Change in the Probable Form of Attack
While the Germans were developing their war-production mainly for ground forces for the Russian Campaign at the expense of their air force, the Royal Air Force was expanding rapidly. In the circumstances a prolonged battle for air supremacy prior to invasion had even less chance of success than in 1940, and the attempt was unlikely to be repeated.
Instead, the C.-in-C. Home Forces believed that the German Command would merge the efforts of its land, air and sea forces into a concentrated attack based upon surprise, speed, ruthlessness, and the rapid exploitation of success, making the fullest use of armoured forces. Of the several possible methods of attack he considered the concentrated effort to be the most probable, and the greatest threat.
One Month’s Warning of Invasion Probable
The Chiefs of Staff accepted the view that the Germans would be unable to disengage from the Russian front and concentrate in the West for an invasion of this country without disclosing their intentions. They accordingly told the Defence Services on the 2nd August that they might expect “from now on to receive one month’s warning from us of impending invasion”.15
Five to Seven Days Possible Delay Before the Full Deployment of the Home Fleet
The Admiralty worked out a programme, given as Appendix 23, on the assumption of three weeks’ notice by which time nearly two thirds (97) of the total of destroyers in Home Commands and 75 per cent. of the total of coastal vessels in the United Kingdom could be transferred to anti-invasion duties, apart from heavier ships of the Home Fleet and vessels which might be recalled from foreign stations.
Such a force would be able to cope without difficulty with the German light escorts, amounting in all to about 17 destroyers, 30 torpedo boats and 60 E-boats, provided their fighter protection could be neutralised.
The Admiralty had, however, to take into account the vital need for escorting our import ships, the reinforcement convoys to the Middle East, and the convoys of war material of all kinds by the Northern route to Russia. In view of the scale of attack on these trade and convoy routes the task was of such urgency that the Admiralty felt that confirmation of actual invasion would have to be awaited before diverting light naval forces and cruisers to counter it.
The fact that an invasion was mounted did not necessarily signify when it would be launched or even that it would be launched at all. Faced with that predicament the Admiralty admitted that five to seven days might elapse after the invasion had started before the Navy in Home Waters could be fully deployed in an anti-invasion role.
The C.-in-C. Home Forces pointed out that it was the first few days of an attempted invasion which were vital, and when navel assistance was most needed. With armoured forces landed in the country five to seven days was a long time, within which it would be perfectly possible to lose this country and the war unless the Home Amy was sufficiently strong to deal with such a threat (see Appendix 22).
Limitations on Air Defence
The Air Staff continued to maintain that until Germany had defeated our fighter force an invasion by sea or air was not a practical operation. The C.-in-C. Home Forces, however, took the view that although the struggle for air supremacy would affect the whole course of invasion the defeat of our fighter force would be unnecessary before German troops could land in this country. They might make the sea crossing under cover of darkness, fog or smoke; and in his opinion the Air Staff calculation gave insufficient attention to the possible absence of naval assistance to our defence during the first vital days, and also to the degree of risk which the German Command would be prepared to accept to gain a decision.
The German expedition would be given the maximum fighter cover, and while our fighter force would be engaged in protecting its own organisation and in taking toll of airborne invasion our bombers would have to face heavy opposition from German fighters and anti-aircraft defence in their efforts to attack the expedition in the embarkation ports, on passage, and on the landing beaches.
In view of the large number of possible assembly ports available to the Germans between Norway and Bordeaux, the C.-in-C. Home Forces considered that the task of disorganising and disintegrating a force of some 40 divisions before it sailed would be beyond the power of our bomber force; and every replacement of merchant shipping by the smaller tank landing-craft and barges would assist the Germans in avoiding the effects of bombardment from the air. The withdrawals of our own forces from Dunkirk, Greece and Crete, undertaken with a varying degree of fighter cover and in the face of heavy bombing, led him to the conclusion that the Royal Air Force could not prevent a large German force being landed in this country.
The Home Army to be Prepared to Defeat the Seaborne Expedition Unaided
While taking into account the casualties which our naval and air action might inflict on an invading force the C.-in-C. Home Forces maintained that for the purpose of the Home Defence plan for the spring of 1942, the sea-barrier, hitherto the country’s main bulwark, would have to be regarded as an undefended no-man’s-land.
In the conditions which might prevail “the Army must be strong enough to deliver a decisive blow against the maximum enemy force which can be transported to this country”. He believed that it would be dangerous to accept any other assumption.
A Pincer Movement on London Expected
The bulk of the German armoured force was expected to be directed against South-Eastern and Eastern Commands, with London and its port as the objective. It was estimated that the maintenance of the German invasion forces when landed would require 8,000 to 10,000 tons of supplies daily, and that the only single port capable of dealing with such a quantity was London.
The Chiefs of Staff had stressed the vital importance of the defence of the Kent coast, bordering the Straits of Dover as being the only place where the Germans had a chance of obtaining an initial and secure line of communications. By capturing the Kent coast and mounting guns on it, or capturing our guns, they could hope to deny the Straits to our naval forces; and they could evade our air force by making the maximum use of the dark hours. It was believed that if the Germans could use the harbours and beaches for landing supplies they would be able to maintain an invading force landed in the Kent area. The Chiefs of Staff were therefore convinced that a vast effort would be made to seize that vital area; and that airborne troops, and possibly gas, might be used to isolate it from the rest of the country.
If bridgeheads were formed in East Anglia and in Kent, the advance would probably take the form of a pincer movement on, or to the west of, London by attacks through the Eastern Counties and through South-East England, combined with a subsidiary attack from landing beaches east of Weymouth. The Joint Intelligence Committee estimated however that the casualties inflicted upon the landing forces would be so heavy that not more than 50 per cent. would advance inland from the beaches.