The Dyle Plan or D Plan was the primary war plan of the French Army to stave off the expected German attack during Fall Gelb. It was conceived by French General Maurice Gamelin in 1940. Named after the Dyle River, which flows from southern Belgium down to Antwerp, the main objective of the plan was to halt the advancing German Army Group B, (which was incorrectly perceived as the strongest), in central Belgium. France had signed a military treaty with Belgium in 1920 to streamline communication and fortification efforts in the event of a German attack, but in October 1936 Belgium changed her policy to one of strict neutrality, limiting coordination of defense plans with France. Gamelin initially proposed the less risky "E (Escaut) Plan", which called for a defense (except for the extreme west in Flanders) based upon a series of fortifications along much of the actual Belgian-French border rather than in Belgium proper. However, Gamelin eventually decided to adopt the Dyle Plan with the argument that the new anti-tank defences built by Belgium along the Dyle and at the Gembloux Gap allowed for a quick entrenchment of the Allied armies. Adopting the Dyle Plan also afforded the French 7th Army an opportunity to link up with the Dutch forces via Breda in The Netherlands, a dangerous gamble since the 7th Army was the French strategic reserve. However, the newly fashioned anti-tank defences at Gembloux proved rather inadequate. Nevertheless, the 1st French army held fast against the 3rd and 4th Panzer Divisions, until forced to retreat towards Lille because of the German penetration in the south. Lord Gort expected that his British Expeditionary Force (BEF) would have two or three weeks to prepare for the Germans to arrive at the Dyle; they were there in four days.
When German leader Adolf Hitler heard of the Allies' advance to the Dyle, he said "I could have wept for joy, they had fallen into the trap". The Dyle Plan played into the hands of the Germans who executed their main attack (the Manstein Plan), through the Ardennes, on the assumption that the Allies would advance into central Belgium. The BEF and the French First and Seventh Armies were surrounded and would have been totally annihilated if it had not been for an impromptu evacuation from Dunkirk. The Dyle Plan was a fundamental flaw in the Allied strategy and one of the decisive factors contributing to the Allied defeat in the Battle of France. According to British historian Julian Jackson, the fall of France can be greatly attributed to the choice by Gamelin to send the French strategic reserve, the 7th army, far from the main German thrust, which made him unable to react to the German plans.
- Julian Jackson, The Fall of France: The Nazi Invasion of 1940 (New York: Oxford University Press, 2003), 28.
- Julian Jackson, The Fall of France: The Nazi Invasion of 1940 (New York: Oxford University Press, 2003), 38.
- Atkin, Ronald (1990). Pillar of Fire: Dunkirk 1940. Edinburgh: Birlinn Limited. pp. 58, 60. ISBN 1 84158 078 3.
- Julian Jackson, The Fall of France: The Nazi Invasion of 1940 (New York: Oxford University Press, 2003), 40.
- Alexander, Don W. (1974). "Repercussions of the Breda Variant". French Historical Studies. 8 (3): 459–88.