In a memorandum to the Chief of the Imperial General Staff, circulated to the War Cabinet, Winston Churchill set out his views on the practicality of an invasion.
His assessment of the prospects of a German landing place great weight on the strength of the Royal Navy. He was encouraged by the fact that the battleships HMS Nelson and HMS Barham would soon be ready for sea and would enable the creation of further battle groups that could break up any invasion force.
He also felt that the largest German ships were under the close surveillance of the RAF and would not be able to mount a surprise breakout. His one caveat is the need for ‘strong Air support’ necessary to protect the Royal Navy during daylight hours.
I FIND it very difficult to visualise the kind of invasion all along the coast by troops carried in small craft, and even in boats. I have not seen any serious evidence of large masses of this class of craft being assembled, and, except in very narrow waters, it would be a most hazardous and even suicidal operation to commit a large army to the accidents of the sea in the teeth of our very numerous armed patrolling forces.
The Admiralty have over 1,000 armed patrolling vessels, of which two or three hundred are always at sea, the whole being well manned by competent seafaring men. A surprise crossing should be impossible, and in the broader parts of the North Sea the invaders should be an easy prey, as part of their voyage would be made by daylight.
Behind these patrolling craft are the flotillas of destroyers, of which 40 destroyers are now disposed between the Humber and Portsmouth, the bulk being in the narrowest waters. The greater part of these are at sea every night, and rest in the day. They would therefore probably encounter the enemy vessels in transit during the night, but also could reach any landing point or points on the front mentioned in two or three hours. They could immediately break up the landing craft, interrupt the landing, and fire upon the landed troops, who, however lightly equipped, would have to have some proportion of ammunition and equipment carried on to the beaches from their boats.
The Flotillas would, however, need strong Air support from our fighter aircraft during their intervention from dawn onwards. The provision of the Air fighter escort for our destroyers after daybreak is essential to their most powerful intervention on the beaches.
Churchill’s conclusion was that the Army had the space and time to regroup and reform inland, rather than be dissipated around the coast:
[I hope] that you will be able to bring an ever larger proportion of your formed Divisions back from the coast into support or reserve, so that their training may proceed in the highest forms of offensive warfare and counterattack, and that the coast, as it becomes fortified, will be increasingly confided to troops other than those of the formed Divisions, and also to the Home Guard.
See TNA CAB 66/9/44
In many ways it is a very re-assuring assessment. It was in fact largely in accord with the assessments being made by the Germans themselves, many of whom, particularly in the Navy, thought that an invasion of Britain in 1940 was an impossibility.
Nevertheless this was a secret assessment. Preparations for a possible invasion were proceeding apace across the country. The mobilisation of tens of thousands of men into the Local Defence Volunteers, soon to be renamed the Home Guard, was transforming the outlook of the civilian population. Churchill was not going to undermine the general stiffening of resolve by going public with the notion that invasion was impracticable.
For more on the Home Guard see The Spitfire Site.