Dutch Defences and Defence Strategy

Dutch Defences and Defence Strategy

Many Dutch defensive works in 1940 have their origins in earlier centuries. Some are updated in the years immediately before the war. Others are hastily constructed on the eve of the German invasion.

The concept of using water as a defensive weapon dates to the seventeenth-century Dutch Republic. Prins Maurits van Nassau conceives the first Hollandse Waterlinie in the 1620’s and 1670’s to protect the province of Holland through the deliberate flooding of polders. The system proves its worth in 1672 when French armies are halted by the inundated zone around the core provinces. In the nineteenth century, Koning Willem I expands and modernises the water line in the aftermath of the Napoleonic Wars. Between 1815 and 1870, the Nieuwe Hollandse Waterlinie is created, extending the inundation zone to cover Utrecht and incorporating modern fortresses at strategic points. Dozens of masonry forts and fortified towns, among them Naarden, Gorinchem, and Muiden, are integral to the line. They are armed with cannon to cover dry approaches and control sluice gates. Between 1880 and 1914, the Stelling van Amsterdam is constructed as a 135-kilometre ring of 45 concrete forts positioned 10 to 15 kilometres around Amsterdam. By the First World War, the Stelling van Amsterdam and the Nieuwe Hollandse Waterlinie together form the nation’s fallback redoubt. Both are partially mobilised in 1914, though neither is tested in combat. These older fortifications still exist in 1940. Many forts are re-garrisoned and receive minor improvements such as added barbed wire or sand berms. Their nineteenth-century armaments and overall design, however, remain largely unchanged and are increasingly obsolete against modern artillery and aircraft.

After 1920, much of the Dutch fortification network is allowed to deteriorate. In 1926, the government disbands or deactivates many fortress units, assuming that static defences are no longer relevant to modern warfare. Key positions such as the Grebbелinie receive minimal maintenance until the growing threat of war in the late 1930s forces a rapid programme of reconstruction. From approximately 1937 to 1939, the Netherlands undertakes a crash programme to renovate existing defences and construct new ones. The Grebbелinie is re-fortified with new trenches, barbed wire, and concrete pillboxes only in the final years before 1940. Even so, many planned works are incomplete when the invasion begins in May 1940. Several pillboxes on the Grebbелinie are still unarmed. A vital pumping station needed to flood the Grebbe plain is not finished in time. Across the country, soldiers spend the months of the Phoney War digging fieldworks and pouring concrete. This comes at the expense of training. Dutch troops in 1940 frequently have more experience handling a shovel than conducting live-fire exercises.

With war imminent, the Netherlands also constructs several entirely new defensive lines. The Peel-Raamstelling is one such project, begun in early 1939 to cover the vulnerable Peel marsh region in Noord-Brabant. Within approximately one year, hundreds of small kazematten are constructed along canals and railway lines. The fortifications at Kornwerderzand on the Afsluitdijk are also recently built, constructed between 1932 and 1936 immediately after the completion of the causeway itself. These bunkers represent the best Dutch construction of the period. They are built of thick reinforced concrete, camouflaged, and fitted with steel doors and firing ports. Smaller defensive lines in Zeeland and across the northern provinces are essentially improvised between 1939 and 1940 using whatever resources are available, including earth trenches, flooded fields, and a small number of concrete shelters. By 1940, the Netherlands fields a patchwork of fortifications ranging from centuries-old brick ramparts to brand-new concrete casemates, all pressed into service in the face of the German onslaught.

Order of Battle

By May 1940, the Netherlands has fully mobilised its armed forces, though their size and equipment remain limited compared to the major powers.

The Koninklijk Nederlandsch-Indisch Leger fields approximately 280,000 soldiers, comprising regular troops and mobilised reservists. They are organised into four Legerkorpsen with a total of nine infantry divisions, eight regular and one light formation. Each division numbers approximately 10,000 to 12,000 men and lacks motor transport. The single Lichte Divisie is a semi-motorised formation intended as a mobile reserve, relying primarily on bicycles and a small number of armoured cars. In addition, independent brigades, Brigade A, Brigade B, and Brigade G, cover specific sectors, and approximately 24 grensbataljons are deployed as forward outposts along the frontiers.

The Luchtvaartafdeling, the army aviation branch, is small and technologically behind its opponents. The Netherlands does not have an independent air force in 1940. On May 10^th, 1940, the LVA has 155 aircraft in its inventory, of which 125 are operational combat types. The fleet includes 28 Fokker G.I twin-engine heavy fighters, approximately 36 single-engine fighters consisting primarily of Fokker D.XXI monoplane fighters and a small number of older Fokker D.XVII biplane fighters, 11 Fokker T.V twin-engine bombers, approximately 35 Fokker C.V and C.X light reconnaissance bombers, and 12 American-built Douglas 8A-3N light attack aircraft delivered just before the outbreak of war. More than half of the Dutch aircraft are biplanes or outdated models. Only 74 are of modern design. The Luchtvaartafdeling is organised into squadrons based at airfields around Den Haag at Ypenburg, Amsterdam at Schiphol, Rotterdam at Waalhaven, and at Soesterberg near Utrecht.

The Koninklijke Marine in 1940 is primarily oriented towards the defence of the Nederlands-Indië, and the bulk of its modern fleet is stationed overseas. In European waters, only a small number of significant warships are available. The newly completed Hare Majesteits Tromp is dispatched to Britain before the invasion. In home waters, the old kanonneerboot Hare Majesteits Johan Maurits van Nassau, armed with 15-centimetre guns, several older torpedoboten, minelayers, and some older O-klasse onderzeeboten are present in Dutch harbours.

Major Defensive Lines and Fortification Systems

To protect Vesting Holland and delay any invasion, the Netherlands establishes a network of defensive lines across the country.

The Grebbелinie is a forward inland defensive line running from the high ground of the Grebbeberg near Rhenen northward to the IJsselmeer. It shields the approaches to the Randstad, the western heartland, via the central Gelderse Vallei. The line is originally constructed in 1745 and is used against the French in 1794. After decades of neglect, it is revived and modernised in the late 1930’s. Its defensive concept is based on the inundation of lowlands, though flooding capabilities are limited by 1940 as a key pumping station remains unfinished. In early 1940, Generaal Winkelman designates the Grebbеlinie the hoofdverdedigingslinie, the main line of resistance, preferring to fight further forward rather than directly on the approaches to the major cities. The Grebbeberg sector is fortified with multiple trench lines and dozens of concrete pillboxes and casemates. Many, however, are poorly sited. Uncleared trees and buildings restrict fields of fire, and some of the weapons installed date to the First World War.

The Peel-Raamstelling is a line constructed in the southeast between 1939 and 1940, positioned just inside the Dutch border in Noord-Brabant. It runs from near the Maas River at Grave southward through the marshy Peel region, using the Peel moors and canals as natural obstacles, to the Belgian border near Weert. In the relatively open northern sector, an artificial defensive canal, the Defensiekanaal, is dug and integrated into the line. The Peel-Raamstelling consists of approximately 300 small bunkers and casemates spaced roughly 200 metres apart, interlinked with barbed wire obstacles. At Mill, a railway crossing is equipped with a vertical rail barricade, a Spargel obstacle, which derails an attacking German train on May 10, 1940. The line is thinly manned, as supporting field battalions are largely redeployed westward just before the invasion. Communications between isolated bunker outposts are poor.

The final redoubt of the Dutch defensive system is the Nieuwe Hollandse Waterlinie, an inundation-based defensive ring encircling the western provinces of Holland and Utrecht. The water line dates to the seventeenth and nineteenth centuries. It is a system of dykes, canals, and sluices capable of flooding a broad belt of polders to create a shallow lake barrier around the core of the country. By flooding low-lying land to a depth of approximately 30 to 50 centimetres, the Dutch create a zone too waterlogged for infantry or vehicles but too shallow for boats. Hidden underwater obstacles including ditches, stakes, and barbed wire, together with fortified towns and forts at strategic points, supplement this natural barrier. The Nieuwe Hollandse Waterlinie, upgraded in the nineteenth century to protect Utrecht in addition to the province of Holland, and the older ring of forts around Amsterdam, the Stelling van Amsterdam built between 1880 and 1914, together form the final Vesting Holland perimeter. These western lines include 45 older concrete forts near Amsterdam and numerous military sluiceworks and inundation basins. During the mobilisation of 1939 and 1940, the waterlinie is activated, sluices are prepared, and land is readied for flooding. The Stelling van Amsterdam is manned.

The Dutch also maintain several secondary and regional defensive lines. Along the eastern frontier, the IJssellinie on the IJssel River and the Maaslinie on the Maas serve as tripwire delay positions intended to slow the initial German advance for a matter of hours. These river lines are only lightly fortified, consisting mainly of improvised barricades and border guard units. In the far northeast, a short line known as the Wons-Stelling guards the approaches to the Afsluitdijk and Friesland. It is an improvised field position that collapses almost immediately under attack. In the southwest, the province of Zeeland, separated from the mainland by wide estuaries, is defended by narrow stop-lines across its causeways. The Bath Line and the Zanddijk Line cross Walcheren and Zuid-Beveland, combining small inundation works and trench lines to block any advance from the east.

Guarding the northern gateway into western Holland is the fortified Afsluitdijk causeway. At the Kornwerderzand sluices, the Dutch construct a strongpoint of seventeen concrete casemates with overlapping fields of fire across the dyke, supplemented by anti-tank obstacles.

The Dutch defensive system in 1940 combines traditional fortification methods, forts, walls, and dykes, with modern fieldworks including bunkers, wire, and mines, supplemented by strategic flooding.

Thousands of small concrete kazematten and pillboxes form the backbone of the 1939 and 1940 Dutch defences. These range from machine-gun emplacements to larger casemates designed for anti-tank or field guns. The Grebbелinie alone contains approximately 380 pillboxes, often arranged in clusters with interlocking fields of fire. Standard designs, including the Dutch S-kazemat, feature walls of reinforced concrete three metres thick and are typically camouflaged as farm buildings or concealed behind dykes. Design flaws, however, are widespread. Many bunkers lack all-round defence and can be approached unseen from the flanks or rear. In a number of cases, civilian buildings and trees are left standing in front of bunker positions due to property considerations, limiting fields of fire and providing cover to attackers. Communication between bunkers in forward zones such as the Peel-Raamstelling is poor. Despite these shortcomings, the kazematten offer determined resistance in battle. At Kornwerderzand on the Afsluitdijk, casemates shelter numerous machine guns and several 5-centimetre anti-tank guns.

A unique feature of Dutch defence is the deliberate use of flooding. The Nieuwe Hollandse Waterlinie and related systems consist of low-lying polders that can be converted into shallow lakes by opening sluices and inundation canals. From 1939 onwards, Dutch engineers prepare detailed flooding plans. They construct or ready sluice gates, pumps, dykes, and culverts to channel water from rivers including the Lek and the Vecht onto surrounding land. The target water depth is approximately 30 to 60 centimetres, sufficient to stop vehicles and infantry but too shallow for boats. Roads and railway causeways crossing the inundation areas are covered by old forts or concrete gun batteries. The kringenwet zones, established clear fields of fire along the dykes, allow trees to be felled as abatis and prohibit the construction of buildings within a set distance of the fortifications. In practice, not all obstructions are cleared in time by May 1940. The flooding defences are partially activated when the invasion begins. Fields around Muiden, Utrecht, and parts of Zuid-Holland are inundated. The lock complexes on the Afsluitdijk are crucial to this system. They control the IJsselmeer water levels used to flood the Grebbелinie area and the Waterlinie and are heavily guarded to prevent capture by German forces.

The Netherlands also incorporates a number of old fortified cities and forts into the 1940 defensive scheme. Towns such as Naarden, with its star-shaped moat and walls, and Muiden sit on the Waterlinie and are retained as strongpoints against infantry. Around Amsterdam, the ring forts of the Stelling van Amsterdam, including Fort Pampus and Fort Spijkerboor, are constructed between the 1890’s and 1900’s. They are armed mainly with antiquated guns of 6-centimetre and 8-centimetre calibre and serve as command posts, magazines, and anti-aircraft positions. In the south, older forts on the Nieuwe Hollandse Waterweg, including Fort Hoek van Holland, and the installations at Den Helder guard the naval ports. These coastal forts possess concrete gun emplacements for medium-calibre naval guns of approximately 15-centimetre. All such installations are mobilised between 1939 and 1940. They are fitted with searchlights, machine guns, and some modern anti-aircraft weapons. Although none faces a direct ground assault in May 1940, they contribute to air defence and local security. The fortress artillery at Den Helder deters any seaborne raid, and several forts near Den Haag assist in repelling German paratroopers.

Anticipating German armour, the Dutch supplement natural barriers with man-made anti-tank obstacles. At key road and dyke chokepoints, removable steel beams known as asperges and concrete tetrahedrons are installed to block vehicles. Bridges and tunnels are wired for demolition. Minefields are laid in selected areas, though a shortage of mines limits their extent. Portions of the Peel marshes and the approaches to the Grebbелinie contain small anti-personnel and anti-tank minefields, often signposted to channel the enemy into prepared kill zones. Barbed wire entanglements are used extensively in front of trench lines, around bunker perimeters, and to seal off gaps in the flooded areas. Combined with the inundations, these passive obstacles are designed to disrupt and delay mechanised units, forcing them into narrow corridors where Dutch weapons can be concentrated.

The Dutch also prepare infrastructure for air defence against Luftwaffe bombers and airborne troops. In late 1939, anti-aircraft gun positions are established around the major cities of Den Haag, Rotterdam, and Amsterdam, and near likely invasion targets such as airfields. The available weapons include a limited number of 20-millimetre Oerlikon and 40-millimetre Bofors guns alongside older 75-millimetre pieces, deployed to cover strategic sites. The Luchtwachtdienst, a volunteer air watch corps, is established to spot low-flying aircraft. Posts equipped with telephones and optics are manned by volunteers to provide early warning of parachute drops or airborne landings. At key government centres in Den Haag and at air bases, additional infantry detachments are held on alert to respond rapidly to airborne attack.