During March 1917, the Admiralty revised the time it would take the Grand Fleet to intervene in any enemy landing operations from 24-28 hours after hostile transports had been sited from the shore to 32-38 hours after hostile transports had been sighted from the shore. The relationship between the submarine campaign and invasion was the reason for this more pessimistic view. The destroyer flotillas were required for two separate and distinct purposes – providing escorts in the Channel, North Sea and the open sea to the westward, and falling with decisive strength upon the enemy transports. Escort duties obviously reduced the available force for coastal defence.
Although the general assessment was that Germany still possessed sufficient troops and transport to land a force of 160,000 troops, it was thought highly unlikely that Germany would undertake such a venture due to the very high risks involved.
The C.I.C Home Forces noted that the defences were now much better than a year ago and in particular the number of machine guns available was much greater. On the other hand he estimated he was short of 60,000 mobile troops of his estimated requirements to meet a force of 160,000. Also the quality of the troops available was generally poor.
Lord Fisher who had previously been convinced invasion was impractical now believed it was feasible. This was for a number of reasons:
The increased number of German submarines and their improved sea-keeping qualities
Gallipoli had demonstrated it was possible to make a landing under fire and maintain the expedition on beaches which were under continuous fire.
The German fleet was effectively under the control of Hindenburg / Ludendorff, who may order the High Seas Fleet to take risks which no naval officer would contemplate.
The risks to the Grand Fleet of mines etc.
The German army had recently increased in size, which rendered the required number of troops available.
That Germany had ample ships, with 45% of the mercantile fleet being at Hamburg and Bremen, as well as suitable small craft based at Emden, suitable for disembarkation.
Lord Fisher also considered that it would be harder to interrupt the enemy’s lines of communication as under modern conditions it would be impossible to maintain surface craft at particular localities to achieve this.
Despite Lord Fisher’s concerns, the general risk of invasion was not revised or preparations to meet it stepped up.
The next major conference (consisting of representatives of the Admiralty, War Office and HQ of Home Forces) held to consider the possibilities of an attack on the United Kingdom was held in December 1917. Since the last conference, there were however a number of factors which reduced the risk of such an attack:
The entry of the United States into the war, which increased the number of ships available for defence and the output of mines
The extension of submarine patrols which would increase the chance of gaining warning of such an attack
The extension of mining in the Heligoland Bight increased the difficulties of assembling and moving convoys without prior mine sweeping, which would be unlikely to go unobserved.
The increase and improvement in the air resource improved the possibilities for over sea reconnaissance and also would ensure overwhelming air superiority against any air force that the enemy could maintain on the British coast. The offensive power of the air force had also increased, with greater opportunity for attacking enemy transports and troops attempting to disembark.
When considering the risk of invasion, the following assumptions were made, based on practical experience gained during the war and on intelligence of current shipping and port facilities for embarkation available to the enemy.
Maximum convoy size:
This was stated to be 32 ships, based on experience. The likely convoy cruising formation would be four columns in line abreast, each column consisting of eight ships.
German Port facilities:
Ample, would not be a limiting factor to the size of any invasion force.
Shipping available to the enemy as at October 1917:
Vessels capable of nine knots and above – 160, aggregating to 800,000 tons.
Three vessels capable of 8 ½ knots speed, aggregating 6,000 tons.
Many vessels below 100 tons.
This was considered sufficient to transport an invasion force of 160,000 men with a strictly limited supply of artillery, ammunition and transport. This would entail five convoys of 32 ships each. Such an operation would be difficult, and unless previously practised, subject to much confusion and many accidents while forming up.
Given that a daylight landing would have to be made, the convoy would have to form up the day before and anchor over night, ready to weight and proceed. Since they could not form up in the dark they would have to leave the embarkation port two days before the invasion, and anchor overnight in cruising formation. The convoy would also be forming up in the British notified mined area, necessitating a very wide swept channel through.
It was again noted that unless the invasion was successful, it would lead to the loss of the entire invasion force. If the invasion force could capture and hold London, this may be enough to end the war. An invasion on the industrial Midlands could be very serious but would unlikely force Britain to make peace.
If sufficient reserves of troops existed in France, it was recognised these could be bought back across the Channel and concentrated around London within a week, providing cross Channel shipping was not interrupted. Germany may, as part of her plans, attempt to disrupt the cross-Channel service. Such disruption would last for about 36 hours before the Grand Fleet intervened. This delay would not be material as this time would be taken up with concentrating the troops at the Channel embarkation ports.
The question of potential invasion beaches was again reviewed. The following areas were ruled out for various reasons:
Beaches to the West of the Straits of Dover: Unlikely as the invasion convoy would have to form up well down into the Narrow Seas the day before, which would give ample warning.
Isle of Thanet: Exists difficult and could easily be held by a small force with machine guns against any force not well equipped with artillery.
Sheppey: Anchorages covered by the guns of Sheerness.
Southend: Covered by the guns of Sheerness.
Foulness: Impracticable for landings.
Latchington Peninsula: Impracticable for landings – area consisted of mudflats, dykes and marshes with the exit from the peninsula a bottle neck of two miles across with un-bridged tidal rivers on either side, flanked by mud flats at low tide.
Mouth of Blackwater River: Impracticable for landings.
Harwich area: Parts of coast impracticable for landings and also covered by the Harwich / Landguard guns.
River Ore to Aldeburgh: Orford beach was backed by the rivers Ore and Alde and the marshes combined to render the area impracticable for landings.
The following beaches were considered suitable for landings, but factors still rendered some unlikely for landings.
Deal to Sandwich: Impracticable for a large force owing to navigational difficulties and the proximity of the naval bases at Sheerness, Dover and Harwich.
Cliff End to Whitstable: As above.
Clacton – Naze: Impracticable for a large force owing to navigational difficulties and the proximity of the naval base at Harwich.
Aldeburgh – Southwold: Suitable for a large landing, but at risk of interference form the naval base at Harwich.
In summary the stretch of coast between Aldeburgh – Southwold was considered the most vulnerable stretch of coast for invasion. But even here any invasion attempt was considered a hazardous operation for the enemy as the beaches were strongly defended by barbed wire and machine guns. Experience at Gallipoli, where naval conditions were favourable, had shown the limited damage caused by ship gunfire on beach defences. Results may be improved with adequate aerial observation, but Britain would have absolute aerial superiority, denying such observation to the enemy. The enemy convoys would also be at risk from attack by submarines, light cruisers and destroyers. Although the escorts protecting the convoy would likely be much stronger than the Harwich force, it was envisaged that an attack could be made on at least part of the invasion fleet. Aircraft could also probably start bombing attacks long before the rear convoys had arrived and anchored (a total of 513 R.F.C. and R.N.A.S. aircraft would be available for bombing attacks, machine gunning enemy troops and transports and engaging enemy scout aircraft). A feint was considered unlikely as this would simply lead to the Grand Fleet intervening earlier.
The conference concluded by recognising what the enemy might hope to achieve by invasion, but against this the following factors the enemy would have to accept:
The certainty of failure if any news leaked out of the plans
The possibility of failure due to a change in the weather.
The possible loss of all transports
The possible loss of a greater part of the High Seas Fleet.
The loss of a large force of men without achieving anything.
Although the Military still considered it possible that the enemy could land 160,000 men on beaches with strictly limited transport and artillery, if unopposed, within 32-38 hours (i.e. the time it would take the Grand Fleet to intervene), the conference concluded in the light of above that only one convoy (maximum of 32 ships carrying 30,000 men) could succeed in landing before any action could be taken but it would be impossible for subsequent convoys to arrive without such action being taken as to ensure the complete failure of the invasion. The Board of the Admiralty did not concur with this conclusion and expressed the opinion that the enemy could reasonably transport a force of 70,000 men.
In light of the conclusion reached by the conference and the desperate need of men for France, the decision was taken to break up the 72nd and 73rd Divisions by withdrawing all Home Service Battalions and transferring the Graduated Battalions to the 68th and 69th Divisions to relieve their Home Service Battalions. At the start of 1917, Home Forces had been reduced from ten divisions to eight with subsequent reductions following, giving an authorised strength of 400, 979 as against the original 500,000.