The 'D-Plan' was the French strategic plan, otherwise known as the 'Dyle Plan', designed to check the German advance expected in 'Gelb' (spring 1940).

The scheme was devised early in 1940 by Général Maurice Gustave Gamelin, the French commander-in-chief, and named for the Dyle river that flows from southern Belgium to the north in the direction of Antwerp. The plan was posited on the belief of the French general staff that 'Gelb' would in essence be a repeat of the 'Schlieffen Plan' of World War I, and was thus to be a huge right-hook movement sweeping through the southern Netherlands, Belgium and north-eastern France to progress as far to the west as Le Havre at the mouth of the Seine river before wheeling to the south and passing to the west of Paris.

The primary task envisaged for the 'D-Plan' was therefore the halting of Generaloberst Fedor von Bock’s Heeresgruppe 'B', incorrectly perceived as the strongest of the three German army groups facing France, in central Belgium along the line of the Dyle river.

In 1920 France and Belgium had signed a military treaty designed to streamline the two nations' fortification and communication efforts in the event of a German attack, but by the 1930s Belgium had changed its policy to one of strict neutrality, and this led to many difficulties in Franco-Belgian co-ordination.

Gamelin initially proposed the less risky 'E-Plan' (otherwise the 'Escaut Plan') for a defence (except for the extreme west in Flanders) based upon a series of fortifications along much of the Franco-Belgian frontier rather than within Belgium proper.

On 24 October 1939 Général d’Armée Gaston Henri Gustave Billotte’s 1er Groupe d’Armées, charged with the defence of north-eastern France in the sector of the front between Longuyon and the North Sea, received orders from Général d’Armée Alphonse Joseph Georges, the commander-in-chief of the Allied forces in north-eastern France, that when ordered to do so by Gamelin, he would advance his forces into Belgium and accept battle on the Escaut river. In the event that this decision was made, Général d’Armée Charles Léon Clément Huntziger’s French 2ème Armée, Général d’Armée André Georges Corap’s French 9ème Armée and Général d’Armée Georges Maurice Jean Blanchard’s French 1^st Armée would remain in position along the French frontier between Longuyon and Maulde sur Escaut. Only General the Lord Gort’s British Expeditionary Force and Général de Corps d’Armée Marie Bertrand Alfred Fagalde’s French XVI Corps d’Armée were to move to the east into Belgium: the former was to take up position around Tournai, and the latter was to move farther downstream on the Escaut, establish a bridgehead at Ghent, and make contact with the Belgian forces as they fell back from the Albert Canal.

On 10 November, one day after an false alert of the start of a German offensive, Général d’Armée Henri Honoré Giraud’s French 7ème Armée, currently in reserve around Reims, was ordered to join the XVI Corps d’Armée.

The weakness of the 'E-Plan' was that it overextended the length of front to be defended, and that the Allied forces on the Escaut river would hardly be assisted by the remnants of the Belgian army falling back from the line of the Albert Canal with the Germans on their heels. For this reason, a secret instruction of 24 October foresaw a considerably deeper Allied advance into Belgium, and Gamelin then decided to adopt the 'D-Plan' with the argument that the new anti-tank defences built by Belgium along the Dyle as the 'KW-Ligne’and at the Gembloux gap would check the Germans for a time long enough for the Allied armed to move into strong defensive positions in central Belgium.

Adoption of the 'D-Plan' also afforded Giraud’s 7ème Armée the opportunity to link with the Dutch forces via Breda in the Netherlands.

In the event the Belgian anti-tank defences at Gembloux proved inadequate and scarcely slowed Generalmajor Horst Stumpff’s 3^rd Panzerdivision and Generalmajor Johann Joachim Stever’s 4^th Panzerdivision of Generaloberst Walter von Reichenau’s 6^th Army, within Heeresgruppe 'B', as they advanced through them.

The 'D-Plan' thus played straight into the hands of the Germans, who had abandoned 'Gelb' in its initial form in favour of the so-called 'Sichelschnitt' for an offensive farther to the south though the Ardennes as the first stage of a left hook up to the English Channel: the Germans had come to believe, quite correctly, that the French response after the German move into Belgium would be an advance to the Dyle river.

Via Georges, Gamelin ordered Billotte to keep his forces in readiness 'for the right circumstances and the order of the general commanding the North-Eastern Theatre of Operations [Georges] and then, while remaining in position on French soil between Rochonvilliers and Revin, to advance in force to the line Louvain-Wavre-Gembloux-Namur. To the south the front will be secure by the occupation of the Meuse between Givet and Namur; in the north the British forces on the Dyle will be in touch on their left with the Belgian forces defending Antwerp.'

This 'D-Plan' gave the Allies a much shorter front to defend, and meant that the Belgians would not have to retreat far before joining the relieving armies. The plan required King Leopold III and the Belgian high command to keep their allies fully informed of their strategic intentions in the event of German aggression.

As soon as the Belgians appealed to the French for assistance in May 1940, therefore, the 'D-Plan' was put into effect under the supervision of Billotte’s 1er Groupe d’Armées: Corap’s 9ème Armée advanced to the line linking Mezières and Namur with its formations to the west of the Meuse river, Blanchard’s 1ère Armée advanced to the line between Namur and Wavre, and Gort’s British Expeditionary Force advanced to the line linking Wavre and Louvain, where it established contact with the Belgians; Giraud’s 7ème Armée was held in reserve in the area to the west of Antwerp.

The British Expeditionary Force and the 7ème Armée were then cut off and surrounded after being outflanked from the south-east by the German advance to the English Channel, and would have been totally destroyed but for the 'Dynamo' evacuation from Dunkirk. The 'D-Plan' was therefore a disaster for Allied strategy, and a decisive factor in the Allied defeat in the Battle of France.