In January 1916 a joint Admiralty-War Office conference again looked at the issue of Home Defence. A number of factors gave rise to doubts as to the estimate of a maximum landing force of 70,000 men. Part of the Reserve Fleet had been sent to the Dardanelles and some ships lost. It was thought that Germany may view invasion as a way out of the deadlock in France. The central conclusions of this conference were that:
Germany may attempt an invasion with up to 160,000 men, landing between the Wash and Straights of Dover.
The Navy would not be able to disrupt enemy operations in less than 24 - 28 hours after the transports were sighted from the shore
The reason underlying these assumptions was a desire not to underestimate Germany’s ability to transport an invasion force to England.
Area in which landings may take place
The area chosen by Germany to mount an invasion or raid would depend upon the distance from her ports, the importance of the objective and the disposition of Britain’s naval force. For an invasion, London was the assumed objective.
The coast north of Cromarty, the whole of the West Coast and the South Coast from Lands End to Selsey Bill was considered more or less immune from invasion but raids of not more than two cruisers or armed merchantmen capable of landing 500-600 men should be considered. A larger force up to 20,000 men could possibly be landed but its re-embarkation would be very improbable.
A landing on the Wash itself was considered unlikely. The coast from the Wash to Dover was considered to be open to attack from any size of force that Germany might be able to land.
Size of the Invasion Force
The limiting factor was the number of transports that could be provided. This was estimated to be one million tons of shipping by the Admiralty’s Director of Transports, Mr Graeme Thompson. The number of men that could be carried by this amount of shipping varied. Mr Thompson assumed that any force would bring motor transport and supplies and stores for a campaign of two months. Given this he assumed a force of not more than three army corps or approx 135,000 men. Others however assumed that Germany would not anticipate a campaign of two months and was also unlikely to bring over a large amount of motor transport. They assumed a force of approx 166,666 men, based on the tonnage per man for short voyages required as shown by recent experience (i.e. 6 tons per man). The enemy invasion fleet would consist of over 200 transports which would require a beach length of about 10 to 13 miles for landing. Several beaches could be utilised if they were sufficiently close to allow the force to join up after landing. As the enemy objective would be London the most likely landing place would be between the Wash and Straights of Dover.
Given the location of the Grand Fleet, it was assumed that a large force of transports could reach England’s shore before being detected. As an example, assuming a landing between Yarmouth and Lowestoft, the only ships close enough that could intervene immediately was the Harwich Force (five light cruisers and two flotillas of destroyers). However a portion of these could well be out on escort duties. As the German transports would be protected by a large covering force, it would be unlikely that the Harwich Force could intervene until dark when they could attack with their torpedoes. A force of nine older destroyers was also at the Humber, but these could also do nothing until after nightfall. Four submarines at Yarmouth could attack immediately. There were another 12 submarines at Harwich but they would take five hours to arrive. Another four submarines were stationed at the Humber but these would take nine hours to arrive.
The Grand Fleet could not be expected to intervene against a landing in the Yarmouth area until after 24 hours, based on the assumption of the Rosyth based ships steaming at 18 knots per hour. The cruisers would then attack the enemy’s main fleet, leaving the accompanying light cruisers and destroyers free to attack the transports, putting an end to all further landings. If the landings were in the Straights of Dover area, it would be 28 hours until ships of the Grand Fleet arrived.
Thus the size and composition of any enemy force would be determined by what it was possible to land in 24 to 28 hours before the Grand Fleet arrived. Recent experience of such landings was limited to Gallipoli where 29,000 men with seven days supplies was landed in 13 ½ hours under heavy enemy fire. In a peace time exercise at Clacton in 1904, 12,000 men, 2,500 horses, 55 guns and 320 vehicles were landed from 10 transports in 48 hours. Neither of these situations would accurately reflect conditions of any invasion on the East Coast. The cyclist battalions and mounted brigades would offer some limited opposition but nothing like that experienced at Gallipoli. It was assumed by the conference that Germany would be able to land in 24 to 28 hours the entire force that she could embark for such an enterprise although guns and transport would be strictly limited.
As has already been noted, the conference assumed the available transport would be the limit to the size of any force for invasion. But what was the actual size of any force that Germany could assemble for such an enterprise? The view of the Director of Operations, War Office, in a statement to the Imperial Defence Committee given on Nov 10th 1915 was accepted - given that Germany was not involved in any major operation of offence or defence, she could easily assemble a force of 10 divisions.
Above: Home Defence troops, Suffolk, March 1916
Throughout the summer of 1916, fresh troops were sent abroad especially to feed the Somme offensive. But the bogey of invasion could not be eradicated. The 62nd West Riding Division, although sent to Salisbury Plain for its final training before dispatch overseas, was retained in the UK to guard the East Coast, the reason given that it was the only fully trained and efficient division in the country and therefore had to be retained for Home Defence.
The question of invasion was looked at again in August 1916. Due to casualties incurred during the fighting on the Somme, fresh troops were desperately needed to replace wastage abroad. There was currently about 480,000 troops committed to Home Defence:
260,000 mobile troops (17 Mounted and Cyclist Brigades, 10 Infantry Divisions, 10 Provisional Brigades, 22 Cyclist Battalions, 10 Heavy Batteries and Lines of Communication and Signal troops)
220,000 sedentary troops (RGA and engineers employed in the defence of ports, AA defence, 10 Home defence Squadrons, 135 Draft-finding battalions and the Royal Defence Corps/Home Service Garrison battalions)
The draft finding units of the sedentary troops could not be reduced because of their role and hence the only option for reducing sedentary troops would be to replace the Royal Defence Corps and Garrison battalions with Volunteers as far as possible. This was considered but the idea rejected.
Of the mobile troops, one division was based in Ireland. Three out of the other nine were ready or nearly ready for dispatch abroad, but still remained available for Home Defence (including the 62nd Div, still stuck at home). By now, 14 out of the 17 Yeomanry Brigades had been converted to cyclists. Twelve of these Brigades and 11 Cyclist battalions were based in the Eastern Command area, to reinforce rapidly any threatened part of the coast. Five Yeomanry brigades and 12 Cyclist battalions were based in Northern and Scottish Command to repel any raids in these areas.
As Germany was now involved in two major defensive operations, on the Somme and on the Eastern Front, as well as committing troops to Verdun for further offensive operations, there was no prospect of her providing the necessary troops to carry out any invasion. As a result, it was decided that 24,000 general service infantry currently allotted to Home Defence could be sent overseas to replace losses. These men were to be replaced by Home Service recruits, who it was expected would be fully trained by the time the enemy was in position to contemplate invasion.
However if conditions should change in favour of Germany, the assumptions arrived at in January remained valid, namely that Germany could transport a force of 160,000 men and that the Navy could not intervene for 24 – 28 hours. Since January, the increase in the size of the Grand Fleet had enabled the Third Battle Squadron to be brought south to the Medway making any landing in the Thames Estuary much more difficult and added to the risk of any invasion force passing through the Straights of Dover. But if the enemy escorts were a superior force, it would still be 24 – 28 hours before the Main Fleet could intervene.