by J E Kaufmann
The Ijssel Line and Maas Line were renovated between 1936 and 1938, when key points received heavy bunkers. However, most construction took place later when many smaller fortifications and obstacles were added to form a continuous line.
The Peel-Raam Line was built at the end of the decade to serve as a possible main line of defense in conjunction with the Grebbe Line, in the event the Belgians decided to secure its right flank.
The first 2.5 km section of the Afsluitdijk (Great Dike) was completed on the island of Wieringen in 1926 and the remaining 25 km were finished in 1932. Manmade islands were created to house the sluices, locks, and two new forts of this line.
Four phases of Dutch construction can be identified in the 1930s. During the first phase, the two new forts were built on the Great Dike in the early 1930s, and new casemates were placed on old forts in 1935. During the second, which lasted from 1936 to 1938, the large anti-tank and machine gun bunkers were built to protect the bridges over the Maas and Ijssel Rivers and other key crossing points. During the third, which lasted from 1939 to 1940, the majority of the bunkers for the Grebbe, Maas, Ijssel, and Peel-Raam Lines were begun and the new armored turrets were put in place. During the fourth and final phase, which took place in 1940, the East and South Fronts of Fortress Holland were improved.
Like the Belgians, the Dutch created a multitude of defensive lines that they could not complete in time for war, nor adequately man with their small army.
DESCRIPTION
1. The Grebbe Line and Fortress Holland
The Dutch army leaders had replaced the New Water Line as the main line of defense for Fortress Holland, with the Grebbe Line in 1940. The line included 144 of the new armored turret positions, which consisted of a fixed turret similar to a cloche but with a distinctive shape and weapons embrasure. The armor made of chromium nickel steel was 100-mm thick for the simplest turrets and up to 170-mm thick for the heaviest type. These turrets were meant to be placed on a small concrete structure and were designed in such a way that they could be placed at different firing angles, depending on the needs of the position. The turret included a gas proof mount for the embrasure and a gas proof door. The intended weapon was a heavy machine gun. A Belgian firm at Liege produced those turrets designed for 47mm and 50-mm fortress anti-tank guns, but most were not delivered in time for installation. The Grebbe Line also included 130 positions, mostly small three-embra sure machine gun bunkers, and only four anti-tank bunkers.
The Grebbe Line consisted of two defensive belts. The area in front of the first belt which could be inundated, varied from several hundred meters to a few kilometers in width. The Grebbe Berg, although only about 37 meters high, dominated the area, and the dense woodlands offered some protection for the troops. The hills averaged about 30 meters in height across most of the position. At one critical point in front of the Grebbe Berg, eighteenth century fortifications reinforced with new works protected the flood gate.
In the spring of 1940, Major William Colbern, American Military Attache to the Hague, who was taken on a tour of the Grebbe Line, described it as a main position protected by an inundation and an outpost line. According to him, the main position followed the west bank of the Een River from the Zuider Zee to Amersfoort, encircling that town on the east, following the west bank of the Luntersche Beek to the confluence of the Grebbe, and then running along the west bank of that river to the Rhine River. An inundation zone, only partially prepared for flooding, occupied the 5 km that separated the outpost line from the main position. Colbern also reported that the area north of Amersfoort could be flooded within six hours, but that the area to the southeast, which was slower to inundate, was already awash in about 45-cm to 60-cm of water. According to Colbern complete inundation was to be achieved in twelve hours at a depth of about 100-cm, covering anti-tank ditches and barbed wire obstacles. The outpost line was not a solid line, but a number of positions placed at intervals of up to 400 meters apart for early warning and mutual support. The defensive positions in the outpost line were not permanent, but instead used wood and sand platforms.
Some of Colbern's descriptions are not confirmed by post-war sources. In addition, a few of his comments obviously indicate that the Dutch had misled him. For instance, he claimed that the main position consisted of three lines about 500 to 800 meters apart, each made up of a continuous trench and of bunkers incorporated in a sand dike. The trenches, reported Colbern, were revetted with wooden planks and without overhead cover and the bunkers were placed about every 300 to 400 meters. Some of the few large bunkers sheltered seven men with a couple of heavy machine guns and an anti-tank gun. Some contained a hand operated air pump for the air filter to prevent infiltration of poison gas. Colbern claimed that the armored turrets were sunk into the dikes without concrete and questioned their stability under bombardment. He also noted that they were not only gas proof, but had bullet-proof glass covers. Most of the work appeared to have been completed at the time he visited the defenses, the positions were camouflaged with earth, bushes, and small trees, and the trenches were covered with green netting interwoven with burlap.
Colbern's evaluation of the entire position reveal some serious weaknesses not always mentioned in more recent sources. Most positions, he correctly concluded, appeared incapable of resisting direct hits from medium artillery. The Dutch reportedly had only four regiments of 105-mm guns that could fire beyond the outpost line from behind the main position since the distance was between 7 to 12 kilometers to that line.
The Grebbe Line was extended to cover the gaps between the Lek and the Waal Rivers and the Waal and Maas Rivers, to form a position called the Ocheten Spa and Putuijk Positions respectively by the Germans in 1940.4 These positions included some of the bunkers, which were included in the count for the Grebbe Line, and inundation mechanisms. An additional position, known as the Waal-Linge Line, extended back from the Grebbe Line to the New Water Line, using inundated areas and the river to close the gap on the exposed southern flank of the Grebbe Line. The small Linge River flowed through the sluices at Fort Asperen where its waters could be backed up to flood the area between the Waal and Lek. The remainder of Fortress Holland was heavily dependent upon its system of inundations and, to a smaller extent, on the numerous small bunkers and other fortifications that had been added to defend key positions. The special armored turret positions that had been installed on the Grebbe Line were placed here too, sometimes on old forts such as Vechten and Rijnauwen near Utrecht. In early 1940 field fortifications were still being prepared and some of the old fortresses such as Muiden, Naarden, Utrecht, Vianen and Gorinchem had been refurbished with the addition of new positions. Only the eastern part of Muiden was modernized between 1934 and 1940: three heavy machine gun casemates were added in 1934, a number of lighter machine gun casemates in 1934, and a number of lighter machine gun casemates with Belgian made cloches between 1939 and 1940.
Fort Kijkuit guarded the route created by the new state highway that ran through the swampy region of the New Water Line from Hilversum to Harlem via Vreeland. Fort Kijkuit and Fort Spion, which occupied positions on artificial islands among the marshes, were over a hundred years old and modernized in the 1880s. Fort Kijkuit received a large reinforced concrete casemate whose roof and some walls were up to 2.5 meters thick, and an armored shield mounting two light guns that covered the causeway. Similar concrete casemates were placed near the road, but only this one was built of reinforced concrete.
In the New Water Line some of the restored forts included Fort Rijnauwen and Fort Vechten. Rijnauwen built in 1876, had old armament that included ten 150mm cannons, six 120-mm cannons, and even older weapons. Special armored turrets for a machine gun were added before the war. Between the world wars reinforced concrete positions were added to cover the road and terrain in front of the fort.'
The newest fort in the Amsterdam Ring of 42 forts, was completed in 1912 with two 105-mm Krupp cannons in a turret. Old Fort Walmer included a casemate for 100-mm guns
. Fort Pampus, built in 1879 and situated on an artificial island outside of Amsterdam, mounted two turrets with 240-mm cannons, two positions with a pair of embrasures for old 57-mm QF guns, and an observation cloche. Other forts of the ring included Fort Hoofdorp, Fort Velsen, and Fort Spijkerboor completed in 1904, 1900 and 1911 respectively. Each mounted two disappearing 57-mm gun turrets and two flanking casemates for a pair of 105-mm guns. Some forts like Spijkerboor had more weapons, in this case a 105-mm gun turret, and Fort Velsen had a 150-mm gun turret. Batteries of two 150-mm howitzers and four 150-mm howitzers occupied positions behind the forts. Many of the Amsterdam forts had a long, thin design and their bomb-proof barracks and associated buildings had concrete walls only 1.9 meters thick. They lacked electricity and running water so the garrisons obtained their drinking water from cisterns that collected runoff from the roof when it rained.
The inundation system at Amsterdam, and other positions was required to produce a sheet of water 50-cm deep, which would make the area impassable to troops using boats. Unfortunately the elevation of the polders varied from .5 meters to 7 meters and in Amsterdam alone required eighty different inundation levels. To protect the flood gates, armored ships were maintained in the vicinity.
The two new forts of Kornwerderzand and Den Over consisted of two lines of fortifications that included a number of bunker positions. Because of the low water table, these bunkers could not be linked by subterranean works, but they represented nonetheless the most modern fortified groups in the Netherlands.
Fort Kornwerderzand comprised seventeen and Fort Den Over, thirteen structures that included machine gun casemates, anti-tank gun casemates, flak storage bunkers, a searchlight bunker, a command post bunker, and a combination infirmary and usine bunker. Fort Kornwerderzand lay on the eastern end of the dike and Fort Den Over on the western end. The forts occupied two islands apiece. The first line of each fort was located on the easternmost island of its group and the second line on the island behind it.
The forts' bunkers were mostly on one level and occupied small sand hills on the man-made islands. The standard roof and wall protection was 2.0 meters of reinforced concrete. More vulnerable positions were 2.5 to 3.0 meters thick, while only the walls of the Flak bunkers had. less than 2.0 meters. The ceilings were lined with teak wood from the Dutch East Indies. The bunkers varied in size and held ten to twenty men. Each fort held a total garrison of about 200 men, which included artillery men, engineers, infantrymen, and signal men. Many of the individual positions were partially surrounded by their own concrete trench which comprised firing steps for one or more machine guns in open positions, especially those found on the end of a jetty. Dutch-made periscopes were used in the roofs of the bunkers for observation. Some bunkers even included large cloches, also manufactured in the Netherlands, that used episcopes through their narrow vision slits.
The anti-tank bunkers mounted one or two guns. The one at Komwerderzand had two floors and a special observation embrasure between the gun rooms. The Flak bunkers housed a small antiaircraft weapon and served as a storage and rest area for the crew. The diesel engines in the usine bunker served as a secondary power source for the locks. All the bunkers had an underground telephone link, and in some cases, trenches were dug to connect them.
The army maintained the Wons Position in front of Fort Kornwerderzand on the Friesland coast, a bridgehead consisting mainly of earth and wooden defenses in the form of field fortifications. The Wons Position consisted of about ninety-five bunkers, a third of which had only rifle embrasures, and only six anti-tank gun bunkers. The Wons Position and the forts on the Great Dike formed the Den Helder Position, a defensive area which included five old forts near Den Oever that protected the naval base and supported the new forts. One of these old forts mounted two 150-mm Krupp guns and ten obsolete 240-mm guns. Another was a panzer fort endowed with two outdated turrets, each mounting a pair of 305-mm guns.
The West Front of Fortress Holland consisted mainly of coastal defenses guarding the access to the river mouths, the ports, and the open coastline. Some of its old forts, the newest of which was Fort Hoek van Holland built in 1888, still maintained a defensive role. Fort Hoek van Holland had two turrets mounting a pair of 240-mm cannons each and a turret with a pair of 150-mm cannons. Its 10-meter wide moat was of limited value since the fort's main protection consisted of an armored gallery with rifle embrasures. Fort Ijmiden, located on a small island near the town of the same name, included a turret with two obsolete 150-mm guns, five emplacements for ancient 240-mm guns, and two positions for 75-mm guns."
The main type of bunker throughout the Dutch defensive lines was the small three man machine gun casemate with walls 0.7 to 1.2 meters thick and roofs 1.0 meter thick. The three-embrasure bunker called the "Spider" or "Porcupine" included firing crenels for light machine guns and had enough space for three men. Unlike the casemate, which gave flanking fires, the "Spider" faced the enemy and was found mostly in the major lines, including those on the frontier. A few larger types of bunkers were also built for machine guns.
The troop shelter or "Pyramid," which served as a shelter and command post, included a defended entrance and a periscope in the roof for observation. Its roof was up to 2.15 meters thick and angled to deflect bombs. It was found in several locations in Fortress Holland and, in some cases, it was placed in lines adjacent to roads where obstacles were prepared to bar an enemy's advance.
On the South Front of Fortress Holland, the bridgehead at Moerdijk included river casemates built early in the 1930s. Later in the decade pyramids were erected behind the dike near its base on the north bank across from the bridgehead. Armored turrets mounting machine guns had their concrete emplacements built into the dike of the north bank. The bridgehead included many special turrets and pyramids and a inundation zone on the south bank.
The inlet sluices on the Lek River at Fort Honswijk controlled the water level in front of the fortifications of Utrecht. However, the new Amsterdam-Rhine Canal, which passed through the New Water Line, could interfere with the flooding. To prevent this problem the Plofsluis sluice was built at the same time as the new canal in 1934. It had five huge containers that could quickly block the canal when their contents of 40,000,000 kilograms of rubble were released, allowing the main sluice to flood the area as planned. The position was bombproof and gas proof so that its garrison could continue to carry out its duties. The Plofsluis also had flood gates to control the flow of water in either direction if the enemy was able to take control of the Lek Dike.
Other key facilities for inundation included the Diefdijk, which regulated the Waal and Lek Rivers with sluices at each end. Fort Everdingen was located at one end of the dike on the Lek, adjacent to the main sluice. Fort Asperen was at the other end of the dike, on the small Linge River. In the 1930s the Diefdijk was broken by a new road that connected Utrecht to 's-Hertogenbosch. A bridge that included a special gate known as a "guillotine," which could be dropped across the road to close the dike, was built at the point where the dike was broken. Thus the area in front of the dike could still be flooded when needed. Across from Fort Everdingen lay Fort Honswijk. Both forts, built in 1860 and modernized in the 1870s, controlled a main sluice on either side of the Lek. Fort Everdingen was given additional concrete shelters during the Great War. Several lines of shelters were added east of Fort Honswijk in 1916, and "pyramids" and two road blocks in 1940.
2. The Frontier Defenses
The Ijssel and Maas Lines included 480 machine gun bunkers (including 337 of the smallest type), 352 armored turrets, and 22 anti-tank gun bunkers. Many of these bunkers had special armored turrets, a few had a ready room and cloche or an anti-tank gun. The large river casemates generally had two levels: one below and one above ground level. There were also four-story bunkers with two floors for embrasures above ground. These large structures usually defended bridges and crossings at places like Zwolle, Deventer, Doesburg, Arnhem, Nijmegen, Venlo, and Roermond. The defensive positions bu
ilt in front of main lines, virtually on the frontier, were rather insignificant, meant mainly to preserve a neutral stance.
Between the frontier and the Maas-Ijssel Lines, roadblocks were readied for quick installation. They included concrete and steel obstacles, and, in some cases, freshly felled trees that were booby trapped. In addition, all the bridges were mined before the war began.
One key position, almost linking the Maas and Ijssel Line, was the old fort of Pannerden, built in 1872 and refurbished between world wars. Its importance lay in the fact that it occupied the point where the Rhine River forked to become the Waal and Neder (Lower) Rhine.' Fort Pannerden included three armored batteries: one of five 150-mm guns, two with two 105-mm guns. The garrison of 300 men occupied a large brick fort with a massive moat that was not bombproof by modern standards. Only the earthen cover and new armored plate for the gun batteries and some new concrete casemates kept the fort from being totally obsolete.
Fort Westervoort, which stood further down the Rhine near Arnhem where the Rhine forked again into the Ijssel, was renovated in the 1930s when several casemates were added to it. Between Arnhem and Nijmegen, the fortifications were extended to link the Ijssel and Maas Lines, taking advantage of the fork of the Rhine. The position was very close to the German border.
The Dutch also resorted to passive resistance, removing all road signs and markers within about 75 kilometers of the frontier months before the invasion in order to confuse the enemy and force him to pass through the rather featureless and unmarked terrain.
The Peel Line, not actually a frontier position, was designed to join with the Grebbe Line. However, since the 1940 strategy was to withdraw into Fortress Holland, it should be included with the border positions. It backed up the Maas Line and ran from the small Raam River which entered the Maas River in the north near Grave and then took a southerly course to Griensaveen and Weert, and then on to the Belgian border. It consisted of three lines. The first included field works and bunkers located at intervals of 300 meters, the second field fortifications, and the third, which followed the Zuid Willems Canal from 's-Hertogenbosch to Nederweert (northeast of Weert), bunkers and field works. The Zuid Willems Canal, which was navigable, had a width ranging from 30 to 35 meters. The first two lines occupied positions among the Peel Marshes. The first line included the newly completed canal from Grave to St. Anthonis to Griendsveen, which was designed for the defense. This defensive canal passed through the marshes to link the Raam River in the north with the Maas in the south. Since the canal had to rise over some of the high ground in the Peel Marshes, it had sluices. The spoil from the 10 meter wide canal were used to build earth works on one of its banks. This canal zig zagged through the swamp land and its sluices could be used to flood large parts of the Peel Marshes. Small porcupine (three embrasure) bunkers were placed along the canal. Machine gun casemates for flanking fire, and armored turret positions were placed on every bend of the canal and covered key points.