British anti-invasion preparations of the Second World War

Invasion of Britain (prospective)
	Part of World War II
	; A pillbox, typical of the hardend field defences constructed in preparation for an expected German invasion.
Date	1940–41
Standort	Vereinigtes Königreich
Result	Invasion never took place

The majority of the fortifications scattered around the landscape of Britain date from World War II. They were constructed in a period of a few months during 1940 from the time of the Dunkirk evacuations. At this time, the threat of invasion, which had for centuries been at most a theoretical contingency, became a serious possibility.

The majority of the anti-invasion preparations no longer exists. Ditches have been filled; wood and metal re-cycled; at the coast pillboxes have tumbled into the sea or sunk into the sands on which they were built; yet other features have succumbed to road improvements or other modern developments.

Early beginnings

As things went badly for the British expeditionary force in France, it became evident that some thought must be given to the possibility of having to resist an attempted invasion by German forces.

On 14 May 1940, Secretary of State for War, Anthony Eden, announced the creation of the Local Defence Volunteers (LDV) - later to become know as the Home Guard. The announcement was met with enthusiasm and far more men volunteered than the government expected; by the end of June there were nearly 1.5 million volunteers. Within days, the evacuation of the British forces from Dunkirk began, most of the personnel were brought back to Britain, but much of the army's vehicles, tanks, guns, amunition and heavy equipment were left behind in France. There were plenty of personnel for the defence of the country, but equipment was in critically short supply.

Lines and islands

A Home Defence Executive was formed under General Sir Edmund Ironside, Commander-in-Chief Home Forces to organise the defence of Britain. At first, defence arrangements were entirely static and focused on the coastline and, in a classic example of defence in depth, on a series of inland defence 'stop' lines. Of these defensive lines, the longest and most heavily fortified was GHQ Line. GHQ Line ran across southern England, wrapped around London and then ran north to Yorkshire. It was intended to protect the national capital and the industrial heartland of England. In all some 50 stop lines were constructed although some were never completed.

Military thinking shifted rapidly. Given the lack of equipment and properly trained men, Ironside had had little choice but to adopt a strategy of static warfare, but it was soon percieved that this would not be sufficient. Ironside has been critisised for having a siege metality, but this is entirely unfair as he always understood the limits of the stop lines and never expected them to hold out indefinitely.

However, some officiers were said to be becoming obsessed with concrete and a new focus was required. On 19th of July 1940, Ironside was replaced by General Brooke, his appointment coincided with more trained men and better equipment becoming available. Under Brook, new strategies and tactics were devised. More concentration was placed on defending the costal crust and inland, a hedgehog defence strategy of defended localities and anti-tank islands were established each having all round defence. Many of these anti-tank islands were established along the existing stop lines, especially at towns and villages where there as a Home Guard to provide personel. However, the stop lines were not entirely abandoned.

The primary purpose of the stop lines and the anti-tank islands that followed was to hold up the enemy, slowing his progress and restricting the route of an attack. It was always recognised that it was not possible to hold off a determined attack forever. The need to prevent tanks from breaking through was of key importance, consequently, the defences generally ran along pre-existing barriers to tanks such as rivers and canals; railway embankments and cuttings; thick woods, and other natural obstacles. Where possible, usually well drained land was allowed to flood making the ground too soft to support even tracked vehicles. Pre-existing obstacles were supplemented with anti-tank obstacles such as ditches, massive concrete blocks (generally cubic or cylindrical) and pyramid shaped concrete blocks known as dragons teeth.

Crossing points in the defence network -- bridges, tunnels and other week spots -- were called nodes or points of resistance. These were fortified with removable road blocks, barbed wire entanglements, and mines. These passive defences were overlooked by trench works, gun and mortar emplacements, and pillboxes. In places entire villages were fortified using barriers of scaffolding, sandbagged positions and loopholes in existing buildings.

Nodes were designated 'A', 'B' or 'C' depending upon how long they were expected to hold out. Home Guard troops were largely responsible for the defence of nodal points and other centres of resistance such as towns and defended viallage. Category 'A' nodal points and anti-tank islands usually had a garrison of regular troops.

Airfields and open areas

Airfields and other open areas were considered vulnerable to invasion from the air: a landing by paratroops, glider bourne troops or even powered aircaft which could land and take off again. Open areas with a straight length of 500 yards (457 m) or more within five miles (8 km) of the coast or an airfield were considered vulnerable. These were blocked by trenches or, more usually, by obstacles such concrete or wooden posts, sections of concrete pipe or even old cars.

Airfields themselves could be vulnerable, these were protected by trenchworks and pillboxes which face not outwards as might be expected, but inwards towards the runway. Many of these fortification were specified by the Air Ministry and the designs are unique to arifields.

Weapons

In 1940, weapons were in critially short supply and the British defence relied heavily on improvisation and ingenuity. The Home Guard arsenel inlcuded weapons that the regular Army no longer required and weapons that could be produced cheaply without consuming materials that were needed to produce armaments for the regular units. These included the Blacker Bombard anti-tank weapon, the Sticky bomb, the Northover Projector (a blackpowder powered mortar), the No 76 Special Incendiary Grenade (a glass bottle filled with highly inflammable material), the No. 73 Grenade (an anti-tank grenade resembling a Thermos flask), and the Smith Gun (a small artillery gun that could be towed by a private motorcar).

A Barrel Flame Trap was a site prepared with a barrel of petrol (gasoline) to be dischaged across a road and then ignited. A Flame Fourgasse was a barrel filled with a mixture of petrol, oil and rubber -- which in time becomes sticky -- to be fired by an explosive charge. At the coast, Petroleum Warfare Sites were prepared where a petrol based mixture could be piped onto the shore, and even into the water, and ignited.

Minefields were laid with both anti-tank and anti-personel mines. Bridges were prepared for demolition at short notice, often by placing explosives into prepared detonation chambers. The Canadian Pipe Mine (also called a McNaughton Tube) was a bored pipe packed with explosives -- once in place this could be use to instantly ruin a road or runway. A Depth Charge Crater was a site prepared with buried explosives that could be detonated to instantly form a deep crater as an anti-tank obstacle.

Barriers and obstacles

Barbed wire

Barbed wire usually took the form of either Wire entanglements in the form of three coils of concertina wire fixed by metal posts -- called Triple Danert Wire Fencing -- or a simple fence of straight wires supported on waist high posts.

Scaffold Fencing

Z1 fence (also recorded as obstacle 2.1) was a fence made of scaffolding poles and was deployed at low water along vulerable beaches. Essentially, it was a fence of scaffolding tubes 9 feet (3 m) high. It was intended to be an obstacle to tanks and was placed at low water so that they could not get a good run at it.

Aniti-tank Ditch

Thousands of miles of anti-tank ditches were dug; usually by mechanical excavators, but frequently by hand. They were typically 18 feet (5.5 m) wide and 11 feet (3.4 m) deep and could be either trapazoidal or triangular in section with the defended side being especially steep and reveted with whatever material was available. Because agricultural land was valuable, the ditches were quickly filled in after the war. Today, hardly anthing remains of these ditches, but at the time they must have been the most conspicuous of all the fortifications. A few remain, much humbled, as field drains or field boundaries whereas others can be seen only as crop marks.

Anti-tank Obstacles

The most common anti-tank obstacles were simple cubes of cylinders of reinforced concrete. The cubes generally came in two sizes: 5 feet (1.5 m) or 3.5 feet (1 m).

Large cylinders were made from a section of sewer pipe 3-4 feet (90-120 cm) in diameter filled with concrete typically to a height of 4 to 5 feet, frequently with a dome at the top. Smaller cylinders cast from concrete are also frequently found.

Pimples, popularly known as Dragons Teeth, were pyramids of concrete designed specifically to counter tanks which, attempting to pass them, would climb up exposing vulnerable parts of the vehicle and possibly slip down with the tracks between the points. They range is size from just 2 feet (60 cm) square at the base to 4 feet (120 cm) square with about 3 feet (90 cm) square being typical. There was also a conical form.

Cubes, cylinders and pimples were deployed in long rows, often several rows deep, to form anti-tank barriers at beaches and inland. They were also used in smaller numbers to block roads. They frequently sport loops at the top for the attachment barbed wire.

Anti-tank walls were also constructed -- essentially continuously abutted cubes.

There was also a tetrahedral or caltrop-shaped obstacle, although it seems these were rare.^[1]

Where natural anti-tank barriers needed to be augmented, concrete or wood posts would suffice.

Road and rail blocks

Roads and railways offered the enemy fast routes to their objectives -- tanks could move easily along railways. Many roadblocks were formed by General Ironside that were semi-permanent, in many cases, General Brooke had these removed altogether as experience had shown they could be as much of an impediment to friends as foes. Brooke favoured removable blocks.

The simplest of the removable roadblocks were massive concrete posts with holes and/or slots to accept horizontal railway lines or RSJs. These blocks would be placed strategically where it was difficult for a vehicle to go around -- anti-tank obstacles and mines being positioned as required. These removable roadblocks could be opened or closed within a matter of minutes.

Socket type roadblocks, also known as 'hedgehog' comprised railway lines or RSJs bent or welded at about a 60 degree angle. These could be placed in pre-prepared sockets placed in the road. When not in use, the sockets were covered by metal covers.

Demolition charges

Bridges and other key points were prepared for demolition at short notice by

Concertina wire.
Anti-tank cubes.
Anti-tank cylinder.
Roadblock at a bridge on the Basingstoke Canal.

Hardend field defences

In May of 1940, the directorate of Fortifications and Works (FW3) was setup at the War Office under the direction of Major-General G. B. O. Taylor. Its purpose was to provide a number of basic pillbox designs which could be constructed by soldiers and local labour at appropriate defensive locations. In the following June and July FW3 issued 7 basic designs for rifle and light machine gun, designated Type 22 to Type 28. In addition, there were designs for medium machine gun emplacements and gun emplacements suitable for either the Ordnance QF 2 pounder and Ordnance QF 6 pounder.

In addition to these designs, the Air Ministry provided designs of fortifications intended to protect airfields from troops landing or parachuting. These would not be expected to face heavy weapons so that the degree of protection was less and there was more emphasis on all-round visibility. Many of these were later reinforced.

There were further designs for pillbox like structures for various purposes including light anti-aircraft positions, observation posts and searchlight positions to illuminate the shoreline. Local commanders introduced their own designs. Unique to the north east of England, were the lozenge and eared pillbox designs. In southern England there are also octagonal pillboxes, the date of their introduction is unclear.

The basic designs were adapted to local circumstances and available building materials such that, outwardly, two pillboxes of the same basic design could look quite different. The height of a pillbox could vary significantly according to local needs, some were half buried so that the embrasures might be as low as ground level, others were raised up to give a better view; those built into hillsides might lack embrasures on some walls; embrasures were generally precast and factory produced to standard designs, but these were in short supply some embrasues were made from brick or concrete paving; the entrance could be moved and its size varied as might be convenient and might have additional walls for protection, a freestanding blast wall or a steel door.

Appearance also varied due to the building materials used, although all are actually formed from reinforced concrete. Where brick was used as a shuttering, the bricks essentially formed a mould into which concrete was poured - the bricks being left in place. Otherwise, the pillbox was formed using shuttering of wood (usually planks, but sometimes plywood) and/or corrugated iron. Wood shuttering was removed, whereas corrugated iron was sometimes left in place.

The degree of protection offered by a pillbox varied considerably. The thickness of the walls and roof varied from just 12 inches (30 cm) to 3.5 feet or more. Generally speaking, the thiner walled pillboxes were intended to be bullet proof whereas the thick walled pillboxes were intended to be shell proof.

Furthermore, some pillboxes were completely ad hoc designs suited to local conditions.

Construction often took advantage of whatever materials were available locally. For example, at the coast beach sand and pebbles would be used. Use of local materials had the added advantage of aiding camoflage. The reinforced concrete used in construction was generally conventional making use of thin steel rebars with floor, walls and roof all mutually bonded. However, at least one case is known where scrap metal had been used: the concrete was reinforced with parts of an old bed.

FW3 pillbox types

Type 22

The type 22 pillbox is a regular hexagon in plan with an embrasure in five of the sides and a entrance in the other. The embrasures are suitable for rifles or light machine guns. Some have a low entrance that allows an extra embrasure above. Each wall is about 6 feet (1.8 m) long and it was generally built to the bullet proof standard of 12 inches (30 cm) thick. Internally there is a Y shaped anti-richochet wall (the top of the Y nearest the entrance), the internal wall also helps support the roof. It is the most common pillbox design in Britain, but is easily confused with the common type 24 which is an irregular hexagon and the less common octagonal.

Type 23

The type 23 pillbox is rectangular in plan -- essentially two squares, one of which is roofed and the other open -- there are three embrasures, one in each of the available sides of the covered section. Suitable for rifles or light machine guns. The open section was for a light anti-aircraft defence: a Bren or Lewis gun on a mounitng. Usually, there is no gound level entrance, to get in one had to climb over the wall into the open section and then pass though a door to the covered section. The walls were 8 feet (2.4 m) wide by 16 feet (4.8 m) long and usually built to a bullet proof standard of 12 inches (30 cm) thick.

Type 24

The type 24 pillbox is an irregular hexagon in plan. The rear wall is the longest at about 14 feet (4.3 m), this has the entrance with a pair of embrasures either side. The other walls vary from 7-8 feet (2.2-2.5 m) each having a single embrasure suitable for rifles. Internally there is a Y shaped anti-richochet wall (the top of the Y nearest the entrance), the internal wall also helps support the roof. The type 24 was always built to at least bullet proof standard of 12 inches (30 cm) thick, but often was thicker. A thick walled variant was introduced to a shell-proof standard, it was larger externally and had walls 36-50 inches (91-127cms), thick. This thick-walled variant is, confusingly, sometimes called a Type 29 by pillbox reseachers, but this is not an official designation and should be avoided.

Type 25

The type 25 pillbox is the only FW3 design that is circular with a diameter of 8 feet (2.4 m). The walls were just 12 inches (30 cm) thick. There were three embrasures and a small entrance like a low windown. This design was made from reinforced concrete shuttered by corrugated iron, this gave the design the popular name Armco after the manufacturer of corrugated iron of that name.

Type 26

The type 26 is a simple square in plan, each wall being 10 feet (3 m) long. There is a door in one side and embrasures in each of the remaining three walls with, possibly, an addition embrasure next to the door. Occasionally, there are two embrasures in one of the walls. Walls are normally to bullet proof standard at about 18 inches (45 cm) thick.

The type 26 also had a important prefabricated variant, the shuttering was provided by precast concrete slabs slotted into reinforced concrete posts.

Type 27

The type 27 is the most varied of the FW3 designs, it may be a octagonal or hexagonal in plan with walls between 9 feet 9 inches (3 m) and 11 feet 7 inches (3.5 m). The outer walls being 3 feet (1 m) thick. Its defining characteristic is a central well open to the sky which could be used as a light anti-aircraft position.

Type 27 is often located on or near airfields.

Type 28

The type 28 is the largest of the FW3 designs and the only one with a specific anti-tank capability. It is almost square in plan with the forward facing corners chamfered. The walls are each about 20 feet by 19 feet (6.1 m by 5.8 m) long constructed to shell-proof specification at about 42 inches (110 cm) thick. There is a very large forward embrasure. It was designed to take a 2 pounder anti-tank gun or other artillery piece, the gun shield of which would lagely filled the appature. There are usually standard embrasures in each of the two side walls.

The rear entrance is very large and was designed for ease of wheeling in and out the gun.

The type 28a is an important and common variant. It is wider than the type 28 and has a side area for an infantry chamber giving a forward facing standard embrasure. This resolved the problem of the type 28 being vulnerable to a head-on infantry attack.

A further variant is the type 28a twin which has two main gun embrasures on adjacent walls giving two possible firing positions for the one main gun and together with two adjacent infantry sections.

The traverse of the gun was limited to about 60°. Generally, these pillboxes were positioned to fire along fixed lines, such as enfilading fire across an anti-tank ditch or at bridge and in such positions the limited traverse of the gun creates no real disadvantage whereas the small size of the embrasure provides greater protection for the gun and its crew.

Vickers MMG emplacement

The Vickers machine gun pillbox is essentially square in plan with the forward facing corners chamfered. The walls are 14 feet (4 m) long and there is geneally a freestanding blast wall covering the entrance on either the left or right side. The walls were constructed to shell proof standard of 3 feet (1 m). There is a large embrasure and inside is a concrete, trapezoidal table on which to mount the weapon's tripod. The other walls would each have a standard sized embrasure.

Vickers machine gun pillboxes are frequently sited in pairs and were often dug-in with overhead earth cover.

Other hardend defences

Lozenge pillbox

The lozenge pillbox is found only in the North East of England. Lozenge pillboxes are an irregular hexagon in plan having two opposite sides significantly longer than the others, this allows space for three forward facing embrasures. There was an exit at each end and more embrasures. It was designed for infantry armed with rifles and/or light machine guns.

Eared pillbox

The eared pillbox is, like the lozenge pillbox, found only in the North East of England. It has a similar irregular hexagon plan with two large embrasures on two of the short sides. There is a bulge at the base of the wall below the large embrasures that is thought to have accomodated the cooling system for the machine gun.

Octagonal pillbox

Dover Quad

The Dover Quad pillbox is a 13ft (4m) square pillbox with an overhanging roof slab and is only found in the Dover area of England.

Somerset Defence Post

The defence post is only found in Somerset, it is about 8 feet (2.4 m) square with walls about 15 inches (38cm) thick and have wide slits extending the full width of three faces. There is a porch covering the entrance. Some have an open section on top reached by rungs and a ladder.

Norcon pillbox

The Norcon was a small circular pillbox named after the company that manufactured it. It was made from a concrete pipe 6 feet diameter and 4 feet high with several cut loopholes, a roof made of timber and corrugated iron, and earth; extra protection was provided by the use of sandbags.

Ruck Machine gun post

A pillbox with a curving concrete shingled roof on a pre-cast conrete fame, build over a rectangular pit and covered with turf and sandbags

Alan Williams Turrett

Pillbox formed by a revolving metal turret set above a steel and brick-lined pit.

Tett Turret

Two-man pillbox with a revolving concrete turret set aboive a cylindrical concrete pit

Spigot mortar emplacement

A spigot mortar emplacement was unroofed, someitmes constructed of brick or concrete, but may be a simple reveted earthwork. Its defining characteristic is a central concrete pedestal with a stainless steel peg (rust free even after more than 60 years). The pedestal was for the a type of Spigot Mortar called the Blacker Bombard -- effective against both tanks at a range of about 100 yards (100 m) and personel at a range of about 500 yards (500 m).

Embrasured walls and buildings

Existing walls and building provided a ready-made alternative to a pillbox. Whatever may be lacking in protection was made up for by speed and convenience.

Image:2-pounder gun.jpg|British 2lb gun. Ordnance QF 2 pounder Ordnance QF 6 pounder

Would the defences have been effective?

The question of whether the defences would have been effective in the event of an invasion is a vexed one. On the one hand, the experiences of the FIrst World War made it clear that assaulting prepared defences with infantry was deadly difficult, but similar preparations in Belgium had been easily overrun by the well equiped German Panzer divisions in the early weeks of 1940. British forces were woefully ill-equiped to take on German armoured divisions. On the other hand, while British preparations for defence were ad hoc, so were the German invasion plans: a fleet of 2,000 or converted barges and other vessels had been hurriedly made available and their fittness for purpose was debatable; the ability of the German forces to land fully equiped divisions with all their heavy equipment in an opposed amphibious assault has to be doubted -- at the time nobody had ever tried it before. The experiences of American forces on Omaha Beach on D-day and taking on Japanesse defenders on Pacific islands clearly showed that under the right conditions, a defender could extract a terrible price from assaulting forces.

To even attempt a landing, local air superiority was essential, but even with that, the RAF would still have been able to operate over friendly territory while the Luftwaffa still had to fly a long distance to reach the English shoreline.

Most significantly of all, the British retained an ace in the form of the Royal Navy home fleet, a force that dwarfed anything the Kriegsmarine could put to sea. The British had 5 capital ships, 11 cruisers and 53 destroyers and many minesweepers and other smaller vessels, the German navy had just 1 capital ship, 1 cruiser and 10 destroyers. The home fleet was based far to the north at Scarpa Flow and, in the event of an invasion would have sailed to the channel -- possibly taking several days to get there. German plans called for the invasion fleet to be screened by blocking the channel with mines, U-boats and torpedo boats. While these naval forces and the Luftwaffe could doubtless have extracted a high price from the Royal Navy they could not have hoped to prevent interference with attempts to land a second wave of troops and supplies that would have been essential to German success -- even if, by then, the Germans had captured a port essential for bringing in signifcant heavy equipment. In this scenario, British land forces would have faced the Germans on more equal terms than they had in the battle of France and it was only necessary to delay the German advance, preventing a colapse until the German land forces were, at least temporarily, isolated by the Royal Navy and then mounting a counter attack. Scholarly consideration of the likely outcome of a German invasion, including wargames held at Sandhurst in 1974 generally agree that while German forces would have been able to land and gain a significant bridgehead, the intervention of the British navy would be decisive and, even with the most optomistic assumptions, the German army would not have gotten further than GHQ Line and would ultimatly have been defeated.

Of course, having failed to gain even local air superiority in the Battle of Britain, Operation Sealion was postponed indefinitely. However, it seems clear that Hitler and his generals were well aware of the problems of mounting an invasion. Hitler was not idiologically committed to a long war with Britain and many commentators have suggested that German invasion plans were a feint never intended to be put into action.

While Britain may have been militarily secure in 1940, both sides were aware of the possibility of a political collapse. If the the Germans had won the Battle of Britain, the Luftwaffe would have been able to strike at will anywhere in southern England and with the prospect of an invasion, the British government would have come under enormous pressure to come to terms: the extensive anti-invasion preparations demonstrated to Germany and to the people of Britain that whatever happened in the air, the United Kingdom was both able and willing to defend itself.

Neglect, rediscovery and reuse

The great majority of Britain's defences have been destroyed, a process that started even before the end of the war. After the war, farmers, across who's land structures had been built were, in additon to receiving compensation, were paid to fill in trenches and demolish pillboxes. In the case of pillboxes, the sum of £5 is sometimes mentioned to pay for demolition, but the challenge of demolishing such structures in considerable and it seems that most farmers pocketed the cash, treating it as compensation.

Today, it is very rare to find any part of Britain's defences other that that comprised of concrete. Immediately after the war, there were more pressing matters to attend to than conserving the detritus of a battle that never happened. For decades, with the sole exception of Pevensey Castle -- where the new fortifications were seen as a part of the buildings history -- there was never even a suggestion that anything should be deliberatley conserved.

As the years passed, errosion and modern construction destroyed many pillboxes; for those that remain neglect and the attentions of nature achieved a degree of camouflage that a soldier of the second world war could only envy. Years after the war, memories faded and in the public mind it became popular to assume that the few pillboxes and other concrete objects were all that was done to defend Britain; that their purpose was just to bolster morale and that there would have been no realistic hope of resisting a German assault. Whereas, in fact, what can easily be seen today are just the robust remains of what was a massive programme of fortification that was likely to have proven highly effective.

Extant war-time records are fairly poor, and nobody could be sure how many pillboxes and related hardend field defences had survived -- or indeed, how many had been constructed in the first place. In the late 1970s, journalist Henry Wills began research on the topic eventually leading to publication of Pillboxes: A Study of UK Defences in 1985. Interest was stimulated, both public and professional. Local surveys were carried out. Surveys culminated in the Defence of Britain Project which took place from 1995 to 2002 attempting to record all know military defence sites. From this and other surveys, it is estimated that some 28,000 pillboxes and other hardend field fortifications were constructed in the United Kingdom of which some 6,500 still survive.

External links

^ "The 'Caltrop' as Anti-Tank Obstacle". Retrieved 2006-03-04.

[1] "The 'Caltrop' as Anti-Tank Obstacle". Retrieved 2006-03-04.

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