The 'Dyle Plan', otherwise known as 'Plan D', was the scheme created by the commander-in-chief of the French army, Général d’Armée Maurice Gamelin, to defeat a German attempt to invade France through Belgium (1940).

The Dyle (Dijle in Dutch) river is 53 miles (86 km) long, extending from Houtain le Val through Flemish Brabant and Antwerp to a confluence with the Nete river at Rumst. In his plan, Gamelin intended French, British and Belgian troops to halt a German invasion force along the line of this river.

The Franco-Belgian Accord of 1920 had co-ordinated communication and fortification efforts of both nations' armies. After the German reoccupation of the Rhineland on 7 March 1936, and the region’s subsequent remilitarisation, 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 capability and reliability of the Belgian army now led to uncertainty about whether or not French troops could move quickly enough into Belgium to avoid an encounter battle and thus be able to fight a defensive battle from prepared positions. The 'Escaut Plan' ('Plan E') and 'Dyle Plan' were both 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 the 'Dyle Plan' for an advance to the line of the Dyle river, which was between 43 and 50 miles (70 to 80 km) shorter. Some officers at the Grand Quartier Général (general headquarters of the French army) doubted that the French could arrive on this line before the Germans did so.

German dissatisfaction with 'Gelb', the plan for the campaign against the Netherlands, Belgium, Luxembourg and north-eastern France, increased during the winter of 1939/40. On 10 January 1940, a German aeroplane inadvertently landed at Maasmechelen in Belgium, one of the officers on board carrying plans for 'Gelb'. This 'Mechelen Incident' was a catalyst for the doubts about 'Gelb' and led to the creation and acceptance of 'Sichelschnitt', which was a bold, almost reckless, gamble for an attack by armoured forces farther to the south through the Ardennes regions of forested hills and valleys. The attack on the Low Countries thus became a decoy to lure the Allied armies to the north, the more easily to outflank them from the south.

Over the winter of 1939/40, Gamelin altered the 'Dyle Plan' with the 'Dyle Plan (Breda variant)', an advance into the Netherlands as far as Breda in North Brabant. The 7ème Armée, the most powerful element of the French strategic reserve, was added to the 1er Groupe d’Armées close to the southern coast of the English Channel, to rush to the Scheldt river estuary and link with the Dutch army at Tilburg or Breda. Some of the best divisions of the French army were moved to the north even as elite German units were being transferred to the south for the 'Sichelschnitt' invasion via the Ardennes.

After the territorial changes imposed by the Treaty of Versailles, signed on 28 June 1919 to bring World War I to an end, had transferred the provinces of Alsace and Lorraine from Germany to France, natural resources, industry and population close to the frontier, all of which 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 (supreme war council), which was revived on 23 January 1920. By 1922, two schools of thought had emerged: that led by Général de Division Edmond Buat, the army chief-of-staff between January 1920 and December 1923, advocated the building of continuous fortifications along the frontier for a relatively static defence; and the other, supported by Maréchal de France Ferdinand Foch and Maréchal de France Philippe Pétain, wanted fortified regions to be built as centres of resistance for offensive action. In this latter concept, armies would manoeuvre around the centres until the most favourable time and conditions for attack. By the end of 1922, majority opinion in the Conseil Supérieur de la Guerre favoured a system that could be used offensively and defensively.

By 1918 the French army was so strapped for men that conscripts received 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 Conseil Supérieur de la Guerre decided on 41 active divisions as well as five Algerian and three colonial divisions, with a mobilisation potential of 80 divisions. However, the French government imposed a 32-division limit with 150,000 full-time soldiers, but in 1926 revised this total to 20 active divisions with 106,000 full-time 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 a future war would be won by a mass army, even if it was full of short-service and indifferently trained men. The period of 12-month conscription lasted from 1928 to 1935. An army of one-year conscripts was accepted by the army as a large and fairly well trained army in wartime was considered more important than a highly trained, rapidly 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 French occupation garrison of 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 men were sufficiently trained for operations. By 1933, there were 320,000 soldiers in mainland France, of whom 226,000 had received more than six months' training. Thus the French army was only twice the size of the German Reichswehr, which comprised soldiers who were highly trained because of the long-term period of service imposed by the Treaty of Versailles.

In September 1920, the Conseil Supérieur de la Guerre 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 north-eastern France could never again be repeated. In September, the Franco-Belgian Accord of 1920 was signed for military co-operation: should international tension increase, the Belgians were to 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 was to forestall the Germans and fight a defensive battle from prepared positions. Motorised transport would be necessary to rush forward French troops, then ferry engineer stores to fortify the positions. The French army created mobile fortification parks stocked with fortification materials ready for movement by road and rail, but should the Belgian army be overwhelmed, the French might be forced into an encounter battle and a war of movement in the Belgian central plain. French strategy was to avoid any early involvement in a decisive battle, after the French 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.

Studies by the French general staff in 1919 were reported to the Conseil Supérieur de la Guerre in 1920 and a commission of 1922, chaired by Maréchal de France 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, a frontier defence commission reported to the Conseil Supérieur de la Guerre that fortifications should be built from Metz to Thionville and Longwy, to protect the Moselle river valley and the mineral resources and industry of Lorraine. The area around the Lauter river, which was the most north-easterly part of the common border with Germany, should be fortified as it was an obvious invasion route, but there was no need to fortify the Rhine river as the Vosges mountains farther to the west and the small number of railways on the German side would prevent German aggression in this region. Belfort was near the Swiss frontier and partly protected by the Rhine river, 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 river 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 Conseil Supérieur de la Guerre 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-eastern frontier, with covered infantry positions between the main fortifications. André Maginot, the minister of war in 1922/24, 1929/30 and 1931/32, became the driving force for obtaining the money to fortify the north-eastern frontier to a degree sufficient to resist a German invasion for three weeks and thereby provide the time needed for the French army to mobilise. Work began in 1929 on the Région Fortifiée de Metz (Metz fortified region) through the Moselle river 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 river at Wittring.

Requirements for the fortifications were the presence of natural cover, nearby sites for observation posts, the minimum of dead ground, maximum arcs of fire, ground suitable for anti-tank obstacles and infantry positions, and ground on which paved roads could be built so that wheel marks would be avoided. Maisons fortes were to be built near the frontier as permanently garrisoned works whose men would alert the army, blow bridges and erect roadblocks, which were tasks for which materials were dumped. About 1.5 to 2 miles (2.4 to 3.2 km) farther to the rear were concrete avant-postes with permanent garrisons armed with 47- or 65-mm guns, intended to delay an attacker so that buried casemates and ouvrages (fortresses) farther back could be manned. There were also to be artificial obstacles of four to six rows of upright railway line, 10 ft (3 m) high set in concrete, of random depth and covered by barbed wire entanglements. A barbed wire obstruction 20 ft (6.1 m) farther 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 river. On other stretches, casemates were interspersed with ouvrages every 3 to 5 miles (4.8 to 8 km). 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.

The Ardennes region was considered to be easily defended and in 1927, the Guillaumat Commission concluded that the few narrow serpentine roads through wooded hills could be blocked easily with felled trees, minefields and roadblocks. The swift advance of a large force, particularly a road-bound one, through natural and artificial obstacles, could thus readily be rendered slow and arduous. Once an invader managed to get through the Ardennes, the depth and width of the Meuse river made it a considerable obstacle. The resources and equipment needed to use the Ardennes route would take so long to assemble and deploy that the French army expected to have ample time to reinforce the area. During the 1930s, the possibility of an attack through the Ardennes was reconsidered, in 1934 Pétain called the area 'not dangerous' and in 1936 Gamelin and the Belgian chief-of-staff, Luitenant-generaal Cumont, the Ardennes were not vulnerable as log as the French held the Arlon shoulder and the Belgians the opposite shoulder at Liége. Compared to the terrain and resources behind the north-eastern border and the lack of defensible ground on the northern frontier, the connecting ground of the Ardennes was less vulnerable to attack.

The Conseil Supérieur de la Guerre considered the defence of the frontier from Luxembourg to Dunkirk to be the most difficult problem, and inseparable from the defence of France’s north-eastern border with Germany. Fortifying this part of the frontier would economise on troops, allowing a larger force to operate on the northern border with Belgium. In the north, the flat and open country on the Franco-Belgian border would need far more extensive fortification than the hill country of Alsace and Lorraine, and the high water table would mean that defences would have to be built upward rather than dug downward. A fortified defence in depth would be impractical because the industrial conurbation of Lille, Tourcoing, Roubaix and Valenciennes and its railway communications obstructed the construction of a prepared battlefield with barbed wire, trenches and tank traps. Along with the lack of geographical obstacles, there were many roads and railways straight to Paris. Fortifying the frontier might also create doubts about French intentions among the Belgians, when the Belgian route was the obvious avenue of invasion, pointing at Paris. From May 1920, the Conseil Supérieur de la Guerre considered Belgium the main route of a possible invasion, particularly as the fortification of the north-eastern frontier would deprive German planners of an alternative and force them into a version of their 1914 invasion.

When France declared war on Germany during 3 September 1939, French military strategy had been settled on the basis of an analysis of geography, resources and manpower. The French army would defend on the right and advance into Belgium on the left, the latter in order to fight forward of the French frontier. The extent of the forward move was dependent on events, which were complicated in 1936 by the Belgian repudiation of the accord of 1920. The Belgian declaration of neutrality made the Belgian government reluctant to co-operate openly with France but it did communicate information on the Belgian defences. By May 1940, there had been an exchange of the general nature of French and Belgian defence plans but little co-ordination, especially against a German offensive westward through Luxembourg and eastern Belgium. The French expected Germany to breach Belgian neutrality first, thus providing a pretext for French intervention or for the Belgians to request support when an invasion was imminent. Most of the French mobile forces were assembled along the Belgian border, ready to make a quick move forward and take up defensive positions before the Germans arrived.

An early appeal for help might give the French time to reach the German/Belgian frontier but there were three feasible defensive lines farther to the rear. There was a practicable line from Givet to Namur, across the Gembloux gap (la trouée de Gembloux), Wavre, Louvain and along the Dyle river to Antwerp, later termed the 'Dyle Plan' or 'Plan D', which could be reached and was 43 to 50 miles (70 to 80 km) shorter than the alternatives. A second possibility was a line from the French border to Condé, Tournai, along the Escaut (Scheldt) river to Ghent and thence to Zeebrugge on the North Sea coast, possibly farther along the Scheldt (Escaut) river to Antwerp, which became the 'Escaut Plan' or 'Plan E'. The third possible defensive line was along field defences on the French border from Luxembourg to Dunkirk. For the first fortnight of the war, Gamelin favoured 'Plan E' because of the example of the fast German advances in Poland after the launch of their 'Weiss' (i) invasion of 1 September 1939. Gamelin and the other French commanders doubted that they could advance any farther forward before the Germans arrived, and late in September Gamelin therefore issued Billotte a directive for 'assuring the integrity of the national territory and defending without withdrawing the position of resistance organised along the frontier'.

The 1er Groupe d’Armées therefore had permission to enter Belgium and deploy along the Escaut river in accord with 'Plan E'. On 24 October, Gamelin directed that an advance beyond the Escaut river could not succeed unless the French moved fast enough to forestall the Germans.

By October 1939, the Germans had prepared 'Gelb' for an offensive in the west over the Belgian plain with the object of inflicting a huge defeat on the Allies and occupying as much as possible of the Netherlands, Belgium and north-eastern France, to undertake an air war against the UK and to protect the Ruhr against any Allied invasion of Germany. Several times during the winter of 1939/40, Adolf Hitler ordered the launch of 'Gelb', with several warning reaching the Allies through agents and signals intelligence. A warning that the German offensive was to start on 12 November was received from various sources, with the main Panzer effort to be directed against the Low Countries, and the Allied forces were alerted accordingly. It was later discovered that Hitler had ordered a state of readiness on 5 November but cancelled it on 7 November. Several other German alerts eluded Allied military intelligence, and the German 'Weserübung' invasion of Denmark and Norway took the Allies by surprise. An agent reported that the German invasion in the west was set for mid-December and was received from a Czechoslovak source in the Abwehr, the German military intelligence apparatus; another Allied alert was called after reports that the attack would begin on 13 January. Hitler ordered the attack on 17 January and then postponed it again. French and British intelligence were certain that the Germans could begin an invasion quickly and that there would be little time, after the first German move forward, to discover the time and the place.

On 10 January 1940 a German aeroplane made a forced landing near Maasmechelen (Mechelen) in Belgium. The aeroplane was carrying an officer passenger who had with him the Luftwaffe’s plans for an offensive through central Belgium to the North Sea. The documents were seized by the Belgian authorities and passed to Allied intelligence, which believed they were a 'plant'. In the full-moon period of April, another Allied alert was called in case of an attack on the Low Countries or only the Netherlands, an offensive through the Low Countries, outflanking the 'Ligne Maginot' from the north, an attack on the 'Ligne Maginot' or an invasion of France through Switzerland. No contingency anticipated a German attack through the Ardennes. The Germans assumed that the captured documents had reinforced the Allied appreciation of their intentions, and on 30 January, some details of the Aufmarschanweisung Nr 3, Fall Gelb were amended. On 24 February, the main German effort was moved south to the Ardennes, and 20 divisions (including seven Panzer and three motorised) were transferred from Heeresgruppe 'B' opposite the Netherlands and Belgium to Heeresgruppe 'A' facing the Ardennes. French intelligence uncovered a transfer of German divisions from the Saar to the area lying to the north of the Moselle river, but failed to detect the redeployment from the Dutch border to the Eiffel/Moselle river area.

By a time late in 1939, the Belgians had improved their defences along the Albert Canal and increased the readiness of their army, and Gamelin and the Grand Quartier Général then began to consider the possibility of an initial advance farther than the Escaut river. By November, the Grand Quartier Général had decided that a defence along the Dyle river line was feasible, despite the doubts of Général d’Armée Alphonse Joseph Georges, the commander of the North-Eastern Front, about reaching the Dyle river before the Germans. The British had been lukewarm about an advance into Belgium, but Gamelin persuaded them and on 9 November, the Dyle river plan was adopted. On 17 November, a session of the Conseil Supérieur de la Guerre decided that it was essential to occupy the Dyle river line, and Gamelin issued a directive that day detailing a line from Givet to Namur, the Gembloux gap, Wavre, Louvain and Antwerp. For the next four months, the Dutch and Belgian armies laboured over their defences, the British Expeditionary Force was enlarged and the French army received more equipment and training.

In May 1940, the 1er Group d’Armées was responsible for the defence of France from the English Channel coast to the north-western end of the 'Ligne Maginot'. Général d’Armée Henri Giraud’s 7ème Armée, the British Expeditionary Force, Général d’Armée Georges Maurice Jean Blanchard’s 1ère Armée and Général d’Armée André Corap’s 9ème Armée were ready to advance to the Dyle river line by pivoting on the 2ème Armée to their right. The 7ème Armée would take over the area to the west of Antwerp, ready to move into the Netherlands, and the Belgians were expected to delay a German advance at the Albert Canal and then retire to the Dyle river between Antwerp and Louvain. On the Belgian right, the British Expeditionary Force was to defend about 12 mile (20 km) of the Dyle river from Louvain to Wavre with nine divisions, and the 1ère Armée on the British Expeditionary Force’s right was to hold 22 miles (35 km) with 10 divisions from Wavre across the Gembloux gap to Namur. The gap from the Dyle river to Namur, to the north of the Sambre river, with Maastricht and Mons on each side, had few natural obstacles and was a traditional route of invasion, and led straight to Paris.

The 9ème Armée was to take position to the south of Namur, along the Meuse river to the left (northern) flank of the 2ème Armée, which was the right (eastern) flank army of the 1er Groupe d’Armées, holding the line from Pont à Bar 3.7 miles (6 km) to the west of Sedan to Longuyon. The Grand Quartier Général considered that the 2ème Armée and the 9ème Armée had the easiest task of the army group since they were in entrenched positions on the western bank of the Meuse river on ground that was easily defended and behind the Ardennes and would have plenty of warning of a German attack in the centre of the French front. After the transfer of the 7ème Armée Army to the 1er Groupe d’Armées, seven divisions remained behind the 2ème Armée and 9ème Armée, and other divisions could be moved from behind the 'Ligne Maginot'. All but one of the division were to each side of the junction of the two armies, the Grand Quartier Général being more concerned about a possible German attack past the northern end of the 'Ligne Maginot' and then to the south-east through the Stenay gap, for which the divisions behind the 2ème Armée were well positioned.

If the Allies could control the Scheldt river estuary, supplies could be transported to Antwerp by ship and contact established with the Dutch army along the river. On 8 November, Gamelin directed that a German invasion of the Netherlands must not be allowed to pass round the west of Antwerp by gaining the southern bank of the Scheldt river. The left flank of the 1er Groupe d’Armées was reinforced by the 7ème Armée, containing some of the best and most mobile French divisions, which moved from the general reserve by December. The role of the army was to occupy the southern bank of the Scheldt river and be prepared to move into the Netherlands and protect the estuary by holding its northern bank along the Beveland peninsula (now the Walcheren-Zuid-Beveland-Noord-Beveland peninsula) in the 'Holland Hypothesis'. On 12 March, Gamelin discounted dissenting opinions at the Grand Quartier Général decided that the 7ème Armée would advance as far as Breda in order to link with the Dutch. Georges was told that the 7ème Armée’s role on the left flank of the Dyle manoeuvre would be linked to it, and Georges notified Billotte that if it were ordered to cross into the Netherlands, the left flank of the army group was to advance to Tilburg if possible and certainly as far as Breda. The 7ème Armée was to take position between the Belgian and Dutch armies, bypassing the Belgians along the Albert Canal and then turning to the east, a distance of some 110 miles (175 km), against German armies only about 55 miles (90 km) distant from Breda. On 16 April, Gamelin also made provision for a German invasion of only the Netherlands by changing the area to be reached by the 7ème Armée. The 'Escaut Plan' was to be followed only if the Germans forestalled the French move into Belgium.

From 01.00 on 10 May, the Grand Quartier Général received information from Brussels and Luxembourg that the German invasion was about to begin, and at 04.35 the invasion of Low Countries and France started. Woken at 06.30, Gamelin ordered the start of the 'Dyle Plan'. Around dawn on 10 May, German bombers attacked targets in the Netherlands and began to drop paratroopers onto airfields. Dutch, French and British aircraft attacked the Luftwaffe on the ground and in the air, but the Germans nonetheless captured several airfields. The 7ème Armée drove forward on the northern flank and some of its advanced elements reached Breda on 11 May, by which time the Germans had captured the north-eastern frontier provinces of the Netherlands, were advancing on The Hague (Den Haag) and fighting in Rotterdam. The French found that the Moerdijk causeway had been captured by German paratroopers, cutting the link between southern and northern Holland, forcing the Dutch army to retire to the north in the direction of Amsterdam and Rotterdam. The French collided with Generalleutnant Alfred Ritter von Hubicki’s 9^th Panzerdivision and the advance of Général de Division Jean Baptiste Emmanuel Molinié's 25ème Division d’Infanterie Motorisée was stopped by German infantry, tanks and Junkers Ju 87 single-engined dive-bombers.

Général de Brigade François Picard’s 1ère Division Légère Mécanique was forced to retreat as the French heavy tanks were still on trains to the south of Antwerp. The Breda variant had been thwarted in fewer than two days, and on 12 May Gamelin ordered the 7ème Armée to cancel the plan and cover Antwerp, retiring from the lie of the Bergen op Zoom-Turnhout Canal, 20 miles (32 km) from Antwerp, to Lierre 10 miles (16 km) away. On 13 May, more German forces were landed at Den Haag and Rotterdam, the German army broke through the Dutch at Wageningen on the northern side of the Waal river and pushed the 7ème Armée back from Breda to Herentals and Bergen op Zoom, where it was met by Belgian troops retreating from Turnhout. By 14 May, much of the Netherlands had been overrun and the 7ème Armée found that fighting in the close country among the canals of southern Holland and north-western Belgium was costly against the German combination of ground and air attack. On the following day, Dutch resistance continued in Zeeland as German troops advanced in Zuid-Beveland and Walcheren, but the Dutch government surrendered at 11.00. Two divisions of the 7ème Armée remained to hold Zeeland and another two held Antwerp as the rest of the army retreated to the south. On 15 May, the rest of the 7ème Armée retreated from Zuid-Beveland under attack from the Luftwaffe, and the Belgian army prepared to retire through Antwerp to hold the Scheldt river estuary and a line to the south along the Willebrook Canal to Brussels. The 7ème Armée kept three divisions on the southern side of the Scheldt river estuary on 17 May, and the Belgians began to retreat from Antwerp toward the Scheldt river; Brussels and Mechelen fell to the Germans that evening.

In Belgium, the Albert Canal defence line was based on Fort Eben Emael. German attacks on this installation began at dawn, dive-bombers and paratroopers attacking the fort’s upper level. By 12.00 on 11 May, the German gliderborne troops on the roof of Fort Eben Emael had forced the garrison to surrender and two bridges over the Maas (Meuse) river at Vroenhoven and Veldwezelt near Maastricht had been captured. The disaster forced the Belgian army to retreat toward the line from Antwerp to Louvain on 12 May, far too soon for the French 1ère Armée to arrive and dig in. Général de Corps d’Armée René Jacques Adolphe Prioux’s French Corps de Cavalerie had reached the Gembloux gap on 11 May, and officers reported that the area had been far less fortified by the Belgians than expected: anti-tank defences had not been built and there were no trenches or concrete fortifications, although there were some cointet-elements (steel barriers) but without any of the anti-tank mines supposed to protect them. Some of the cointet-elements were so poorly sited that a French officer wondered if the Germans had been asked where to put them. Prioux tried to persuade Billotte and Georges to scrap the Dyle Plan and revert to the Escaut Plan but with the 1er Groupe d’Armées on the move, Georges decided against changing the plan; Blanchard was ordered to accelerate the advance of the 1ère Armée to arrive on 14 May, a day early.

The Corps de Cavalerie made contact with the Germans at 13.00 and fought a delaying action against General Erich Hoepner’s XVI Corps (mot.) in the 'Battle of Hannut' (12/14 May). Hannut was the first tank-against-tank encounter of the campaign and the French Somua S35 cavalry tank proved superior to the German tanks in firepower and armour protection. The Corps de Cavalerie then withdrew behind the 7ère Armée, which had now reached the line of the Dyle river. The corps suffered 105 tank casualties against 165 German tanks knocked out, but the French left their damaged tanks behind, allowing the Germans to recover and repair 100 of their own tank losses. Belgian troops were retiring to the area between Louvain and Antwerp, filling the gap between the British Expeditionary Force and the 7ème Armée. There was a lull along the Belgian army positions from Wijnegem to Lier and Louvain and the front held by the British Expeditionary Force. To the south of this last, the 1ère Armée tried to dig in from Wavre to Gembloux and Namur but closer to the Belgian/French frontier to the south, the Germans got across the Maas (Meuse) river at Houx and from Douzy to Vrigne sur Meuse in France. On 15 May, the British Expeditionary Force counterattacked at Louvain and the Germans attacked the 1ère Armée along the Dyle river, resulting in the encounter engagement which Gamelin had tried to avoid. The 1ère Armée repulsed the XVI Corps (mot.) during the 'Battle of the Gembloux Gap' (14/15 May), which had followed the 'Battle of Hannut', but by this time the French had realised that the main German attack had come farther to the south through the Ardennes.

The 1ère Armée began to pull back toward Charleroi as the French success in Belgium was contributing to the disaster on the Meuse river at Sedan, and on 16 May Blanchard was ordered to retreat to the French border. The British also began to retreat to the Escaut river and the 1ère Armée was pushed back closer to the Charleroi-Brussels Canal. On the next day, parts of the British Expeditionary Force started to retreat toward the Dender river as the British reorganised to face the threat on their right flank against the Germans who had broken through to the south of the 1ère Armée. The 1ère Armée retreated to a line from Ath southward to Lens to link with the remainder of the 9ème Armée at Mons, and only small forces remained between Maubeuge and Attigny. The Allied forces in Belgium continued the retreat on 18 May to the Escaut river and toward the French border farther to the south. The Germans followed the Allied retreat on 19 May but made a much greater effort in the south from the Meuse river, along the Somme river valley, toward the English Channel coast. Parts of the 7ème Armée began to assemble from Péronne along the Somme and Ailette rivers, across the Oise river to Coucy le Chateau.

Small German units were flown to the Belgian Ardennes to seize key road junctions, and other German troops advanced into Luxembourg as the five Panzer divisions of General Ewald von Kleist’s Panzergruppe 'von Kleist' advanced through the Ardennes, the XIX Corps (mot.) with three Panzer divisions on the southern flank toward Sedan against the 2ème Armée and the XLI Corps (mot.) with two Panzer divisions on the northern flank toward Monthermé against the 9ème Armée. The XV Corps (mot.) moved through the upper Ardennes with two Panzer divisions toward Dinant as a flank guard against any counterattack from the north. From 10 to 11 May, the XIX Corps (mot.) engaged the two cavalry divisions of the 2ème Armée, surprised them with a far larger force than expected and forced the French back. The 9ème Armée to the north had also sent forward its two cavalry divisions, which were withdrawn on 12 May before they met German troops. Corap needed the cavalry divisions to reinforce the defences on the Meuse river as some of the infantry had not arrived. The most advanced German units reached the Meuse river during the afternoon, but the local French commanders thought that they were far ahead of the main body and would wait before trying to cross the Meuse river.

From 10 May, Allied bombers had been sent to raid northern Belgium to delay the German advance while the 1ère Armée moved up, but the attacks on the bridges at Maastricht had been costly failures, the Royal Air Force’s 135 day bombers being reduced to 72 operational aircraft by 12 May. Georges changed air force priority from the 1ère Armée to the 2ème Armée on 12 May, but Billotte diverted only one- third of the air effort. Georges also began to reinforce the 2ème Armée by ordering the 3ème Division Cuirassée, which was a reserve armoured division, and five other divisions from the general reserve but with no urgency. The reinforcements moved as transport arrived from 11 to 13 May and were positioned to stop a German wheel to the south-east against the rear of the 'Ligne Maginot'. Despite the precautions taken against a German attack through the Ardennes, Georges and Gamelin remained more concerned about events in Belgium, and on 13 May, when the Germans were across the Meuse river at three points, the Grand Quartier Général reported that it was too soon to predict the main German attack. At 07.00 on 13 May, the Luftwaffe began bombing the French defences around Sedan and continued for eight hours with about 1,000 aircraft, the biggest air attack in history to that time.

Little material damage was done to the 2ème Armée, but its morale collapsed. In the 55ème Division at Sedan, some troops began to straggle to the rear, and then during the evening panic spread throughout the division. German troops attacked across the river at 15.00 and secured three footholds on the western bank by the fall of night. The French and British air forces managed to fly 152 bomber and 250 fighter sorties against the Sedan bridges on 14 May, but only in formations of between 10 and 20 aircraft. The attackers suffered losses of 11%, the RAF losing 30 of 71 aircraft and the French being reduced to sending obsolete bombers to attack in the afternoon, also with many losses. The 1ère Division Cuirassée, which was to have become part of the 1ère Armée reserve, was sent to Charleroi on the northern side of the German salient on 10 May. Billotte was still unsure of the main German effort and hesitated to direct the division to the 9ème Armée until 14 May, when it was the afternoon before the relevant order arrived, after which the divisional progress was hindered by the presence on the relevant roads of hordes of refugees. When the 4ème Division d’Infanterie Nord Africaine counterattacked that day, the 1ère Division Cuirassée was still struggling forward and was caught refuelling by the 7^th Panzerdivision. The 1ère Division Cuirassée knocked out about 100 German tanks but was defeated in detail and ceased to exist as a formation. The 9ème Armée had been bypassed on both flanks and was ordered to retreat from the Meuse river to a line between Charleroi and Rethel. The French held the Meuse river about 5 miles (8 km) to the south of Namur, but the German crossings of the Meuse river farther south, between Dinant and Stenay, continued with a swift advance past Mézières. On the southern side of the German salient, on the 2ème Armée’s right flank, it took until 15 May for the 3ème Division Cuirassée to attack at Stonne and again the attacks were piecemeal, lasting for several days but having only local effect. On 16 May, the Germans reached Hirson and pushed beyond Montcornet toward Laon, with little opposition to their westward advances. The 1er Group d’Armées was ordered to retreat from the 'Dyle Line', to avoid being trapped by the German breakthrough against the 2ème Armée and 9ème Armée. A defensive line was to be created from Maubeuge along the Sambre and Oise rivers, but German troops got across the Sambre river at Landrecies and the Oise rivers at several points on 18 May, and by evening had reached St Quentin and advanced toward Cambrai, which fell on 19 May, followed by Amiens on 20 May. The Germans reached Abbeville on the English Channel coast and closed on Montreuil and Boulogne, thereby cutting off the Allies' northern armies.

By choosing the 'Dyle Plan' and imposing the Breda variant, Gamelin had completed the evolution of army planning for the defence of France that began in 1920. Staff studies of the Breda variant caused some senior French generals to question the plan: Georges requested that the 7ème Armée be replaced by two divisions and returned to the general reserve, and warned against sending the bulk of the French mobile forces against an attack on the northern flank that was a diversion for a German attack through the centre. It has been claimed that Gamelin gained confidence in the capacity of the Allied armies, adopting a grand strategy of dubious value over the objections of some of the most senior French generals. For control of the Scheldt river estuary and the possibility of adding 10 Dutch divisions to the Allied order of battle, Gamelin committed the best divisions of the general reserve, leaving little to confront a German surprise. Gamelin imposed the Breda variant unilaterally, without consultation with the governments of Belgium and the Netherlands, which refused to make detailed arrangements for joint military action unless invaded.

Gamelin was an officer who had risen through the ranks of the French army with a reputation for caution, yet he took a great gamble with the 'Dyle Plan', which was not inherently reckless until the adoption of the Breda variant. It has been suggested that had they been aware of the French plan, it would have greatly relieved the Germans of apprehensions about their huge gamble in the Ardennes. Some of the best French divisions were wasted on the Breda variant, leaving few reserves from the Rhine river to the English Channel, the best divisions on the flanks, leaving France vulnerable to an attack through the centre. Georges was responsible for the placement of the divisions behind the 1er Groupe d’Armées, but Gamelin devised the Breda variant and forced it on some reluctant subordinates. The 'Dyle Plan' was laid down in thick document volumes for each headquarters, Prioux complaining of 'enormous dossiers…full of corrections, additions, annexes, appendices, etc.'. Motorised units in the 7ème Armée and 1ère Armée received orders for vehicle speeds, distances to be maintained and the formalities to be observed with the Belgian authorities. Had the divisions followed their instructions, the rapid deployment to the 'Dyle Line' would have been reduced to 10 miles (16 km) per day.

In the 'Battle of Hannut', the 2ème Division Légère Mécanisée and 3ème Division Légère Mécanisée of the Corps de Cavalerie, with 239 Hotchkiss light tanks and 176 Somua S35 cavalry tanks, had faced the 3^rd Panzerdivision with 280 tanks and the 4^th Panzerdivision with 343 tanks. The German units had only 73 PzKpfw III medium and 52 PzKpfw IV battle tanks, while the French also had 90 Panhard 178 armoured cars armed with 25-mm SA 35 anti-tank guns capable of penetrating the armour of any German tank. The 37-mm gun carried by the PzKpfw III was ineffective against the armour of the French tanks and the 75-mm gun carried by the PzKpfw IV could penetrate the armour of the S35 only at short range. By fighting on the defensive, the French tanks also had the advantage of being able to hide in villages and engage from cover. The French lack of operational radios was a tactical disadvantage, however, and a report of the 35^th Panzerregiment called the French 'leaderless, aimless, poorly led and tactically inferior'. The French still managed to inflict a considerable number of tank casualties on the Germans at Hannut (and later at Gembloux), the 4^th Panzerdivision being reduced on 16 May to 137 operational tanks including only four PzKpfw IV machines, a reduction of 45 to 50%. The 3^rd Panzerdivision lost between 20 and 25% of its tanks, and despite the fact that lightly damaged tanks could quickly be repaired, the fighting power of the XVI Corps (mot.) was substantially reduced.

The 'Battle of Hannut' was a French tactical success, the stand of the Corps de Cavalerie providing time for the rest of the 1ère Armée to dig on the 'Dyle Line' by 14 May, the fifth day of the campaign. The German attack on the 'Dyle Line' could not be organised in any strength until the following day. At the operational level, the fact that the 'Battle of Hannut' had been fought at all was a significant success for the German decoy operation in central Belgium, which made the French victory irrelevant in the context of the campaign. The Corps de Cavalerie, with its superior organisation and equipment, would have been invaluable for a French counterattack against the German divisions over the Meuse river at Sedan. When local French counterattacks at Sedan failed on 14 May, Gamelin contemplated ordering the Corps de Cavalerie to counterattack southward, but the XVI Corps (mot.) and the Luftwaffe had inflicted losses so severe that the corps was incapable of effecting such a manoeuvre. With no forces available against the penetration at Sedan, the XVI Corps (mot.) was no longer needed for the feint in Belgium and was transferred to Heeresgruppe 'A' on 18 May.