The knock out air blow did not materialize. The German air striking force had not been assembled to deliver a knock out blow by a bombing campaign but rather to clear the way for the rapid advance of the army through enemy territory. The German occupation of Norway and Denmark in April 1940 and Holland in May resulted in a revision of the Chiefs of Staff assumptions. They admitted that their assessment of the small risk faced by invasion either airborne or seaborne was no longer valid. The East coast mine barrier (laid off Scotland to the Humber to prevent invasion from German ports only) was extended southwards to include the Thames, and all potential landing grounds within five miles of ports and possible landing beaches and airfields up to 20 miles from the East coast were to be obstructed.
The forces available to deal with parachutists were also considered inadequate and on May 13th the War Cabinet authorized the formation of the Local Defence Volunteers to ensure that parachutists could be dealt with immediately. In view of the fear of Fifth Columnists, all enemy aliens were interned. Civil Defence measures were also heightened with voluntary evacuation from East and South coast towns encouraged with the rest of the population expected to stay put. Measures were also put in place to deny to the enemy the use of petroleum, food and commodities.
Despite these measures, invasion was not considered imminent and on May 11th the 1st Armoured Division was ordered to France at the request of the C.I.C. British Expeditionary Force. Embarked with the Division was the entire force of medium and cruiser tanks leaving only 168 light tanks armed with only machine guns in Britain.
On May 14th the main German attack commenced on Belgium and France and by the night of May 27th evacuation of the remaining Expeditionary Force along with large numbers of the French Army was in full swing. On May 28th C.I.C Fighter Command told the War Cabinet that the fighter force had been “reduced almost to cracking point” due to its commitment to protect Allied Forces on the Continent. On May 29th the Chiefs of Staff warned the War Cabinet that it was highly probable the Germans were planning a full-scale attack on Britain. Churchill agreed and on Jun 12th declared “we must now concentrate everything on the defence of this island”.
The Chiefs of Staff did not so much fear an imminent invasion – this would take time to put in place, but rather a large scale raid, either airborne or seaborne or both, which if it established a footing might be followed by invasion. Such a raid could take the form of a seaborne force landed by a fleet of up to 200 fast motor boats carrying 100 men each launched from Dutch and Belgium ports. Such a force could easily slip through in the dark or thick weather. The boats being of shallow draught could run up on open beaches. A simultaneous landing of up to 20,000 parachutists could also be expected. It was also noted that the Germans had organized a force of ships at Vigo which may be significant given a reported gap in laid mines near the English coast. In either case it was also admitted that if the force established itself, Home Forces “had not got the offensive power to drive it out”.
Recent experience on the Continent resulted in a revision of the risk posed by parachute troops. The Dutch experience showed that parachute troops could not be dealt with rapidly by ground forces armed with rifle alone. It was also now considered possible that up to 500 parachute troops could be landed close to each of the vital South-eastern sector fighter headquarters (Tangmere, Kenley, Biggin Hill, Hornchurch, Northweald, Debden and Duxford) – such a force would be capable of destroying aircraft, hangers, control rooms etc and paving the way for sustained bomber attacks on these airfields and gaining air superiority over London and South-east England. As a result aerodrome defences were increased by arming RAF personnel with rifles and mounting Bren guns in mobile blockhouses – i.e. ‘Bison’ lorries. The Air Staff had wanted the Army to guard aerodromes but these were maintained in their original role of guarding key points and in forming mobile counter attack reserves.
The seaborne threat was also revised – C.I.C Home Forces stated on May 31st “We are at the present time very ill-prepared to meet a German offensive which may have an initial strength of 20,000 air-borne and 20,000 seaborne troops who will be relevantly well trained. A large part of the Army at Home is as yet insufficiently trained and equipped with artillery and armoured fighting vehicles to take the offensive, and we must act on the defensive in prepared positions”.
Above: The 'Hostile Coast' facing Britain after the Battle of France
Due to the priority in men and equipment going to the Expeditionary Force, the Home Force Divisions were under strength in manpower, artillery and machine guns. Mobility relied upon civilian motor coaches which would take eight to 24 hours to collect. The successful evacuation of 350,000 troops from Dunkirk (out of 400,000 sent over) greatly eased the manpower crises but not the equipment crises as virtually all the modern tanks, artillery, machine guns and transport had either been destroyed or abandoned in France.
This state of affairs required a revision of the Julius Caesar Plan, which had assumed a landing in the vicinity of a port. As a consequence many miles of open beach were undefended. The C.I.C Home Forces had now to decide how to dispose his ill-equipped force to meet a landing on open beaches.
Defence Plans for the United Kingdom, TNA
Cabinet Minutes, TNA