The Dyle Plan or Plan D was the plan of the Commander-in-Chief of the French Army, Maurice Gamelin, to defeat a German attempt to invade France through Belgium. The Dyle (Dijle) river is 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. After the German Remilitarization of the Rhineland on 7 March 1936, the Belgian government abrogated the accord and substituted a policy of strict neutrality, now that the German Army () was on the German–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 shorter. Some officers at (GQG, general headquarters of the French Army) doubted that the French could arrive before the Germans.
German dissatisfaction with (Case Yellow), the campaign plan against France, Belgium and the Netherlands, increased over the winter of 1939–1940. On 10 January 1940, a German aircraft landed at Maasmechelen in Belgium, carrying plans for the invasion. The Mechelen Incident was a catalyst for the doubts about and led to the Manstein Plan, a bold, almost reckless, gamble for an attack further south through the Ardennes. The attack on the Low Countries became a decoy to lure the Allied armies northwards, more easily to outflank them from the south.
Over the winter of 1939–1940, Gamelin altered Plan D with the Breda variant, an advance into the Netherlands to Breda in North Brabant. The Seventh Army, the most powerful element of the French strategic reserve, was added to the 1st Army Group close to the coast, to rush to the Scheldt Estuary, link with the Dutch Army at Tilburg or Breda. Some of the best divisions of the French army were moved north, when units of the German Army were being transferred south for the new version of , an invasion through the Ardennes.
Background
French defence policy
After the territorial changes in the Treaty of Versailles (28 June 1919) transferred the provinces of Alsace and Lorraine to France, natural resources, industry and population close to the frontier, vital for the prosecution of another war of exhaustion, meant that the French army would not be able to gain time by retreating into the interior as it had in 1914. By the 1930s, the importance of the two provinces and north-eastern France to the French economy had grown. The French Army was responsible for frontier protection under the (CSG, Supreme War Council), which was revived on 23 January 1920. By 1922, two schools of thought had emerged, one led by General Edmond Buat that advocated the building of continuous fortifications along the frontier for a relatively static defence and one supported by Marshal Ferdinand Foch and Marshal Philippe Pétain, which wanted fortified regions to be built as centres of resistance for offensive action. Armies would manoeuvre around the centres until the most favourable time and conditions for attack. By the end of 1922, majority opinion in the CSG favoured a system that could be used offensively and defensively.