The 'E-Plan' was the French initial strategic plan for implementation in the event of a German invasion but soon succeeded by the 'D-Plan' (1939/40).
In 1920 France and Belgium had signed a military treaty designed to streamline the two nations' fortification and communication efforts against the possibility or in the event of a German attack on them, 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.
The French commander-in-chief, Général d’Armée Maurice Gustave Gamelin, initially proposed this 'E-Plan' (otherwise '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 should move 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 1ème 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 river, establish a bridgehead at Ghent, and make contact with the Belgian forces as they fell back from the Albert Canal.
The day after an alert on 9 November, Général d’Armée Henri Honoré Giraud’s French 7ème Armée, currently in reserve around Reims, was ordered to join the XVI Corps d’Armée in the move into Belgium.
The weakness of the 'Escaut Line' was that it overstretched the length of the front that was 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 German forces 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 river and at the Gembloux gap would check the Germans for a time long enough for the Allied armed forces to move into strong defensive positions in central Belgium.