The Julius Caesar Plan

A camouflaged defensive position constructed in the north wall of Pevensey Castle, East Sussex, during the Second World War. One report states: ‘At the time of the construction of the defence works in the walls of Pevensey Castle, from late July 1940 through August and September, the infantry regiment at Pevensey had been the 4th Bn. Duke of Cornwall’s Light Infantry, and the commander of this battalion, Lt. Col. Harrowing, appears to have been responsible for the siting of the machine gun emplacements, and for organising the strengthening of various of the dungeons and towers of the medieval castle to serve as headquarters buildings. This work was carried out by 562nd Field Company Royal Engineers.’

27 October 1939

Air Power Considered to Have Made the Risk of Invasion Negligible

The advent of air-power had changed the prospects for an invasion of this country. It was believed that the preparation of an expeditionary force would be unable to escape the watch of our air-reconnaissance patrols, and that the expedition could be bombed and shelled to destruction before reaching these shores.

Coastal Command R.A.F. was responsible for reconnaissance over possible invasion ports on the Continent (17 of the 19 squadrons approved for Coastal Command were ready to operate at the outbreak of war); and Bomber Command had an adequate striking force to attack any concentration of shipping. Our naval supremacy in Home waters was guaranteed by the Naval Pact with Germany in 1935.

In the circumstances the Committee of Imperial Defence had approved that “so long as our Navy and Air Force are in being, a sea-borne invasion could be defeated without the help of land forces … and the danger of airborne attack on a large scale is negligible”. The land forces to be retained in the United Kingdom needed to be adequate only “to man the anti-aircraft ground defences and to maintain order and essential services in the event of major and sustained air attacks”.

In accordance with their assurances all the Regular divisions in the country were sent to France as soon as mobilised, to be followed by Territorial divisions as they became fit for service.

Not only was the Home Army reduced to a token force of semi-trained troops, but priority was given to the Field Force in France for trained officers and the full output of equipment, artillery and transport from production. For the same reason – the belief that preparations against invasion were unnecessary – coast defence had come last in priority in defence measures, and the 28 “defended ports” were far below the approved requirement in armament.

Civil Invasion Preparations Cancelled

As the War Office did not propose to make specific preparations to deal with large scale seaborne or airborne raids, or invasion, civil defence schemes to meet such a contingency were “unnecessary and, indeed, impracticable”.

Entries in the Government War Book for civil anti-invasion measures, such as the evacuation of the population from coastal areas, removal of supplies, etc. inserted purposely after the 1914/18 War to ensure that they were not overlooked, had been cancelled in 1937.

Risk of a Large Scale Raid Considered

During the first weeks of the war the activities of German submarines off the North and West coasts resulted in a reduction of our light naval force in the North Sea to provide escorts for trade protection. When the nights began to lengthen in October, the War Cabinet agreed that a convoy of German transports might slip through our naval and air patrols, and land an armed force on the coast. The Chiefs of Staff were accordingly asked to reconsider the risk of a large scale raid, and to take the necessary steps to meet it. Our naval and air forces could be rapidly strengthened sufficiently to intercept any reinforcements of troops and supplies; but even a local success, such as the destruction of a port or of some vital objective near the coast, might have a political and moral effect sufficient to tie up many more troops at home. The requirement therefore was to destroy the landing force as soon as possible before any serious damage could be done.

The Commander-in-Chief, Home Forces, was asked to “prepare immediate plans to meet an invasion on a large scale, based on a course of enemy action which had previously been ruled out an unlikely”. The outcome of that request was the “Julius Caesar” plan produced by G.H.Q. Home Forces on the 27th October

The maximum German force which might evade our sea control was estimated at a division, or 15,000 fully equipped troops, in twenty 4,000– 5,000 ton transports supported by 10,000 airborne troops in 1,000 civil aircraft.

Until the airborne troops had captured a port from the landward side, cleared opposition from the vicinity of the docks and anchorages, and from ground commanding the entrance to the port, it was considered to be “supremely dangerous for a seaborne landing to be attempted”; so that “if the initial air landing operation is a failure the operation as a whole cannot proceed and has definitely failed”. Consequently the defeat of the airborne force was the principal aim of the plan. With equipment limited to rifles and light machine guns, and a restricted ammunition supply, the airborne troops were expected to have little staying power unless quickly supported from the sea.

The most likely objective was an aerodrome, or landing grounds, near a port of considerable size, such as Harwich or the Humber, where a number of quays, wharves and cranes were available for rapid disembarkation; but defence precautions were taken at all ports between Peterhead and Newhaven where ships could come alongside, in particular Aberdeen, Dundee, Yarmouth, Lowestoft and Ramsgate.

The main defence was by fighter aircraft and anti-aircraft guns which would destroy the troop-carrying aircraft in the air; but parties of aircraft might evade the air defence, or might land before daybreak or in bad visibility. The ground defence was based chiefly on the location of mobile reserves within call; and the success of the plan would depend upon the ability of the local coastal forces to “pin down” the German airborne formations, and upon the time taken by the mobile reserves to reach the area of operations.

To give the earliest possible warning additional air reconnaissance and naval patrols were to be ordered to cover the German coastal and North Sea areas by day and on moon-light nights; and certain bomber squadrons were to be maintained in immediate readiness to bomb ship concentrations. With those precautions it was estimated that a minimum of eight hours’ notice could be given of any attempted large scale raid. The code-word “Julius”, denoting that an invasion was impending, brought the Home Defence forces to a state of readiness at eight hours’ notice; the code-word “Caesar” signified that an invasion was imminent.

It was expected that the landing force, seaborne and airborne, would be eliminated within seven days. That calculation provided the basis for the short-term period for the immobilisation of ports and the denial of facilities to the enemy. The civil population not in immediate danger were to be encouraged to remain in their homes; but the exodus of those persons in the danger zone was to be controlled and directed so that military two-way roads into the area of operations was kept clear of all civil traffic.

The C.-in-C. Home Forces put the minimum Army requirement for the plan at seven divisions: two for the Eastern and one each for the Northern and the Scottish Commands, and three in G.H.Q. Reserve. In Yorkshire, Lincolnshire and East Anglia armoured detachments were to be ready to move at once to break up the landing force before a port could be seized. The dispositions of Home Forces early in May 1940, when nine divisions were available for the plan.

The Julius Caesar plan may be regarded as an annex to the Record of Home Defence Measures; and together they formed the foundation of the plans for Home Defence during the first Winter of the War.