable airfields and might have become a trap for the assault troops, since the enemy could, with relatively light forces, defend the neck of the peninsula, bottling Allied troops within the beachhead and denying them any expansion into the interior of France.

In the Caen sector the defenses were relatively light and the beaches were of high capacity and sheltered from the prevailing winds. The terrain, moreover, was suitable for airfield development and for the consolidation and subsequent expansion of the beachhead. Air support to the initial area at such a distance from the English coast presented the chief difficulty. From a beachhead in the Caen sector it was believed that it would be possible to seize the Brittany ports between Cherbourg and Nantes, and through them to build up sufficient forces for the subsequent advance eastward.

In view of these considerations, it was decided that the initial landing on the Continent should be effected in the Caen area with the subsequent seizure of a lodgement area comprising the Cherbourg-Brittany ports.

The assault itself, according to the COSSAC plan, was to be launched with a short air bombardment of the beach defenses, after which three assault divisions would be landed on the Caen beaches, followed by the equivalent of two tank brigades and a regimental combat team. At the same time, airborne forces were earmarked for the capture of the town of Caen, while subsidiary operations were to be undertaken by commandos and airborne units to neutralize certain coast defenses and to seize important river crossings. The object of the as- 2 sault forces was to occupy the general line Grandcamp - Bayeux- Caen.

Action subsequent to the assault and early build-up was to take the form, under this initial plan, of a strong thrust southward and southwestward with a view to destroying enemy forces, acquiring sites for airfields, and gaining depth for a turning movement into the Cotentin Peninsula directed on Cherbourg. When sufficient depth had been gained, a force was to advance into the Cotentin and seize Cherbourg. At the same time a thrust was to be made to deepen the beachhead southeastward to cover the establishment of additional airfields in the area southeast of Caen. It was estimated that within 14 days of the initial assault Cherbourg would be taken and the beachhead extended to include the general line Trouville-Alen~on-Mont-St-Michel. By that time it should have been possible to land some 18 divisions and to have in use about 14 airfields from which 28 to 33 fighter-type squadrons could operate.

Later operations based upon this plan were necessarily to be dictated to a large extent by enemy reactions. If he proved sufficiently weak, an immediate advance might be undertaken to sieze Le Havre and Rouen. More probably, however, it was felt that it would be necessary to take and bring into use the Brittany ports first in order to build up forces sufficient to breach the line of the Seine upon which it was expected the enemy would make a stand with the bulk of his forces. For this purpose a thrust southward would be made to seize Nantes and St-Nazaire, followed by Brest and the smaller Brittany ports. The beachhead would be consolidated on the left flank along the Eure River from Dreux to Rouen and thence along the Seine to the sea while Chartres, Orleans, and Tours were occupied. As soon as lines of communications in this area were developed and sufficient air forces established, operations would be instituted against Paris aad the Seine ports, with subsidiary operations to clear the Biscay ports for the reception of additional United States troops and supplies.

In broad outline, this was the plan proposed for the assault upon Nazi Europe. Following approval by the Combined Chiefs of Staff and in accordance with their instructions, the COSSAC organization proceeded with more detailed planning, and by 29 November sufficient progress had been made to permit of directives being issueed to 21 Army Group and to the United States First Army.

Arrow  D-Day: Development of Overlord



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