In the 1930s, the Dutch government pursued a policy of strict neutrality. After World War I, the Dutch parliament supported a disarmament policy because it was generally thought that World War I had been "the war to end all wars". When the threat of Nazi Germany became more apparent the Dutch government decided to reinforce and retrain their Armed Forces. In case of a violation of neutrality by Germany, the strategy of the Army Command was to fall back on the Water Line, which formed part of Fortress Holland, the Dutch national redoubt and to await Allied assistance from France and the United Kingdom. To defend the redoubt, it was necessary to slow the German advance down in order to give as many Dutch forces as possible the chance of assembling in Fortress Holland. To this effect, several defensive lines had been constructed throughout the country. The Maas Line and the IJssel Line had been constructed along the Maas and IJssel rivers and served to detect German incursions into Dutch territory and to delay the Germans in the first hours of an invasion. The fortress at Kornwerderzand on the narrow Afsluitdijk guarded the northern approach to Fortress Holland while the Peel-Raam Line in North Brabant guarded the southern approach. Any attempt to approach Fortress Holland through the central part of the country would be delayed at the Grebbe line. At the beginning of 1940, Chief of Staff General Henri Winkelman redesignated the Grebbe Line the Main Defence Line, because defending the East Front of Fortress Holland would bring the major city of Utrecht into the frontline and the enemy too close to the Dutch capital Amsterdam.
The Grebbe Line was built in 1745 and had been used for the first time in 1794 against the French. It was maintained throughout the 19th century, but had been neglected ever since because it was thought to have become obsolete. In 1926, most fortifications were disbanded. When Germany became a potential threat the Dutch government had the Line recommissioned. At the end of the 1930s, a series of pillboxes and casemates were constructed in the area south of the IJsselmeer and north of the Rhine. The Line was constructed according to French military principles from World War I which had proven to be successful then, but had, unknown at the time of construction, become obsolete. There were major flaws in the design of the pillboxes, which were difficult to defend against attack from the flanks and rear. The (fixed) weapons were antiquated, many of them dating back to World War I. Because the Dutch government did not want to antagonise local residents, permission to remove buildings and trees in the line of fire was refused, which greatly reduced the effectiveness of the defences and gave attackers plenty of cover. The trench system was also based on World War I principles. It consisted of a line of outposts (voorpostenlijn), a Frontline (frontlijn), a Stopline (stoplijn) and a Final Line (ruglijn). Another dangerous mistake was the lack of serious security measures at the construction sites. The government did not want to interrupt tourism as the local economy of Rhenen was dependent on revenues from the Ouwehands Dierenpark, a zoo located on a hill near Rhenen, the Grebbeberg. In the months leading up to the invasion, German officers in civilian clothes visited the zoo and used its lookout tower to survey the local defences. The government estimated that the Line would be completed in November 1940 and in May 1940 the bomb-proof pumping station at the Grebbeberg—which was necessary for the control of local flooding—had not been completed. Because of the lack of inundation, the German spies realised that the Grebbeberg would be a vulnerable spot in the Grebbe Line.
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