British anti-invasion preparations of the Second World War entailed a large-scale division of military and civilian mobilisation in response to the threat of invasion (Operation Sea Lion) by German armed forces in 1940 and 1941. The British Army needed to recover from the defeat of the British Expeditionary Force in France, and 1.5 million men were enrolled as part-time soldiers in the Home Guard. The rapid construction of field fortifications transformed much of the United Kingdom, especially southern England, into a prepared battlefield. Sea Lion was never taken beyond the preliminary assembly of forces. Today, little remains of Britain's anti-invasion preparations, although reinforced concrete structures such as pillboxes and anti-tank cubes can still be commonly found, particularly in the coastal counties.
Political and military background On 1 September 1939, Germany invaded Poland; two days later, Britain and France declared war on Germany, launching the Second World War. Within three weeks, the Red Army of the Soviet Union invaded the eastern regions of Poland in fulfilment of the secret Molotov–Ribbentrop Pact with Germany. A British Expeditionary Force (BEF) was sent to the Franco-Belgian border, but Britain and France did not take any direct action in support of the Poles. By 1 October, Poland had been completely overrun. There was little fighting over the months that followed. In a period known as the Phoney War, soldiers on both sides trained for war and the French and British constructed and manned defences on the eastern borders of France.
However, the British War Cabinet became concerned about exaggerated intelligence reports, aided by German disinformation, of large airborne forces which could be launched against Britain. At the insistence of Winston Churchill, then the First Lord of the Admiralty, a request was made that the Commander-in-Chief, Home Forces, General Sir Walter Kirke, should prepare a plan to repel a large-scale invasion. Kirke presented his plan on 15 November 1939, known as "Plan Julius Caesar" or "Plan J-C" because of the code word "Julius" which would be used for a likely invasion and "Caesar" for an imminent invasion. Kirke, whose main responsibility was to reinforce the BEF in France, had very limited resources available, with six poorly trained and equipped Territorial Army divisions in England, two in Scotland and three more in reserve. With France still a powerful ally, Kirke believed that the eastern coasts of England and Scotland were the most vulnerable, with ports and airfields given priority.
On 9 April 1940, Germany invaded Denmark and Norway.War crimes trial judgement on the invasion of Norway, via the Avalon Project – accessed 14 January 2008 This operation preempted Britain's own plans to invade Norway. Denmark surrendered immediately, and, after a short-lived attempt by the British to make a stand in the northern part of the country, Norway also fell. The invasion of Norway was a combined forces operation in which the German war machine projected its power across the sea; this German success would come to be seen by the British as a dire portent. On 7 and 8 May 1940, the Norway Debate in the British House of Commons revealed intense dissatisfaction with, and some outright hostility toward, the government of Prime Minister Neville Chamberlain. Two days later Chamberlain resigned and was succeeded by Churchill.
On 10 May 1940, Germany invaded France. By that time, the BEF consisted of 10 infantry divisions in three corps, a tank brigade and a Royal Air Force detachment of around 500 aircraft. The BEF and the best French forces were pinned by the German attack into Belgium and the Netherlands, but were then outflanked by the main attack that came behind them through the Ardennes Forest by highly mobile Panzer divisions of the Wehrmacht, overrunning any defences that could be improvised in their path. In fierce fighting, most of the BEF were able to avoid being surrounded by withdrawing to a small area around the French port of Dunkirk. With the Germans now on the coast of France, it became evident that an urgent reassessment needed to be given to the possibility of having to resist an attempted invasion of Britain by German forces.
British armed forces
British Army
The evacuation of British and French forces (Operation Dynamo) began on 26 May with air cover provided by the Royal Air Force at great cost. Over the following ten days, 338,226 French and British soldiers were evacuated to Britain. Most of the personnel were brought back to Britain, but many of the army's vehicles, tanks, guns, ammunition and heavy equipment and the RAF's ground equipment and stores were left behind. Some soldiers even returned without their rifles. A further 215,000 were evacuated from ports south of the Channel in the more organised Operation Aerial during June.
In June 1940 the British Army had 22 infantry divisions and one armoured division. The infantry divisions were, on average, at half strength, and had only one-sixth of their normal artillery. Over 600 medium guns, both 18/25-pdr and 25-pounders, and 280 howitzers were available, with a further hundred 25-pounders manufactured in June. Over 300 4.5-inch howitzers900 were modified in 1940 aloneand some 60-pounder howitzers and their modified 4.5-inch version as well as antiquated examples of the 6-inch howitzer were recovered from reserve after the loss of current models in France. These were augmented with several hundred additional 75 mm M1917 guns and their ammunition from the US. Some sources also state the British army was lacking in transport (just over 2,000 Universal Carriers were available, rising to over 3,000 by the end of July). There was a desperate shortage of ammunition such that little could be spared for training.
Records show that the British possessed over 290 million rounds of .303 ammunition of various types on 7 June, rising to over 400 million in August. VII Corps was formed to control the Home Forces' general reserve, and included the 1st Armoured Division. In a reorganisation in July, the divisions with some degree of mobility were placed behind the "coastal crust" of defended beach areas from The Wash to Newhaven in Sussex. The General Headquarters Reserve was expanded to two corps of the most capable units. VII Corps was based at Headley Court in Surrey to the south of London and comprised the 1st Armoured Division and 1st Canadian Division with the 1st Army Tank Brigade. IV Corps was based at Latimer House to the north of London and comprised 2nd Armoured Division, the 42nd (East Lancashire) Infantry Division and the 43rd (Wessex) Infantry Division. VII Corps also included a brigade, which had been diverted to England when on its way to Egypt, from the 2nd New Zealand Expeditionary Force. Two infantry brigades and corps troops including artillery, engineers and medical personnel from the 6th Australian Division were also deployed to the country between June 1940 and January 1941 as part of the Second Australian Imperial Force in the United Kingdom.
The number of tanks in Britain increased rapidly between June and September 1940 (mid-September being the theoretical planned date for the launch of Operation Sea Lion) as follows:
DateLightCruiserInfantry+Tanks in Britain10 Jun 1940 292 0 741 Jul 1940 2651181194 Aug 1940 336 173 189To Egypt−52−52−5027 Aug 1940 295 138 18515 Sept 1940 306 154 224 These figures do not include training tanks or tanks under repair.
The light tanks were mostly the Mark VIB and the cruiser tanks were Mark I , Mark II and Mk IVs. The infantry tanks included 27 obsolete Matilda Is but the rest were almost all the capable Matilda II.Appendix 7 to 13 of David Newbold`s "British planning and preparations to resist an invasion on land Sept 1939 to Sept 1940" British Library EThOS ID 241932 The first Valentine infantry tanks were delivered in May 1940 for trials and 109 had been built by the end of September. In the aftermath of Dunkirk some tank regiments, such as the 4th/7th Royal Dragoon Guards, were expected to go into action as infantry armed with little more than rifles and light machine guns. In June 1940 the regiment received the Beaverette, an improvised armoured car developed by order of the Minister of Aircraft Production Lord Beaverbrook, and former holiday coaches for use as personnel carriers. It did not receive tanks until April 1941 and then it was the problematic Covenanter cruiser tank.
Churchill stated "in the last half of September we were able to bring into action on the south coast front sixteen divisions of high quality of which three were armoured divisions or their equivalent in brigades". It is significant that the British Government felt sufficiently confident in Britain's ability to repel an invasion (and in its tank production factories) that it sent 154 tanks (52 light, 52 cruiser and 50 infantry) to Egypt in mid-August. At this time, Britain's factories were almost matching German output in tanks, that by 1941, they surpassed.
Home Guard
On 14 May 1940, Secretary of State for War Anthony Eden announced the creation of the Local Defence Volunteers (LDV)later to become known as the Home Guard. Far more men volunteered than the government expected and by the end of June, there were nearly 1.5 million volunteers. There were plenty of personnel for the defence of the country, but there were no uniforms (a simple armband had to suffice) and equipment was in critically short supply. At first, the Home Guard was armed with guns in private ownership, knives or bayonets fastened to poles, Molotov cocktails and improvised flamethrowers.
By July 1940 the situation had improved radically as all volunteers received uniforms and a modicum of training. 500,000 modern M1917 Enfield Rifles, 25,000 M1918 Browning Automatic Rifles and millions of rounds of ammunition were bought from the reserve stock of the U.S. armed forces, and rushed by special trains directly to Home Guard units. New weapons were developed that could be produced cheaply without consuming materials that were needed to produce armaments for the regular units. An early example was the No. 76 Special Incendiary Grenade, a glass bottle filled with highly flammable material of which more than six million were made.
The sticky bomb was a glass flask filled with nitroglycerin and given an adhesive coating allowing it to be glued to a passing vehicle. In theory, it could be thrown, but in practice it would most likely need to be placedthumped against the target with sufficient force to stickrequiring courage and good fortune to be used effectively. An order for one million sticky bombs was placed in June 1940, but various problems delayed their distribution in large numbers until early 1941, and it is likely that fewer than 250,000 were produced.Anti-tank measures Sticky Bomb adoption and production – WO 185/1. The Catalogue, The National Archives
A measure of mobility was provided by bicycles, motorcycles, private vehicles and horses. A few units were equipped with armoured cars, some of which were of standard design, but many were improvised locally from commercially available vehicles by the attachment of steel plates. By 1941 the Home Guard had been issued with a series of "sub-artillery", a term used to describe hastily produced and unconventional anti-tank or infantry support weapons, including the Blacker Bombard (an anti-tank spigot mortar throwing a 20 lb 'bomb'), the Northover Projector (a black-powder mortar), and the Smith Gun (a small smoothbore artillery gun that could be towed by a private motorcar).
Royal Air Force