page_title: French war planning 1920–1940
section_title: Maginot Line
hierarchical_section_title: French war planning 1920–1940 / Background / Maginot Line
context_page_description: 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 43–50 mi shorter.
context_section_description: 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 River 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. The Minister of War André Maginot (1922–1924, 1929–1930 and 1931–1932) 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 (Metz Fortified Region) through the Moselle valley to the Nied at Teting, 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 region from Bitche to the Sarre at Wittring.
  The 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 1.5–2 mi (2.4–3.2 km) back were concrete Avant-postes with permanent garrisons armed with 47 mm or 65 mm guns, intended to delay an attacker so that buried casemates and ouvrages (fortresses) further back could be manned. Artificial obstacles of 4–6 rows of upright railway line, 10 ft (3.0 m)-long set in concrete and of random depth and covered by barbed wire. A barbed wire obstruction 20 ft (6.1 m) 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 3–5 mi (4.8–8.0 km). Interval Troops of infantry, gunners, engineers and mechanised light cavalry with field artillery, could manoeuvre between the fortifications, advancing to defend casemate approaches and relieve outposts or retiring to protect fortress entrances, the troops provided continuity, depth and mobility to the static defences.
caption: Map of the principal fortified section of the Maginot Line
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