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 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 Grand Quartier Général doubted that the French Army could arrive before the Germans.
German dissatisfaction with Fall Gelb, 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 Mechelen in Belgium, carrying plans for the invasion. The Mechelen Incident was a catalyst for the doubts about Fall Gelb to become overwhelming 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 and link with the Dutch Army at Tilburg or Breda. Some of the best divisions of the French army were moved north, when elite units of the German Army were being transferred south for the new version of Fall Gelb, an invasion through the Ardennes.
Background
French defence policy
After the territorial changes in the Treaty of Versailles 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 Conseil supérieur de la guerre, 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.
Manpower
By 1918, French conscripts were receiving no more than three months' training and after the war, it was considered that the size of the army should be determined by the number of divisions needed for security. The number of professional soldiers and conscripts necessary was derived by multiplication and the quantity of men was more important than their education or training. In 1920, the CSG decided on 41 active divisions, plus five Algerian and three colonial divisions, with a mobilisation potential of 80 divisions. The government imposed a 32-division limit with 150,000 full-time soldiers but in 1926, the government set the size of the army at 20 active divisions, with 106,000 professional soldiers, to comprise a reservoir of trained men on which reservists could form a mobilised wartime army. Reducing the size of the active army allowed a reduction in the number of conscripts and the term of compulsory service from two years to one year by 1928. In 1928 a comprehensive series of laws had been passed for the recruitment and organisation of the army, which determined its peacetime nature; the cadre of professionals maintained the army ready for the mobilisation of a mass of reservists.The French army expected that another war would be won by a mass army, even if it was full of short service and sketchily trained men; the period of twelve-month conscription lasted from 1928 until 1935. A force of one-year conscripts was accepted by the army, because a big, fairly well-trained army in wartime was considered more important than a highly trained, quick-responding and offensively-minded army in peacetime. From 220,000 to 230,000 men were trained each year, half being called up every six months, the previous group moving to the active army as the new men began training. The 106,000-man regular army was capable only of manning frontier defences, training recruits and providing planning staffs; when the garrison in the Rhineland returned, the army lost the capacity for independent or limited action in Europe without mobilisation. In the context of the later 1920s, the decline in the readiness of the standing army did not seem to be a disadvantage. By the time the one-year law affected numbers in 1932, there were 358,000 soldiers in metropolitan France, of whom 232,000 were sufficiently trained for operations. By 1933, there were 320,000 soldiers in mainland France, with 226,000 having had more than six months' training; the French army was only twice the size of the German Reichswehr, which comprised highly trained soldiers, because of the long term period of service imposed by the treaty of Versailles.
Belgium
In September 1920, the CSG made a strategic decision that the defence of the northern frontier must begin with a rush into Belgium. The French army never deviated from the belief that the loss of agricultural, mining and industrial resources in 1914 could never be repeated. In September, the Franco-Belgian Accord of 1920 was signed for military co-operation; if international tension increased, the Belgians would request assistance and the French would send an army to the Belgian–German border, making it the main line of French resistance to a German attack. As the policy was studied, it became clear that a force moving to the Belgian–German border would have to be mobile if it were to forestall the Germans for a defensive battle from prepared positions.Motorised transport would be necessary to rush forward French troops, then ferry engineer stores to fortify their positions. The French army created mobile fortification parks stocked with engineer stores for fortification, ready to be moved by road and rail but if the Belgian army was overwhelmed, the French might be forced into an encounter battle and a war of movement in the central Belgian plain. French strategy was to avoid a decisive battle early on, after the disaster of the Battle of the Frontiers in 1914 but the necessity of avoiding a war on French soil meant that a forward move could not be avoided.
Maginot Line
Studies made by the General Staff in 1919 were reported to the CSG in 1920 and a commission of 1922, chaired by Marshal Joseph Joffre reported in December 1925, in favour of centres of resistance built in peacetime, not a continuous fortified front. From 17 December 1926 to 12 October 1927, the Frontier Defence Commission reported to the CSG that fortifications should be built from Metz to Thionville and Longwy, to protect the Moselle Valley and the mineral resources and industry of Lorraine. The area around the Lauter, the most north-eastern part of the common border with Germany, should be fortified as an obvious invasion route but there was no need to fortify the Rhine, because of the Vosges mountains further west and the small number of railways on the German side. Belfort was near the Swiss frontier and partly protected by the Rhine but there was an avenue of invasion to the west, which should be protected. The commission gave emphasis to defence against a surprise attack, with the limited objective of capturing the Metz and Lauter areas.The commission recommended that priority be given to protecting the resources and industries of Lorraine that were vital for the French economy and would become more important for a war economy. The nature of fixed defences was debated during the 1920s, with advocates of the offensive use of fortifications, deep or shallow defences and centralised and decentralised designs. On 12 October 1927, the CSG adopted the system recommended by Pétain, of large and elaborately fortified defences from Metz to Thionville and Longwy, at Lauter and Belfort, on the north-east frontier, with covered infantry positions between the main fortifications. André Maginot, the Minister of War became the driving force for obtaining the money to fortify the north-eastern frontier, sufficient to resist a German invasion for three weeks, to give time for the French army to mobilise. Work began in 1929 on the Région Fortifiée de Metz through the Moselle valley to the Nied at Teting-sur-Nied, then the Région Fortifiée de Lauter, east of Hagenau from Bitche to the Rhine, the extension of the Metz region to Longuyon and the Lauter river region from Bitche to the Sarre at Wittring.
Requirements for the fortifications were natural cover, sites nearby for observation posts, the minimum of dead ground, a maximum arc of fire, ground suitable for anti-tank obstacles and infantry positions and ground on which paved roads could be built, to eliminate wheel marks. Maisons Fortes were to be built near the frontier as permanently garrisoned works, whose men would alert the army, blow bridges and erect roadblocks, for which materials were dumped. About back were concrete Avant-postes with permanent garrisons armed with intended to delay an attacker so that buried casemates and ouvrages further back could be manned. Artificial obstacles of 4 to 6 rows of upright railway line, -long set in concrete and of random depth and covered by barbed wire. A barbed wire obstruction further back covered a field of anti-tank mines overlooked by twin machine-guns and anti-tank guns in casemates. The casemates were distributed in series and were the only defensive works along the Rhine; on other stretches, casemates were interspersed with ouvrages, every. Interval Troops of infantry, gunners, engineers and mechanised light cavalry with field artillery, could manoeuvre between the fortifications, advancing to defend casemate approaches to relieve outposts or retiring to protect fortress entrances; the troops provided continuity, depth and mobility to the static defences.