COSSAC plan voor operatie Overlord (15-07-1943)

The Secretary,
Chiefs of Staff Committee,
Offices of the War Cabinet.
15th July, 1943.

1. In my original Directive (C.O.S. (43) 215 (O)) I was charged with the duty of preparing a plan for a full scale assault against the Continent in 1944 as early as possible.

2. This part of my Directive was subsequently amplified (see C.O.S. (43) 113th Meeting (O), Item 4), in that I was ordered to submit an outline plan for an assault, with certain specified forces, on a target date the 1st May, 1944, to secure a lodgement on the Continent from which further offensive operations can be carried out. It was indicated to me, in the course of this amplification, that the lodgement area should include ports that, suitably developed, could be used by ocean-going ships for the build-up of the initial assault forces from the United Kingdom, and for their further build-up with additional divisions and supporting units that might be shipped from the United States or elsewhere.

3. I have the honour now to report that, in my opinion, it is possible to under-take the operation described, on or about the target date named, with the sea, land and air forces specified, given a certain set of circumstances in existence at that time.

4. These governing circumstances are partly within our direct control and partly without. Those within our control relate first to the problem of beach maintenance, and secondly to the supply of shipping, naval landing craft and transport aircraft. Wherever we may attempt to land, and however many ports we capture, we cannot escape the fact that we shall be forced to main-tain a high proportion of our forces over the beaches for the first two or three months while port facilities are being restored; and that, in view of the vari-ability of the weather in the Channel, this will not be feasible unless we are able rapidly to improvise sheltered anchorages off the beaches. New methods of overcoming this problem are now being examined. There is no reason to suppose that these methods will be ineffective, but I feel it my duty to point out that this operation is not to be contemplated unless this problem of pro-longed cross-beach maintenance and the provision of artificial anchorages shall have been solved.

5. As regards the supply of shipping, naval landing craft and transport air-craft, increased resources in these would permit of the elaboration of alterna-tive plans designed to meet more than one set of extraneous conditions, whereas the state of provision herein taken into account dictates the adoption of one course only, or none at all. In proportion as additional shipping, land-ing craft and transport aircraft can be made available, so the chances of suc-cess in the operation will be increased. It seems feasible to contemplate addi-tions as a result either of stepped-up production, of strategical re-allotment or, in the last resort, of postponement of the date of assault.

6. I have come to the conclusion that, in view of the limitations in resources imposed by my directives, we may be assured of a reasonable chance of success on the 1st May, 1944, only if we concentrate our efforts on an assault across the Norman beaches about Bayeux.

7. As regards circumstances that we can control only indirectly, it is, in my opinion, necessary to stipulate that the state of affairs existing at the time, both on land in France and in the air above it, shall be such as to render the assault as little hazardous as may be so far as it is humanly possible to calcu-late. The essential discrepancy in value between the enemy's troops, highly organised, armed and battle-trained, who await us in their much vaunted impregnable defences, and our troops, who must of necessity launch their assault at the end of a cross-Channel voyage with all its attendant risks, must be reduced to the narrowest possible margin. Though much can be done to this end by the means available and likely to become available to us in the United Kingdom to influence these factors, we are largely dependent upon events that will take place on other war fronts, principally on the Russian front, between now and the date of the assault.

8. I therefore suggest to the Chiefs of Staff that it is necessary, if my plan be approved, to adopt the outlook that Operation "Overlord" is even now in progress, and to take all possible steps to see that all agencies that can be brought to bear are, from now on, co-ordinated in their action as herein below described, so as to bring about the state of affairs that we would have exist on the chosen day of assault.

9. Finally, I venture to draw attention to the danger of making direct com-parisons between operation "Husky" and operation "Overlord." No doubt the experience now being gained in the Mediterranean will prove invaluable when the detailed planning stage for "Overlord" is reached, but viewed as a whole the two operations could hardly be more dissimilar. In "Husky," the bases of an extended continental coastline were used for a converging assault against an island, whereas in "Overlord" it is necessary to launch an assault from an island against an extended continental mainland coastline. Further-more, while in the Mediterranean the tidal range is negligible and the weather reasonably reliable, in the English Channel the tidal range is considerable and the weather capricious.

10. Attached hereto are papers setting forth the plan that I recommend for adoption.

F.E. MORGAN, Lieutenant-General,
Chief of Staff to the Supreme Commander (Designate).

H.Q., C.O.S.S.A.C.,
Norfolk House,
St. James's Square, S.W.1.

Object.

1. The object of Operation "Overlord" is to mount and carry out an opera-tion, with forces and equipment established in the United Kingdom, and with target date the 1st May, 1944, to secure a lodgement on the Continent from which further offensive operations can be developed. The lodgement area must contain sufficient port facilities to maintain a force of some twenty-six to thirty divisions, and enable that force to be augmented by follow-up ship-ments from the United States or elsewhere of additional divisions and sup-porting units at the rate of three to five divisions per month.

Selection of a Lodgement Area.

2. In order to provide sufficient port facilities to maintain these large forces, it will be necessary to select a lodgement area which includes a group of major ports. We must plan on the assumption that ports, on capture, will be seri-ously damaged and probably blocked. It will take some time to restore nor-mal facilities. We shall thus be forced to rely on maintenance over beaches for an extended period.

3. A study of the beaches on the Belgian and Channel coasts shows that the beaches with the highest capacity for passing vehicles and stores inland are those in the Pas de Calais and the Caen-Cotentin area. Of these, the Caen beaches are the most favourable, as they are, unlike the others, sheltered from the prevailing winds. Naval and air considerations point to the area between the Pas de Calais and the Cotentin as the most suitable for the initial landing, air factors of optimum air support and rapid provision of airfields indicating the Pas de Calais as the best choice, with Caen as an acceptable alternative.

4. Thus, taking beach capacity and air and naval considerations together, it appears that either the Pas de Calais area or the Caen-Cotentin area is the most suitable for the initial main landing.

5. As the area for the initial landing the Pas de Calais has many obvious advantages such that good air support and quick turn round for our shipping can be achieved. On the other hand, it is a focal point of the enemy fighters disposed for defense, and maximum enemy air activity can be brought to bear over this area with the minimum movement of his air forces. Moreover, the Pas de Calais is the most strongly defended area on the whole French coast. The defenses would require very heavy and sustained bombardment from sea and air: penetration would be slow, and the result of the bombardment of beach exits would severely limit the rate of build-up. Further, this area does not offer good opportunities for expansion. It would be necessary to develop the bridgehead to include either the Belgian ports as far as Antwerp or the Channel ports Westwards to include Havre and Rouen. But both an advance to Antwerp across the numerous water obstacles, and a long flank march of some 120 miles to the Seine ports must be considered unsound operations of war unless the German forces are in a state not far short of final collapse.

6. In the Caen-Cotentin area it would be possible to make our initial landing either partly on the Cotentin Peninsula and partly on the Caen beaches, wholly in the Cotentin or wholly on the Caen beaches. An attack with part of our forces in the Cotentin and part on the Caen beaches, is, however, consid-ered to be unsound. It would entail dividing our limited forces by the low-lying marshy ground and intricate river system at the neck of the Cotentin Peninsula; thus exposing them to defeat in detail.

7. An attack against the Cotentin Peninsula, on the other hand, has a reason-able chance of success, and would ensure the early capture of the port of Cherbourg. Unfortunately, very few airfields exist in the Cotentin, and that area is not suitable for rapid airfield development. Furthermore, the narrow neck of the Peninsula would give the Germans an easy task in preventing us from breaking out and expanding our initial bridgehead. Moreover, during the period of our consolidation in the Cotentin the Germans would have time to reinforce their coastal troops in the Caen area, rendering a subsequent amphibious assault in that area much more difficult.

8. There remains the attack on the Caen beaches. The Caen sector is weakly held; the defenses are relatively light and the beaches are of high capacity and sheltered from the prevailing winds. Inland the terrain is suitable for airfield development and for the consolidation of the initial bridgehead; and much of it is unfavourable for counter-attacks by panzer divisions. Maximum enemy air opposition can only be brought to bear at the expense of the enemy air defense screen covering the approaches to Germany; and the limited number of enemy airfields within range of the Caen area facilitates local neutraliza-tion of the German fighter force. The sector suffers from the disadvantage that considerable effort will be required to provide adequate air support to our assault forces and some time must elapse before the capture of a major port.
After a landing in the Caen sector it would be necessary to seize either the Seine group of ports or the Brittany group of ports. To seize the Seine ports would entail forcing a crossing of the Seine, which is likely to require greater forces than we can build up through the Caen beaches and the port of Cher-bourg. It should, however, be possible to seize the Brittany ports between Cherbourg and Nantes and on them build up sufficient forces for our final advance Eastwards.
Provided that the necessary air situation can first be achieved, the chances of a successful attack and of rapid subsequent development are so much greater in this sector than in any other that it is considered that the advantages far outweigh the disadvantages.

The Lodgement Area Selected.

9. In the light of these factors, it is considered that our initial landing on the Continent should be effected in the Caen area, with a view to the eventual seizure of a lodgement area comprising the Cherbourg-Brittany group of ports (from Cherbourg to Nantes).

Opening Phase up to the Capture of Cherbourg.

10. The opening phase in the seizing of this lodgement area would be the effecting of a landing in the Caen sector with a view to the early capture and development of airfield sites in the Caen area, and of the port of Cherbourg.

11. The main limiting factors affecting such an operation are the possibility of attaining the necessary air situation; the number of offensive divisions which the enemy can make available for counter attack in the Caen area; the availability of landing ships and craft and of transport aircraft; and the capac-ity of the beaches and ports in the sector.

12. Although the strength of the G.A.F. available in 1944 on the Western front cannot be forecast at this stage, we can confidently expect that we shall have a vast numerical superiority in bomber forces. The first-line strength of the German fighter force is, however, showing a steady increase and although it is unlikely to equal the size of the force at our disposal, there is no doubt that our fighters will have a very large commitment entailing dispersal and operations at maximum intensity. Our fighters will also be operating under serious tactical disadvantages in the early stages, which will largely offset their numerical superiority. Before the assault takes place, therefore, it will be necessary to reduce the effectiveness of the G.A.F., particularly that part which can be brought to bear against the Caen area.

13. The necessary air situation to ensure a reasonable chance of success will therefore require that the maximum number of German fighter forces are contained in the Low Countries and North-West Germany, that the effective-ness of the fighter defense in the Caen area is reduced and that air reinforce-ments are prevented from arriving in the early stages from the Mediterranean. Above all, it will be necessary to reduce the overall strength of the German fighter force between now and the date of the operation by destruction of the sources of supply, by the infliction of casualties by bringing on air battles, and, immediately prior to the assault, by the disorganization of G.A.F. instal-lations and control system in the Caen area.

14. As it is impossible to forecast with any accuracy the number and location of German formations in reserve in 1944, while, on the other hand, the forces available to us have been laid down, an attempt has been made in this paper to determine the wisest employment of our own forces and then to determine the maximum number of German formations which they can reasonably overcome. Apart from the air situation, which is an over-riding factor, the practicability of this plan will depend principally on the number, effective-ness, and availability of German divisions present in France and the Low Countries in relation to our own capabilities. This consideration is discussed below (paragraph 35).

15. A maximum of thirty and a minimum of twenty-six equivalent divisions are likely to be available in the United Kingdom for cross-Channel operations on the 1st May 1944. Further build-up can be at the rate of three to five divi-sions per month.

16. Landing ships and craft have been provided to lift the equivalent of three assault divisions and two follow-up divisions, without "overheads," and it has been assumed that the equivalent of an additional two divisions can be afloat in ships.

17. Airborne forces amounting to two airborne divisions and some five or six parachute regiments will be available, but, largely owing to shortage of transport aircraft, it is only possible to lift the equivalent of two-thirds of one airborne division simultaneously, on the basis of present forecasts.

18. Even if additional landing ships and craft could be made available, the beaches in the Caen area would preclude the landing of forces greater than the equivalent of the three assault and two follow-up divisions, for which craft have already been provided. Nevertheless, an all-round increase of at least 10 per cent. in landing ships and craft is highly desirable in order to provide a greater margin for contingencies within the framework of the exist-ing plan. Furthermore, sufficient lift for a further assault division could most usefully be employed in an additional landing on other beaches.

19. There is no port of any capacity within the sector although there are a number of small ports of limited value. Maintenance will, therefore, of neces-sity be largely over the beaches until it is possible to capture and open up the port of Cherbourg. In view of the possibilities of interruption by bad weather it will be essential to provide early some form of improvised sheltered waters.

20. Assuming optimum weather conditions, it should be possible to build up the force over the beaches to a total by D plus 6 of the equivalent of some eleven divisions and five tank brigades and thereafter to land one division a day until about D plus 24.

Proposed Plan.

Preliminary Phase.

21. During the preliminary phase, which must start forthwith, all possible means including air and sea action, propaganda, political and economic pressure, and sabotage, must be integrated into a combined offensive aimed at softening the German resistance. In particular, air action should be di-rected towards the reduction of the German air forces on the Western front, the progressive destruction of the German economic system and the under-mining of German morale.

22. In order to contain the maximum German forces away from the Caen area diversionary operations should be staged against other areas such as the Pas de Calais and the Mediterranean Coast of France.

Preparatory Phase.

23. During this phase air action will be intensified against the G.A.F., par-ticularly in North-West France, with a view to reducing the effectiveness of the G.A.F. in that area, and will be extended to include attacks against com-munications more directly associated with movement of German reserves which might affect the Caen area. Three naval assault forces will be assem-bled with the naval escorts and loaded at ports along the South Coast of Eng-land. Two naval assault forces carrying the follow-up forces will also be assembled and loaded, one in the Thames Estuary and one on the West Coast.

The Assault.

24. After a very short air bombardment of the beach defenses three assault divisions will be landed simultaneously on the Caen beaches, followed up on D Day by the equivalent of two tank brigades (United States regiments) and a brigade group (United States regimental combat team). At the same time, airborne forces will be used to seize the town of Caen; and subsidiary opera-tions by commandos and possibly by airborne forces will be undertaken to neutralize certain coast defenses and seize certain important river crossings. The object of the assault forces will be to seize the general line Grandcamp-Bayeux-Caen.

Follow-up and Build-up Phase.

25. Subsequent action will take the form of a strong thrust Southwards and South-Westwards with a view to destroying enemy forces, acquiring sites for airfields, and gaining depth for a turning movement into the Cotentin Penin-sula directed on Cherbourg. When sufficient depth has been gained a force will advance into the Cotentin and seize Cherbourg. At the same time a thrust will be made to deepen the bridgehead South-Eastwards in order to cover the construction and operation of additional airfields in the area South-East of Caen.

26. It is considered that, within fourteen days of the initial assault, Cherbourg should be captured and the bridgehead extended to include the general line Trouville-Alencon-Mont St. Michel. By this date, moreover, it should have been possible to land some eighteen divisions and to have in operation about fourteen airfields from which twenty-eight to thirty-three fighter-type squad-rons should be operating.

Further Developments after Capture of Cherbourg.

27. After the capture of Cherbourg the Supreme Allied Commander will have to decide whether to initiate operations to seize the Seine ports or whether he must content himself with first occupying the Brittany ports. In this decision he will have to be guided largely by the situation of the enemy forces. If the German resistance is sufficiently weak, an immediate advance could be made to seize Havre and Rouen. On the other hand, the more prob-able situation is that the Germans will have retired with the bulk of their forces to hold Paris and the line of the Seine, where they can best be covered by their air forces from North-East France and where they may possibly be reinforced by formations from Russia. Elsewhere they may move a few divi-sions from Southern France to hold the crossings of the Loire and will leave the existing defensive divisions in Brittany.
It will therefore most probably be necessary for us to seize the Brittany ports first, in order to build up sufficient forces with which we can eventually force the passage of the Seine.

28. Under these circumstances, the most suitable plan would appear to be to secure first the left flank and to gain sufficient airfields for subsequent opera-tions. This would be done by extending the bridgehead to the line of the River Eure from Dreux to Rouen and thence along the line of the Seine to the sea, seizing at the same time Chartres, Orleans and Tours.

29. Under cover of these operations a force would be employed in capturing the Brittany ports; the first step being a thrust Southwards to seize Nantes and St. Nazaire, followed by subsidiary operations to capture Brest and the various small ports of the Brittany Peninsula.

30. This action would complete the occupation of our initial lodgement area and would secure sufficient major ports for the maintenance of at least thirty divisions. As soon as the organization of the L. of C. in this lodgement area allowed, and sufficient air forces had been established, operations would then be begun to force the line of the Seine, and to capture Paris and the Seine ports. As opportunity offered, subsidiary action would also be taken to clear the Germans from the Biscay ports to facilitate the entry of additional Ameri-can troops and the feeding of the French population.

Command and Control.

31. In carrying out Operation "Overlord" administrative control would be greatly simplified if the principle were adopted that the United States forces were normally on the right of the line and the British and Canadian forces on the left. Major Conditions Affecting Success of the Operation.

32. It will be seen that the plan for the initial landing is based on two main principles--concentration of force and tactical surprise. Concentration of the assault forces is considered essential if we are to ensure adequate air support and if our limited assault forces are to avoid defeat in detail. An attempt has been made to obtain tactical surprise by landing in a lightly defended area-presumably lightly defended as, due to its distance from a major port, the Germans consider a landing there unlikely to be successful. This action, of course, presupposes that we can offset the absence of a port in the initial stages by the provision of improvised sheltered waters. It is believed that this can be accomplished.

33. The operation calls for a much higher standard of performance on the part of the naval assault forces than any previous operation. This will depend upon their being formed in sufficient time to permit of adequate training.

34. Above all, it is essential that there should be an over-all reduction in the German fighter force between now and the time of the surface assault. From now onwards every practical method of achieving this end must be employed. This condition, above all others, will dictate the date by which the amphibi-ous assault can be launched.

35. The next condition is that the number of German offensive divisions in reserve must not exceed a certain figure on the target date if the operation is to have a reasonable chance of success. The German reserves in France and the Low Countries as a whole, excluding divisions holding the coast, G.A.F. divisions and training divisions, should not exceed on the day of the assault twelve full-strength first-quality divisions. In addition, the Germans should not be able to transfer more than fifteen first-quality divisions from Russia during the first two months. Moreover, on the target date the divisions in reserve should be so located that the number of first-quality divisions which the Germans could deploy in the Caen area to support the divisions holding the coast should not exceed three divisions on D Day, five divisions on D plus 2, or nine divisions by D plus 8.

During the preliminary period, therefore, every effort must be made to dissi-pate and divert German formations, lower their fighting efficiency and dis-rupt communications.

36. Finally, there is the question of maintenance. Maintenance will have to be carried out over beaches for a period of some three months for a number of formations, varying from a maximum of eighteen divisions in the first month to twelve divisions in the second month, rapidly diminishing to nil in the third month. Unless adequate measures are taken to provide sheltered waters by artificial means, the operation will be at the mercy of the weather. Moreover, special facilities and equipment will be required to prevent undue damage to craft during this extended period. Immediate action for the provision of the necessary requirements is essential.

37. Given these conditions--a reduced G.A.F., a limitation in the number or effectiveness of German offensive formations in France, and adequate ar-rangements to provide improvised sheltered waters--it is considered that Operation "Overlord" has a reasonable prospect of success. To ensure these conditions being attained by the 1st May, 1944, action must start now and every possible effort made by all means in our power to soften German resis-tance and to speed up our own preparations.

Offices of the War Cabinet, S.W.1,
30th July, 1943.