The Chiefs of Staff assessment (18th Nov 1939)considered only a large scale raid or full scale invasion as the threat of small scale raids was considered to carry little threat. To mount a full scale invasion would require the Germans to gain control of the sea route for the duration of the invasion and neutralizing Britain’s air force.
Neutralizing the air force could involve:
1. Extended operations against aerodromes, front line aircraft and aircraft factories
2. A sudden and concentrated effort on front line fighter and bomber bases immediately prior to invasion.
Gaining control of the invasion sea route would involve a bombing campaign against British Naval forces although even if this had some success submarines and lighter vessels would still be available to attack any invasion fleet.
It was also assumed that Germany would violate the Low Countries neutrality either by flying over their air space but more likely by occupying them to gain forward invasion bases and allowing her to employ her short range dive-bombers.
It was considered that any invasion force would not be less than 20,000 conveyed in a fleet of 20-25 ships of approx 5,000 tons. A fleet of this size would be vulnerable to Britain’s light naval forces. The invasion may be preceded by an airborne attack by parachutists or air landing battalions with the objective of seizing a port for the use of disembarkation or a landing ground to land further troops. It was considered Germany had between 3,500 and 4,000 trained parachutists and 5,000 to 6,000 air landing troops. The aircraft used for both parachutists and transport was the Ju52 which had a low performance and was expected to suffer heavy losses from fighters if the landings were attempted during daylight. It was considered too difficult for the Germans to land a sufficient airborne force during darkness and the whole operation was considered to be a “desperate mission which the participants are unlikely to survive”.
A night passage of the invasion fleet was expected to allow disembarkation at dawn. The most likely beaches selected for invasion would be those between Yarmouth and Harwich. Although suitable beaches were available between Flamborough Head and Spurn and just south of the Humber, it was considered that these would be less likely targets due to the strength of the Humber defences and the many shoals and sandbanks which would make navigation of the invasion fleet at night very difficult.
Although there was the risk of the first flight slipping through on dark nights or in conditions of low visibility the subsequent maintenance flights could be harried by air and naval attacks.
German air support could include up to 250 long range fighters (ME110’s) if she violated Dutch air space and with the use of Dutch bases short range fighters could also be provided for support. These fighters would not only have to protect the invasion beaches but also the transports approaching and leaving the beaches. It was expected that heavy losses could be inflicted on the enemy air force if Britain concentrated her fighter forces on East coast stations in event of any invasion.
It was considered that German troops may gain some short term success but that maintaining the invasion force would be untenable due to harassment from air and sea. The chiefs of Staff concluded no reason to change the deployment of the Expeditionary Force as long as the remaining Home Forces could rapidly concentrate and deploy along the East Coast.
Defence Plans of the United Kingdom, TNA