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, incorrectly perceived as the strongest, in central Belgium. France had signed a military treaty with Belgium in 1920 so as to streamline communication and fortification efforts in the event of a German attack, but in October 1936 Belgium changed its 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 defence (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, The Netherlands. Ironically, the newly fashioned anti-tank defences at Gembloux proved rather inadequate, thus allowing the German 3rd and 4th Panzer divisions to easily traverse them.
The Dyle Plan played into the hand of the Germans who in fact executed their main attack (Manstein Plan) through the Ardennes on the assumption that the Allies would advance into central Belgium. The British Expeditionary Force 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 an Allied defeat in the Battle of France. According to British historian Julian Jackson, the Fall of France can be greatly attributed to the poor strategic planning of the French High Command.
See also 
- 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.
- Julian Jackson, The Fall of France: The Nazi Invasion of 1940 (New York: Oxford University Press, 2003), 40.