French war planning 1920–1940

The Dyle Plan or Plan D was the plan of the Commander-in-Chief of the French Army, Général d'armée Maurice Gamelin to defeat a German attempt to invade France through Belgium. The Dyle river is 86 km long, from Houtain-le-Val through Flemish Brabant and Antwerp; Gamelin intended French, British and Belgian troops to halt a German invasion force along the line of the river. The Franco-Belgian Accord of 1920 had co-ordinated communication and fortification efforts of both armies. The Belgian government let the accord lapse after the German Remilitarization of the Rhineland on 7 March 1936, to adopt a policy of strict neutrality, with the German Army on the Belgian border.

French doubts about the Belgian army led to uncertainty about whether French troops could move fast enough into Belgium to avoid an encounter battle and fight a defensive battle from prepared positions. The Escaut Plan/Plan E and Dyle Plan/Plan D were devised for a forward defence in Belgium, along with a possible deployment on the French–Belgian border to Dunkirk. Gamelin chose the Escaut Plan, then substituted Plan D for an advance to the line of the Dyle, which was 70–80 km shorter. Some officers at Grand Quartier Général doubted that the French could arrive before the Germans.

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