The Julius Caesar Plan assumed the following:
1. A force of 4,000 parachutists might attempt to seize an aerodrome or landing ground in order to land further troops.
2. A force of 1,000 civil aircraft capable of carrying 16 men (although only a trained air landing force of 4,000 men).
3. A division could be embarked in 20 transports of 4,000 to 5,000 tons and any crossing would take at least 20 hours (at 15 knots per hour).
4. The force would be escorted by 25 to 30 modern destroyers.
5. A landing on a beach in winter was not possible – the more likely approach would be to attempt to enter a port with transports that had already been seized by parachutists. The following were considered to be the most likely ports: Humber and Harwich with other possible landing places being Aberdeen, Dundee, Lowestoft and Ramsgate.
6. Any such invasion attempt would be accompanied by a heavy air attack against the fleet and air force and other strategic targets. Germany had an estimated 1,750 long range bombers; many more bombers could be used if aerodromes in Holland were used.
7. Any invasion attempt would be much facilitated by the previous capture of Holland.
The feasibility of any such operation would depend on the success of the parachutists in capturing a port. But parachutists would be lightly armed and would take time to assemble and move on a port. Also any such move would disclose the objective of the seaborne invasion which it was considered could not get under way until the parachutists were successful in seizing a port. Even if this was the case it was a complex operation to land a division – it was expected to take several days.
It was assumed that at least eight hours notice would be given on any invasion attempt by reconnaissance in the North Sea to detect concentrations of ships at port or convoys at sea. Troops prepared for active service were to be stationed along the Scottish and English East Coasts. Portions of these troops were to be made mobile by the provision of transport. Those in an advance state of training in Aldershot and Southern Command would be in GHQ reserve. Troops would be employed on guarding vulnerable points including aerodromes.
Any landing at a point protected by fixed defences (although in reality many ports were well below the allocated gun defences in 1939) could only succeed if the fire of these defences had been eliminated by shell fire or bombing – expected to be a lengthy affair. They may also be neutralized by an attack by troops from the flank or rear, landed by parachute or motor boats. Particular attention was to be given to the defence of the Humber and Harwich. Scottish Command was to provide 18 pounder gun protection for Aberdeen and Dundee and Eastern Command for Lowestoft and Ramsgate.
In the event of a landing two bomber squadrons would be made available for the support of land forces.
The main plan was as follows:
1. Inflict maximum damage on parachutists or airborne troops as they landed and collected.
2. If airborne forces became established to draw a cordon around them in order to immobilize them until counter attack troops could move up to eliminate them.
3. If enemy forces landed in Yorkshire, Lincolnshire or East Anglia armoured or horsed troops under the command of Northern or Eastern Command should move forward and attack the enemy while they were still endeavoring to disembarked and concentrate.
4. The larger formations in Scottish, Northern and Eastern Command would be provided with transport and concentrate in the area of operation. Transport would be hired as necessary.
5. If required the GHQ reserve could be deployed.
Home Forces available for the Julius Caesar plan, early May 1940
If an invasion was considered likely the code word “Julius” would be issued. All leave would be stopped and Regional Commissioners notified. If an invasion was imminent the code word “Caesar” would be issued – ammunition would be distributed, all troops warned for immediate action and harbour authorities warned to provide standby parties for the immobilization of cranes and other harbour facilities.
It was not considered feasible to halt the exodus of civilians from areas of active operations – any such movements should be controlled/diverted so that military two-way roads into the area were kept open. The troops earmarked to give aid to the Civil Authorities could be called upon if required. The population not in immediate danger should be encouraged to stay at home (for e.g. by radio broadcast). Telephone exchanges in operational areas were to be protected.
Defence Plans for the United Kingdom, TNA