The 'Battle of France' was fought between Germany (and later Italy) against the Allied powers for France, Belgium, Luxembourg and the Netherlands (10 May/25 June 1940).

On 3 September 1939, France and the UK declared war on Germany following the German 'Weiss' (i) invasion of Poland. Early in this same month France launched its limited 'Saar Offensive' and by the middle of October had withdrawn to its start lines. German armies invaded the Netherlands, Belgium and Luxembourg 10 May 1940 in 'Gelb'. Italy entered the war on 10 June 1940 and attempted an invasion of south-eastern France. France and the Low Countries were conquered, ending land operations on the Western Front until the 'Neptune' (iii) Allied landings in Normandy started 'Overlord' on 6 June 1944.

In 'Sichelschnitt', major German armoured formations made a surprise push through the hilly and forested Ardennes region and then swept forward into the Somme river valley before advancing down this valley to the southern side of the English Channel, thereby cutting off and surrounding the most northern Allied armies that had advanced into Belgium to meet the German armies. British, Belgian and French forces were pushed back to the sea by the Germans. The British and French navies evacuated the encircled elements of General the Lord Gort’s British Expeditionary Force and parts of the French and Belgian armies from Dunkirk in 'Dynamo'.

German forces then began 'Rot' (iii) on 5 June 1940. The 60 surviving French divisions and the two British divisions still in France made a determined stand on the Somme and Aisne irvers, but were defeated by the German combination of air superiority and armoured mobility. German armies outflanked the intact 'Ligne Maginot' and pushed deep into France, occupying Paris unopposed on 14 June. After the flight of the French government to Bordeaux and the collapse of the French army, German commanders met with French officials on 18 June to negotiate an end to the fighting.

On 22 June, the Second Armistice at Compiègne was signed by France and Germany. The neutral Vichy French government led by Maréchal de France Philippe Pétain replaced the Third Republic, and German military occupation began along the French North Sea and Atlantic coasts and their hinterlands. The Italian invasion of France over the Alps took a small amount of ground and, after the armistice, Italy occupied a small area in the south-east. The increasingly collaborationist Vichy French régime retained the zone libre (free zone) in the south. Following the Allied invasion of French Africa in November 1942, the Germans and Italians took control of the zone until it was liberated by the Allies in 1944.

During the 1930s, the French built the 'Ligne Maginot' as a continuous fortification along the border with Germany. The line was intended to economise on manpower and deter a German invasion across the Franco-German border by diverting it into Belgium, which could then be met by the French army’s best divisions. The French believed that the war would therefore take place outside French territory, avoiding the destruction that had been visited on north-eastern France by World War I. The main section of the 'Ligne Maginot' extended from the junction of the French, German and Swiss borders in the south, and ended at Longwy, neat the border of France, Belgium and Luxembourg, in the north. The forested hills and valleys of the Ardennes region were thought to cover the area just to the north. Now retired but still as hero of France for his leadership in World War I, Pétain declared the Ardennes to be 'impenetrable' as long as 'special provisions', i.e. a pincer attack, were taken to destroy an invasion force as it emerged from the Ardennes. The French commander-in-chief, Général d’Armée Maurice Gamelin, also believed the area to be safe from attack, noting that it 'never favoured large operations'. French war games in 1938, and based on a hypothetical German armoured attack through the Ardennes, left the army with the impression that the region was still largely impenetrable and that this, together with the obstacle of the Meuse river, would slow the Germans for a time sufficient to allow the French to bring up troops into the area for a counter-offensive.

In 1939, France and the UK offered military support to Poland in what was in all probability a German invasion. At dawn on 1 September 1939, the German 'Weiss' (i) invasion of Poland began. France and the UK declared war Germany on 3 September, after an ultimatum for Germany immediately to withdraw its forces from Poland was not even answered. Australia and New Zealand also declared war Germany on on 3 September, South Africa on 6 September and Canada on 10 September. While British and French commitments to Poland were met at the political level, the Allies failed to fulfil their military obligations to Poland, later called the Western betrayal by the Poles. The possibility of Soviet assistance to Poland had ended with the Munich Agreement of 1938, after which the USSR and Germany eventually negotiated the German-Soviet Pact, which included an agreement for the two nations to partition Poland. The Allies settled on a long-war strategy in which they would complete their belated rearmament plans of the 1930s while fighting a defensive land war against Germany and weakening its war economy with a trade blockade, ready for an eventual invasion of Germany.

On 7 September, in accordance with the Franco-Polish alliance, France began the 'Saar Offensive' with an advance from the 'Ligne Maginot' 3.1 miles (5 km) into the Saar. France had mobilised 98 divisions (all but 28 of them reserve or fortress formations) and 2,500 tanks against a German force consisting of 43 divisions (32 of them reserve formations) and no tanks. The French advanced until they met the thin and undermanned 'Siegfried-Linie'. On 17 September, Gamelin gave the order for the French forces to withdraw to their starting positions, and the last French troops left Germany on 17 October. Following the 'Saar Offensive', a period of inaction, which the British called the Phoney War the French the drôle de guerre (joke war) and the Germans the Sitzkrieg (sitting war), developed between the belligerents. on the Western Front Adolf Hitler had hoped that France and the UK would acquiesce to the conquest of Poland and quickly make peace. On 6 October, he made a peace offer to both Western powers.

On 9 October 1939, Hitler issued his Führerweisung Nr 6. Hitler recognised the necessity of military campaigns to defeat the Western European nations as an inevitable preliminary to the conquest of territory in eastern Europe, and thus avoid a two-front war, but these intentions were absent from Führerweisung Nr 6. The plan was based on the seemingly more realistic assumption that German military strength would have to be increased over a period of several years. Only limited objectives could be envisaged to improve Germany’s ability to survive a long war in the west. Hitler ordered a conquest of the Low Countries to be executed at the shortest possible notice to forestall the French and to prevent Allied air power from threatening the Rudr region, which was the German industrial heartland. The German offensive was also provide the basis for a long-term air and sea campaign against the UK. There was no mention in the directive of a consecutive attack to conquer the whole of France, although the directive ordained that as large an area of the north-eastern French border areas as possible should be occupied.

On 10 October, the UK refused Hitler’s offer of peace and two days later so too did France. The pre-war German codename of plans for a campaign in the Low Countries was Aufmarschanweisung Nr 1, Fall Gelb (Deployment Instruction No. 1, Case Yellow). General Franz Halder, chief of the Oberkommando des Heeres’s staff, presented the first plan for 'Gelb' on 19 October, detailing an advance through the middle of Belgium: Aufmarschanweisung Nr 1 therefore envisaged a frontal attack, at a cost of half million German men, to attain the limited goal of throwing the Allies back to the Somme river. German strength in 1940 would then be spent, and only in 1942 could the main attack against France begin. When Hitler raised objections to the plan and wanted an armoured breakthrough, as had happened in the invasion of Poland, Halder and Generaloberst Walther von Brauchitsch, commander-in-chief of the German army, attempted to dissuade him, arguing that while fast-moving mechanised tactics were effective against a 'shoddy' eastern European army, they would not work against a first-rate military such as that of France.

Hitler was disappointed with Halder’s plan and initially reacted by deciding that Germany should attack early, ready or not, in he hope that Allied unreadiness might bring about an easy victory. Hitler therefore proposed an invasion starting on 25 October, but then accepted that the date was probably unrealistic. On 29 October, Halder presented Aufmarschanweisung Nr 2, Fall Gelb, with a secondary attack on the Netherlands. On 5 November, Hitler informed von Brauchitsch that he intended the invasion to begin on 12 November, to which the German army’s commander-in-chief responded that the military had yet had time to recuperate from the Polish campaign. von Brauchitsch’s offer to resign was refused, but two days later Hitler postponed the attack, citing poor weather as the reason for the delay, and further postponements followed as commanders persuaded Hitler to delay for a few days or weeks in order to remedy some defect in the preparations or to wait for better weather. Hitler also tried to alter the plan, which he found unsatisfactory: the German leader’s weak understanding of how poorly prepared Germany was for war and how it would cope with losses of armoured vehicles were not fully considered. Although Poland had been quickly defeated, many armoured vehicles had been lost and were hard to replace. This led to the German effort becoming dispersed: the main attack was to remain in central Belgium, while secondary attacks were to be undertaken on the flanks. Hitler made such a suggestion on 11 November, pressing for an early attack on unprepared targets.

Halder’s plan satisfied no one. Generaloberst Gerd von Rundstedt, the commander of Heeresgruppe 'A', recognised that the plan did not adhere to the classic principles of Bewegungskrieg (war of manoeuvre) that had guided German strategy since the 19^th century. A breakthrough was needed to encircle and destroy the main body of Allied forces. The most practical place to achieve this would be in the region of Sedan, which lay in sector allocated to Heeresgruppe 'A'. On 21 October, von Rundstedt agreed with his chief-of-staff, Generalleutnant Erich von Manstein, that an alternative operational plan to reflect these principles was needed, by making Heeresgruppe 'A' as strong as possible at the expense of Generaloberst Fedor von Bock’sHeeresgruppe 'B' farther to the north.

While von Manstein was formulating new plans in Koblenz, Generalleutnant (from 1 November General) Heinz Guderian, the commander of the XIX Corps (mot.), was lodged in a nearby hotel. von Manstein was initially considering a move to the north from Sedan, directly in the rear of the main Allied mobile forces in Belgium. Invited to contribute to the plan during informal discussions, Guderian proposed that most of the Panzerwaffe (armoured arm) should be concentrated at Sedan. This concentration of armour would advance to the west to the coast of the English Channel without waiting for the main body of infantry divisions. This might lead to a strategic collapse of the Allies, thereby avoiding the relatively high number of casualties normally caused by a Kesselschlacht (encirclement battle).

So risky an independent use of armour had been widely discussed in Germany before the war, but the Oberkommando des Heeres doubted that such an operation could work. von Manstein’s general operational ideas won immediate support from Guderian, who understood the terrain, having experienced the conditions with the German army between 1914 and 1918. von Manstein wrote his first memorandum outlining the alternative plan on 31 October, and in it avoided all mention of Guderian and downplayed the strategic part of the armoured formations to avoid unnecessary resistance. Six more memoranda followed between 31 October and 12 January 1940, each more radical than its predecessor. All were rejected by the Oberkommando des Heeres, and nothing of their content reached Hitler.

On 10 January 1940, a German aeroplane, carrying a staff officer with the Luftwaffe plans for an offensive through central Belgium to the North Sea, force-landed near Maasmechelen (Mechelen) in Belgium. The documents were captured, but Allied intelligence doubted that they were genuine. In the full-moon period in April 1940, another Allied alert was called against the possibility of an attack on the Belgium or the Netherlands as an offensive through the Low Countries to outflank the 'Ligne Maginot' from the north, an attack on the 'Ligne Maginot', or an invasion through Switzerland. None of the contingencies anticipated a German attack through the Ardennes, but after the loss of the Luftwaffe plans, the Germans assumed that the Allied appreciation of German intentions would have been reinforced. Aufmarschanweisung Nr 3, Fall Gelb, an amendment to the plan on 30 January, was only a revision of details. On 24 February, however, the decision was made to shift the main German effort south to the Ardennes. Twenty divisions, including seven Panzer and three motorised divisions, were transferred from Heeresgruppe 'B' opposite the Netherlands and Belgium to Heeresgruppe 'A' facing the Ardennes. French military intelligence uncovered a transfer of German divisions from the Saar to the north of the Moselle river, but failed to detect the redeployment from the Dutch frontier to the Eifel-Moselle area.

On 27 January, von Manstein was sacked as chief-of-staff of Heeresgruppe 'A' and appointed commander of a corps in East Prussia: in order to silence von Manstein, Halder had instigated his transfer to Stettin on 9 February. von Manstein’s staff brought his case to Hitler, who had independently suggested an attack at Sedan, against the advice of the Oberkommando des Heeres. On 2 February, Hitler was told of von Manstein’s plan and on 17 February he summoned von Manstein, Oberst Rudolf Schmundt, the chief of personnel of the German army, and Generalleutnant Alfred Jodl, the chief of operations of the Oberkommando der Wehrmacht, to a conference. On the following day, Hitler ordered the adoption of von Manstein’s thinking as it offered the possibility of decisive victory. Hitler recognised the breakthrough at Sedan only in tactical terms, whereas von Manstein his 'Sichelschnitt' plan as a means to an end. He envisaged a high-speed advance to the English Channel and the encirclement of the Allied armies in Belgium, and thus, should it succeed, his plan could have a strategic effect.

Halder then went through an 'astonishing change of opinion', accepting that the Schwerpunkt (point of main effort) should be Sedan. He had no intention of allowing an independent strategic penetration by the seven Panzer divisions of Heeresgruppe 'A'. Much to the dismay of Guderian, this element was absent from the new plan, Aufmarschanweisung Nr 4, Fall Gelb, issued on 24 February. The bulk of the German officer corps was appalled and called Halder the 'gravedigger of the Panzer force'. Even when adapted to more conventional methods, the new plan provoked a storm of protest from the majority of German generals. They thought it utterly irresponsible to create a concentration of forces in a position that was impossible impossible to resupply adequately along routes that could be cut easily by the French. If the Allies did not react as expected, the German offensive could end in catastrophe. The generals' objections were ignored and Halder argued that, as Germany’s strategic position seemed hopeless anyway, even the slightest chance of decisive victory should be grasped. Shortly before the invasion, Hitler, who had spoken to forces on the Western Front and who was encouraged by the success in Norway, confidently predicted the campaign would take only six weeks. Personally, he was most excited over the planned the 'Granit' gliderborne attack on Fort Eben-Emael in Belgium.

On 3 September 1939, French military strategy had been settled, taking in analyses of geography, terrain, resources and manpower. The French army would defend in the east (right flank) and attack on the west (left flank) by advancing into Belgium, to fight forward of the French frontier. The extent of the forward move was dependent on events, which were complicated when Belgium ended the Franco-Belgian Accord of 1920 after the German remilitarisation of the Rhineland on 7 March 1936. Its status as a neutral state made Belgium reluctant openly to co-operate with France, but information was communicated about Belgian defences. By May 1940, there had been an exchange of the general nature of French and Belgian defence plans, but little in the way of co-ordination against a possible German offensive to the west through Luxembourg and eastern Belgium. The French expected Germany to breach Belgian neutrality first, providing a pretext for French intervention, or that the Belgians would request support when an invasion was imminent. Most of the French mobile forces were assembled along the Belgian border, ready to forestall the Germans.

An early appeal for help might give the French time to reach the German-Belgian frontier but if not, there were three feasible defensive lines farther to the rear. A practicable line existed from Givet to Namur, across the 'Gembloux gap', Wavre, Louvain and along the Dyle river to Antwerp, which was some 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 further along the Scheldt (Escaut) river to Antwerp, which became the 'Escaut Plan' or 'Plan E'. The third possibility was along field defences of 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. Gamelin and other senior French commanders doubted that they could move any farther forward before the Germans arrived. Late in September, Gamelin issued a directive to Général d’Armée Gaston Billotte, commander of the 1er Groupe d’Armées: '…assuring the integrity of the national territory and defending without withdrawing the position of resistance organised along the frontier…' giving the 1er Groupe d’Armées authorisation to enter Belgium, to deploy along the Escaut according to 'Plan E'. On 24 October, Gamelin directed that an advance beyond the Escaut river was only feasible if the French moved fast enough to forestall the Germans.

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 began to consider the possibility of advancing farther to the east than the Escaut river. By November, the GQG had decided that a defence along the 'Dyle Line' was feasible, despite the doubts of Général d’Armée Alphonse Georges, commander of the North-Eastern Front, about the possibility of reaching the Dyle river before the Germans. The British had been lukewarm about an advance into Belgium, but Gamelin persuaded them to accede. On 9 November, the 'Dyle Plan' was adopted. On 17 November, a session of the Supreme War Council deemed it essential to occupy the 'Dyle 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 expanded and the French army received more equipment and training. Gamelin also considered a move towards Breda in the Netherlands: if the Allies prevented a German occupation of the Netherlands, the 10 divisions of the Dutch army would join the Allied armies, control of the North Sea would be enhanced, and the Germans would be denied bases for attacks on the UK.

By May 1940, the 1er Groupe d’Armées was responsible for the defence of France from the English Channel coast as far to the south as the northern end of the 'Ligne Maginot'. The Seventh Army (Général d’Armée Henri Giraud’s 7ème Armée, General the Lord Gort’s British Expeditionary Force, Général d’armée Georges Maurice Jean Blanchard’s 1ème Armée and Général d’Armée André Corap’s 9ème Armée were ready to advance to the 'Dyle Line' by pivoting on the right (southern) 2ème Armée. The 7ème Armée would take over in the area to the west of Antwerp, ready to move into the Netherlands, and the Belgians were expected to delay a German advance, then retire from the Albert Canal to the Dyle river, from Antwerp to Louvain. On the Belgian right, the British Expeditionary Force was to defend about 12.5 miles (20 km) of the Dyle river between Louvain and Wavre with nine divisions, and the 1ème Armée, on the right of the British Expeditionary Force, 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 north of the Sambre river, with Maastricht and Mons on either side, had few natural obstacles and was a traditional route of invasion, leading 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.

The 2ème Armée 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. GQG considered that the 2ème Armée and 9ème Armée had the army group’s easiest task, dug in on the west bank of the Meuse river on ground that was easily defended and behind the Ardennes, a considerable obstacle whose crossing would give plenty of warning of a German attack in the centre of the French front. After the transfer from the strategic reserve of the 7ème Armée to the 1er Groupe d’Armées, seven divisions remained behind the 2ème Armée and 9ème Armée, and more could be moved from behind the 'Ligne Maginot'. All but one division were either side of the junction of the two armies, GQG 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 placed.

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 progress around the west of Antwerp and gain 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 had moved from the general reserve by December. The role of the army was to occupy the southern bank of the Scheldt river and be ready to move into the Netherlands to protect the estuary by holding its northern bank along the Beveland peninsula (now the Walcheren, Zuid-Beveland and Noord-Beveland peninsula) in the 'Holland Hypothesis'.

On 12 March, Gamelin discounted dissenting opinion at GQG and decided that the 7ème Armée would advance as far as Breda, to link with the Dutch. Georges was told that the task of the 7ème Armée on the left flank of the Dyle river manoeuvre would be linked to it, and Georges notified Billotte that if it was ordered to cross into the Netherlands, the left flank of the army group was to advance to Breda and, if possible, Tilburg. The 7ème Armée was to take position between the Belgians and Dutch by passing the Belgians along the Albert Canal and then turning to the east, a distance of 109 miles (175 km), when the Germans were only 56 miles (90 km) distant from Breda. On 16 April, Gamelin also made provision for a German invasion of the Netherlands but not Belgium, by changing the deployment area to be reached by the 7ème Armée: the 'Escaut Plan' would be followed only if the Germans forestalled the French move into Belgium.

During the winter of 1939/40, the Belgian consul general in the German city of Köln had anticipated the angle of advance that von Manstein was planning. Through intelligence reports, the Belgians deduced that German forces were concentrating along the Belgian and Luxembourg frontiers. In March 1940, Swiss intelligence detected six or seven Panzer divisions on the German-Luxembourg-Belgian border, and more motorised divisions were detected in the area. French intelligence was informed through aerial reconnaissance that the Germans were constructing pontoon bridges about halfway over the Our river on the Luxembourg-German border. On 30 April, the French military attaché in the Swiss city of Bern warned that the centre of the German assault would come on the Meuse river at Sedan sometime between 8 and 10 May. These reports had little effect on Gamelin, as did similar reports from neutral sources such as the Vatican and a French sighting of a 60-mile (100-km) column of German armoured vehicles on the Luxembourg border trailing back inside Germany.

Germany had mobilised 4.2 million men of the army, 1 million in the Luftwaffe, 180,000 of the Kriegsmarine and 100,000 of the Waffen-SS. When consideration is made for those deployed on occupation duty in Poland, Denmark and Norway, the army had 3 million men available for the offensive on 10 May 1940. This manpower formed the basis of 157 divisions, of which 135, including 42 reserve divisions, were earmarked for the offensive. The German forces in the west in May and June deployed some 2,439 tanks and 7,378 pieces of artillery. In 1939/40, 45% of the army’s manpower was at least 40 years old, and 50% of all the soldiers had just a few weeks' training. The German army was far from motorised: 10% of the army was motorised in 1940 and could muster only 120,000 vehicles, compared with the 300,000 of the French army. The whole of the British Expeditionary Force was motorised. Most of the German logistical transport was based on horse-drawn vehicles. Only 50% of the German divisions available in 1940 were wholly it for operations, often being worse equipped than the German army of 1914 or their French and British equivalents. In the spring of 1940, the German army can thus be classified as only semi-modern, with the 'small number of the best-equipped and 'elite divisions…offset by many second and third rate divisions'.

Heeresgruppe 'A' comprised 45.5 divisions, including seven Panzer divisions, and was to execute the main movement effort through the Allied defences in the Ardennes. (The manoeuvre carried out by the Germans is sometimes known as 'Sichelschnitt', the German translation of the phrase 'sickle cut' coined by Winston Churchill after the event.) It involved three armies in the form of Generaloberst Günther von Kluge 4^th Army, Generaloberst Wilhelm List’s 12^th Army and General Ernst Busch’s 16^th Army, and had three Panzer corps. General Hermann Hoth’s XV Corps (mot.) had been allocated to the 4^th Army but General Geirg-Hans Reinhardt’s XLI Corps (mot.) and Guderian’s XIX Corps (mot.) were united with General Gustav von Wietersheim’s XIV Corps (mot.) of two motorised infantry divisions on a special independent operational level in General Ewald von Kleist’s Panzergruppe 'Kleist', otherwise the XXII Corps (mot.).

Heeresgruppe 'B' comprised 29.5 divisions including three armoured, and was to advance through the Low Countries and lure the northern units of the Allied armies into a pocket. It was composed of Generaloberst Walter von Reichenau’s 6^th Army and General Georg von Küchler’s 18^th Army. Generaloberst Wilhelm Ritter von Leeb’s Heeresgruppe 'C' had 18 divisions in General Erwin von Witzleben’s 1^st Army and General Friedrich Dollmann’s 7^th Army, and was to prevent a flanking movement from the east and with launching small holding attacks against the 'Ligne Maginot' and French forces along the upper reaches of the Rhine river.

Radio proved essential to German success in the 'Battle of France'. German tanks had radio receivers that allowed them to be directed by platoon command tanks, which had voice communication with other units. Radio allowed tactical control and far quicker improvisation than was possible for the Allied armoured forces. Some commanders regarded the ability to communicate to be the primary method of combat and radio drills were considered to be more important than gunnery. Radio allowed German commanders to co-ordinate their formations, bringing them together for a mass firepower effect in both attack or defence. The French numerical advantage in heavy weapons and equipment, which was often deployed in 'penny packets' (i. e. dispersed as individual support weapons), was thereby offset. Most French tanks also lacked radio equipment and orders between infantry units were typically passed by telephone or verbally.

The German system also permitted a degree of communication between air and ground forces. Attached to Panzer divisions were the Fliegerleittruppen (tactical air control troops) in wheeled vehicles. There were too few SdKfz 251 command vehicles for all of the army, but the theory allowed the army in some circumstances to call Luftwaffe units to summon a supporting air attack. General Wolfram Freiherr von Richthofen’s VIII Fliegerkorps, equipped with Junkers Ju 87 single-engined dive-bombers as well as attack aircraft and medium bombers, was to support the dash to the English Channel if Heeresgruppe 'A' broke through the Ardennes, and therefore kept a Ju 87 and a fighter group on call. On average, these aircraft could arrive to support armoured units within 45 to 75 minutes of orders being issued.

The German army had undertaken considerable combined arms training in operations with mobile offensive units, with balanced numbers of well-trained artillery, infantry, engineer and tank formations, integrated into Panzer divisions. The various elements were united by wireless communication, which enabled them to work together at a quick tempo and exploit opportunities more rapidly than the Allies could implement any effective reaction. Panzer divisions could conduct reconnaissance, advance to contact or defend and attack vital positions and weak spots. Captured ground was occupied by infantry and artillery as pivot points for further attacks. Although many German tanks were outgunned by their opponents, they could lure Allied tanks onto the divisional anti-tank guns. The avoidance of tank-versus-tank engagements conserved German tanks for the next stage of the offensive. Units carried supplies for three to four days' operations. The Panzer divisions were supported by motorised and infantry divisions. German tank battalions were to have been entirely equipped with PzKpfw III medium and PzKpfw IV battle tanks by 1940, but delays in deliveries led to the continued retention of numbers of PzKpfw II light tanks and even lighter PzKpfw I vehicles.

The German army lacked a heavy tank like the French Char B1, and in general French tanks were better designs, more numerous and with superior armour and armament, but on the other hand slower and with inferior mechanical reliability than the German designs. Although the German army was outnumbered in artillery and tanks, it nonetheless did possess some advantages over its opponents. The newer German tanks had a crew of five (commander, gunner, loader, driver and mechanic), and the availability of a trained individual for each task allowed a logical division of labour. French tanks had smaller crews, thus the commander had to load the main gun, distracting him from observation and tactical deployment. The Germans enjoyed an advantage through the concept of Auftragstaktik (mission command) by which officers, non-commissioned officers and men were expected to use their initiative and had control over supporting arms, whereas the Allies still relied on the slower, top-down method.

Heeresgruppe 'B' was supported by 1,815 combat, 487 transport and 50 glider aircraft, while another 3,286 combat aircraft supported Heeresgruppe 'A' and Heeresgruppe 'C', and at this time the Luftwaffe was the most experienced, well-equipped and well-trained air force in the world. The Allied combined Allied was 2,935 aircraft, about half the size of the Luftwaffe. The Luftwaffe could provide close support with dive-bombers and medium bombers, but was a broadly based force intended to support national strategy, and could carry out tactical, operational and strategic bombing operations, whereas the Allied air forces available for service over the Western Front were intended mainly for army co-operation. The Luftwaffe could fly air superiority missions, medium-range interdiction, strategic bombing and close air support operations, depending on circumstances. It was not a Panzer spearhead arm, since in 1939 fewer than 15% of Luftwaffe aircraft were designed for close support as this was not the service’s main role.

The Germans also had an advantage in Fliegerabwehrkanone (Flak) anti-aircraft guns. The totals of 2,600 880mm (3.465-in) heavy Flak guns and 6,700 37-mm (1.46-in) and 20-mm (0.79-in) light Flak guns. The armies which invaded the west had 85 heavy and 18 light batteries of the Luftwaffe, 48 companies of light Flak guns integral to army divisions and 20 companies of light Flak allocated as army troops, a reserve in the hands of headquarters above corps level: this amounted to about 700 88-mm (3.465-in) and 180 37-mm (1.46-in) guns manned by Luftwaffe ground units, and 816 20-mm (0.79-in) guns manned by army troops.

France had spent a higher percentage of its gross national product between 1918 and 1935 on its military than other great powers, and the government had added a major rearmament effort in 1936. France mobilised about one-third of the male population between the ages of 20 and 45, bringing the strength of its armed forces to 5 million men. Only 2.24 million of these served in army units in the north. The British contributed a total strength of 897,000 men in 1939, rising to 1.65 million by June 1940. The Dutch and Belgian manpower reserves amounted to 400,000 and 650,000 men respectively.

The French raised 117 divisions, of which 104 (including 11 in reserve) were for the defence of the north. The British contributed 13 divisions, of which three were untrained and poorly-armed labour divisions, in the British Expeditionary Force. Totals of 22 Belgian, 10 Dutch and two Polish divisions were also part of the Allied order of battle. The British artillery strength amounted to 1,280 pieces, Belgium fielded 1,338 guns, the Dutch 656 guns and France 10,700 guns, giving an Allied total of about 14,000 pieces of artillery, 45% more than the German total. The French army also possessed a higher degree of motorisation than the German army which, as noted above, still relied on horses. Although the Belgians, British and Dutch had few tanks, the French had 3,254 tanks, more than the Germans possessed.

The French army was of mixed quality. The mechanised light and heavy armoured divisions (Divisions Légères Mécaniques and Divisions Cuirassées respectively) were new and not thoroughly trained. Reserve B divisions were composed of reservists, above 30 years old and poorly equipped. A serious qualitative deficiency was a lack of anti-aircraft artillery, mobile anti-tank artillery and radio despite the Gamelin’s efforts to produce mobile artillery units. Only 0.15% of military spending between 1923 and 1939 had been on radio and other communications equipment, and to maintain signals security Gamelin used telephones and couriers to communicate with field units.

French tactical deployment and the use of mobile units at the operational level of war was also inferior to that of the Germans. The French had 3,254 tanks on the north-eastern front on 10 May, against 2,439 German tanks. Much of the armour was distributed for infantry support, each army being assigned a tank brigade (groupement) of about 90 light infantry tanks. With so many tanks available, the French could still concentrate a considerable number of light, medium and heavy tanks in armoured divisions, which in theory were as powerful as Panzer divisions. Only French heavy tanks generally carried radio, and the equipment fitted was unreliable, which hampered communication and made tactical manoeuvre difficult by comparison with those of the German forces. In 1940, French military theorists still considered tanks mainly as infantry support vehicles, and with the exception of the SOMUA S.35 French tanks were slow by comparison with their German opponents, enabling German tanks to offset their disadvantages by out-manoeuvring French tanks. On several occasions, moreover, the French were not able to achieve the same tempo as German armoured units. The state of training was also unbalanced, with the majority of personnel trained only to man static fortifications. Minimal training for mobile action was carried out between September 1939 and May 1940.

The French army consisted of three groupes d’armées (army groups). The 2ème Groupe d’Armées and 3èmw Groupe d’Armées defended the 'Ligne Maginot' to the east, while Billotte’s the 1er Groupe d’Armées was on the western (left) flank and would execute the movement forward into the Low Countries. Initially positioned on the left flank near the coast, the 7ème Armée, reinforced by one division légère mécanique (mechanised light division), was intended to move to the Netherlands via Antwerp. To the south of the 7ème Armée were the motorised divisions of the British Expeditionary Force, which was to advance to the 'Dyle Line' on the right flank of the Belgian army, from Leuven (Louvain) to Wavre. The 7ème Armée, reinforced by two divisions légère mécanique and with a division cuirassée (armoured division) in reserve, was to defend the 'Gembloux gap' between Wavre and Namur. The southernmost army involved in the move forward into Belgium was the 7ème Armée, which had to cover the Meuse river sector between Namur to the north of Sedan.

Gort, commander of the British Expeditionary Force, expected to have two or three weeks to prepare for the Germans to advance 60 miles (100 km) to the Dyle river, but the Germans arrived in four days. The 2ème Armée was expected to form the hinge of the Allied movement and remain entrenched. In the event it was this army which was in fact to face the Panzer divisions in their attack at Sedan, but had been given only a low priority for manpower, anti-aircraft and anti-tank weapons, and air support. The army comprised five divisions, of which two were over-age 'Serie B' reservist formations and the 3ème Division Nord-Africaine. Considering their training and equipment, these divisions had to cover a long front and formed a weak point in the French defensive arrangements. This stemmed from the French high command’s belief that the forested Ardennes was impassable to tanks, even though intelligence from the Belgian army and from its own intelligence services warned of long armour and transport columns crossing the Ardennes and being stuck in a huge traffic jam for some time. French war games in 1937 and 1938 had shown that the Germans could penetrate the Ardennes and Corap called it 'idiocy' to think that the Germans could not get through. Gamelin ignored the evidence as it did not accord with his strategy.

The Armée de l’Air had 1,562 aircraft, RAF Fighter Command 680 and RAF Bomber Command could contribute about 392 aircraft. Some Allied types, such as the Fairey Battle single-engined light bomber, were approaching obsolescence. In the single-engined fighter force, only the British Hawker Hurricane, the US Curtiss Hawk 75 and the Dewoitine D.520 were a match for the Germans' Messerschmitt Bf 109, the D.520 being more manoeuvrable although slightly slower. On 10 May 1940, however, only 36 examples of the D.520 had been delivered. The Allies outnumbered the Germans in fighter aircraft, with a total of 1,105 such aircraft (81 Belgian, 261 British and 764 French) against 836 Bf 109 machines. The French and British had more aircraft in reserve.

Early in June 1940, the French aviation industry was producing a considerable number of aircraft, with an estimated reserve of nearly 2,000, but an acute lack of spare parts and essential equipment crippled this fleet. Only about 599 (29%) of the aircraft were serviceable, of which 170 were bombers. The Germans had six times more medium bombers than the French. Despite its disadvantages, the Armée de l’Air performed far better than could have been expected, destroying 916 German aircraft in air-to-air combat, a kill ratio of 2.35/1. Almost one-third of the French victories were accomplished by French pilots flying the Hawk 75, which accounted for 12.6% of the French single-seat fighter force.

In addition to 580 13-mm (0.51-in) machine guns assigned to civilian defence, the French army had 1,152 25-mm (0.98-in) anti-aircraft guns, with 200 20-mm (0.79-in) auto-cannon in the process of delivery and 688 75-mm (2.95-in) guns and 24 90-mm (3.54-in) guns, the last having problems with barrel wear. Also available were also 40 105-mm (4.13-in) anti-aircraft guns of World War I vintage. The British Expeditionary Force had 10 regiments of 3.7-in (94-mm) heavy anti-aircraft guns, the most advanced weapons of their type in the world, and 7.5 regiments of Bofors 40-mm light anti-aircraft guns, for a total of about 300 heavy and 350 light anti-aircraft guns. The Belgians had two heavy anti-aircraft regiments and were introducing Bofors guns for divisional anti-aircraft troops. The Dutch had 84 75-mm (2.95-in), 39 elderly 60-mm (2.36-in), seven 100-mm (3.94-in), 232 20-mm (0.79-in) and 40-mm (1.57-in) anti-aircraft guns and several hundred World War I vintage Spandau M.25 machine guns on anti-aircraft mountings.

At 21.00 on 9 May, the code word 'Danzig' was relayed to all German army divisions to trigger 'Gelb'. Security was so tight that many officers, as a result of the constant delays, were away from their units when the order was received. German forces occupied Luxembourg virtually unopposed. Heeresgruppe 'B' launched its feint offensive during the night into the Netherlands and Belgium. On the morning of 10 May, Fallschirmjäger (paratroopers) of Generalleutnant Kurt Student’s (from 14 May Generalleutnant Richard Putzier’s) 7^th Fliegerdivision and air-landed troops of Generalleutnant Hans Graf von Sponeck’s 22^nd Division (Luftlande) executed surprise landings at The Hague, on the road to Rotterdam and against the Belgian fortress of Eben-Emael, which helped the advance of Heeresgruppe 'B'. The French command reacted immediately, sending the 1er Groupe d’Armées north in accordance with 'Plan D'. This move committed the best French forces, diminishing their fighting power by the partial disorganisation it caused and their mobility by depleting their fuel stocks. By the time it crossed the Dutch border, the 7ème Armée found the Dutch already in full retreat and withdrew into Belgium to protect Antwerp.

The Luftwaffe effort over the Netherlands comprised 247 medium bombers, 147 fighters, 424 Junkers Ju 52/3m three-engined transport and glider-tug aircraft and 12 Heinkel He 59 twin-engined floatplanes. The Dutch air arm had a strength of 144 combat aircraft, half of which were destroyed on the first day. The remainder of the Dutch aircraft were dispersed and accounted for only a handful of Luftwaffe aircraft shot down. The Dutch air arm managed to fly 332 sorties, losing 110 aircraft. General Georg von Küchle’s 18^th Army captured bridges during the 'Battle of Rotterdam', bypassing the New Water Line from the south and penetrating 'Vesting Holland'. A separate operation organised by the Luftwaffe, the 'Battle for The Hague', failed. Airfields around the city (Ypenburg, Ockenburg and Valkenburg) were captured in a costly success, with many transport aircraft lost, but the Dutch army had recaptured the airfields by the end of the day. Some 96 aircraft in all were lost to Dutch artillery fire. Operations by the Luftwaffe’s Transportgruppen had thus cost the service 125 Ju 52/3m aircraft destroyed and 47 damaged, a 50% loss. The airborne operation also cost 50% of the German paratroopers: 4,000 men, including 20 per cent of its non-commissioned officers and 42% of its officers. Of these casualties, 1,200 were taken prisoner and evacuated to the UK.

The French 7ème Armée failed to block the German armoured reinforcements of Generalleutnant Alfred Ritter von Hubicki 's 9^th Panzzerdivision, which reached Rotterdam on 13 May. That same day in the east, following the 'Battle of the Grebbeberg', in which a Dutch counterattack to contain a German breach failed, the Dutch retreated from the 'Grebbelinje' to the New Water Line. Still largely intact, the Dutch army surrendered in the evening of 14 May after the bombing of Rotterdam by Heinkel He 111 twin-engined medium bombers of Oberst Walter Lackner’s Kampfgeschwader 54 'Totenkopf' in a 'terror bombing' action which has remained controversial. The Dutch army considered its strategic situation to have become hopeless and feared destruction of other Dutch cities. The capitulation document was signed on 15 May but Dutch forces continued fighting in the 'Battle of Zeeland' with the 7ème Armée and in the colonies. Queen Wilhelmina unwillingly left the country and in the UK established a government-in-exile. Dutch casualties amounted to 2,157 army, 75 air force and 125 navy personnel; 2,559 civilians were also killed.

The Germans quickly established air superiority over Belgium. Having completed thorough photographic reconnaissance, they destroyed 83 of the 179 aircraft of the Aeronautique Militaire within the first 24 hours of the invasion. The Belgians flew 77 operational missions but this contributed little to the air campaign, and the Luftwaffe was assured of air supremacy over the Low Countries. Because the composition of Heeresgruppe 'B' had been so weakened by comparison with its strength in earlier plans, the feint offensive by Generaloberst Walter von Reichenau’s 6^th Army was in immediate danger of stalling as he Belgian defences on the Albert Canal position were very strong. The main approach route was blocked by the fortress of Eben-Emael, which was then generally considered the most modern in Europe and which controlled the junction of the Meuse river and the Albert Canal. Delay might endanger the outcome of the entire campaign as it was essential that the main Allied forces be engaged before Heeresgruppe 'A' established bridgeheads. To overcome this difficulty, the Germans resorted to unconventional means in the 'Battle of Fort Eben-Emael'. In the early hours of 10 May, DFS 230 gliders landed on top of the fort and disgorged assault teams which disabled the main gun cupolas with hollow-charge explosives. The bridges over the canal were seized by German paratroopers. The Belgians launched considerable counterattacks which were broken up by the Luftwaffe. Shocked by a breach in its defences just where they had seemed the strongest, the Belgian supreme command withdrew its divisions to the 'KW-Ligne' five days earlier than planned. Similar operations against the bridges in the Netherlands, at Maastricht, failed. All were blown up by the Dutch and only one railway bridge was taken, which held up the German armour on Dutch territory for a short time.

The British Expeditionary Force and the French 7ème Armée were not yet entrenched and the news of the defeat on the Belgian border was unwelcome. The Allies had been convinced that Belgian resistance would have given them several weeks to prepare a defensive line at the 'Gembloux gap'. Hoepner’s XVI Corps (mot.), comprising Generalmajor Horst Stumpff’s 3^rd Panzerdivision and Generalmajor Johann Joachim Stever’s 4^th Panzerdivision, was launched over the newly captured bridges in the direction of the 'Gembloux gap'. This seemed to confirm the expectations of the French supreme command that this would be the location of the German Schwerpunkt. Gembloux is located between Wavre and Namur, on flat terrain ideal for armoured operations, and also an unfortified part of the Allied line. To gain time to dig in there, Général de Corps d’Armée René Prioux, commanding the Corps de Cavalerie of the , ps of the French 1ère Armée, sent the 2^nd DLM and 3^rd DLM toward the German armour at Hannut, to the east of Gembloux. These two divisions were thus to provide a screen to delay the Germans and allow sufficient time for the 1ère Armée to dig in.

The 'Battle of Hannut' on 12/13 May was the largest tank battle yet fought, with about 1,500 armoured fighting vehicles involved. The French knocked out about 160 German tanks for a loss of 105 of their own machines including 30 SOMUA S.35 tanks. The Germans were left in control of the battlefield after the French made a planned withdrawal, however, and were able to recover and repair many of their knocked-out tanks. The net German loss amounted to 20 tanks of the 3^rd Panzerdivision and 29 of the 4^th Panzerdivision. Prioux had achieved a tactical and operational success for the French by fulfilling his objective of delaying the Panzer divisions until the 1ère Armée had time to arrive and dig in. The German attack had engaged the 1ère Armée to the north of Sedan, which was the most important objective that Hoepner had to achieve, but had failed to forestall the French advance to the Dyle river or to destroy the F1ère Armée. On 14 May, having been held up at Hannut, Hoepner attacked again, against orders, in the 'Battle of Gembloux'. This was the only occasion when German tanks made a frontal assault on a fortified position during the campaign. The 1ère Division marocaine repulsed the attack and another 42 of the 4^th Panzerdivision's tanks were knocked out, 26 then being written off. This second French defensive success was nullified by events farther to the south at Sedan, however.

The advance of Heeresgruppe 'A' was to be delayed by Belgian motorised infantry and French divisions légères de cavalerie (mechanised cavalry divisions) advancing into the Ardennes. The main resistance came from the Belgian 1/Chasseurs Ardennais, the 1ère Division de Cavalerie, reinforced by engineers and the French 5ère Division Légère de Cavalerie. The Belgian troops blocked roads, held up Generalleutnant Friedrich Kirchner’s 1st Panzerdivision at Bodange for about eight hours and then retired northward too quickly for the French, who had not arrived. The Belgian barriers proved ineffective when not defended, so the German engineers were not disturbed as they dismantled the obstacles. The French had insufficient anti-tank capacity to block the surprisingly large number of German tanks they encountered and quickly gave way, withdrawing behind the Meuse river.

The German advance was hampered by the number of vehicles trying to force their way along the poor road network. The Panzergruppe 'von Kleist' had more than 41,140 vehicles, for which only four march routes through the Ardennes were available. French reconnaissance aircraft had reported German armoured convoys by the night of 10/11 May, but at higher command levels this was assumed to be secondary to the main attack in Belgium. On the next night, a reconnaissance pilot reported that he had seen long vehicle columns moving without lights; another pilot sent to check reported the same and that many of the vehicles were tanks. Later that day, photographic reconnaissance and pilot reports were of tanks and bridging equipment. On 13 May, the Panzergruppe 'von Kleist' caused a traffic jam about 160 miles (250 km) long between the Rhine and Meuse rivers on just one route. While the German columns were sitting targets, the French bomber force attacked the Germans in northern Belgium during the 'Battle of Maastricht' and had failed with heavy losses: in a mere two days, the French bomber force was reduced from 135 to 72 aircraft.

On 11 May, Gamelin ordered reserve divisions to begin reinforcing the Meuse river sector. Because of the danger the Luftwaffe posed, movement over the railway network was limited to night-time operation, slowing the reinforcement. The French felt no sense of urgency, as they believed the build-up of German divisions would be correspondingly slow, and the French army did not undertake river crossings unless assured of heavy artillery support. While they were aware that the German tank and infantry formations were strong, they were confident in their strong fortifications and artillery superiority. The capabilities of the French units in the area were dubious, however, as in particular their artillery was designed for use against infantry and was therefore short of anti-aircraft and anti-tank guns. The German advance forces reached the line if the Meuse river late in the afternoon of 12 May, and in order to allow each of the three armies of Heeresgruppe 'A' to cross, three bridgeheads were to be established: these were at Sedan in the south, which was the target of Guderian’s XIX Corps (mot.) spearheading the 16^th Army; Monthermé in the north-west, which was the target of Reinhardt’s XLI Corps (mot.) spearheading the 12^th Army; and Dinant still farther to the north, which was the target of the Hoth’s XV Corps (mot.) spearheading the 4^th Army. The first German units to arrive possessed not even local numerical superiority, and while the German artillery had an average of 12 rounds per gun per day, the French artillery had 30 rounds per gun per day.

At Sedan, the line of the Meuse river was strongly held by a defensive belt 3.7 miles (6 km) deep, laid out according to the modern principles of zone defence on slopes overlooking the Meuse river valley. It was strengthened by 103 pillboxes, manned by the 147ème Régiment d’Infanterie de Forteresse. Deeper positions were held by the 55ème Division d’Infanterie, a 'Serie B' reserve division. On the morning of 13 May, the 71ème Division d’Infanterie was inserted to the east of Sedan, allowing the 55ème Division d’Infanterie to decrease the width of its front by one-third and deepen its position to more than 6.2 miles (10 km). The division had a superiority in artillery to the German units present. On 13 May, the Panzergruppe 'von Kleist' forced three crossings near Sedan, executed by Kirchner’s 1^st Panzerdivision, Generalleutnant Rudolf Veiel’s 2^nd Panzerdivision and Generalleutnant Ferdinand Schaal’s 10^th Panzerdivision. These groups were reinforced by the elite Infanterieregiment 'Grossdeutschland'. Instead of slowly massing artillery as the French expected, the Germans instead concentrated most of their air power on smashing a hole in a narrow sector of the French lines by carpet bombing and dive-bombing. Guderian had been promised extraordinarily heavy air support during a continual eight-hour air attack, from 08.00 until dusk.

The Luftwaffe executed the heaviest air bombardment the world had yet experienced and the most intense by the Germans during the war. Two Sturzkampfgeschwadern (dive-bomber groups) attacked, flying 300 sorties against French positions. A total of 3,940 sorties was flown by nine Kampfgeschwadern (bomber groups). Some of the forward French pillboxes were undamaged and the garrisons repulsed the crossing attempts of the 2^nd Panzerdivision and 10^th Panzerdivision. The morale of the men of the 55ème Division d’Infanterie farther to the rear was broken by the air attacks and French gunners fled. At the cost of a few hundred casualties, the German infantry penetrated up to 5 miles (8 km) into the French defensive zone by the middle of the night. Even by then, most of the infantry had not crossed, and much of the German success was attributable to the actions of just six German platoons, mainly of assault engineers.

The disorder that had begun at Sedan spread farther. At 19.00 on 13 May, men of the 295ème Régiment of the 55ème Division d’Infanterie were holding the last prepared defensive line at the Bulson ridge 6.2 miles (10 km) behind the river, were panicked by alarmist rumours that German tanks were already behind them and fled, creating a gap in the French defences before any tanks had even crossed the river. This so-called Panic of Bulson also involved the divisional artillery: the Germans had not attacked their position and would not do so until 12 hours later, at 07.20 on 14 May. Recognising the gravity of the defeat at Sedan, Billotte, the commander of 1er Groupe d’Armées, whose right flank pivoted on Sedan, urged that the bridges across the Meuse river be destroyed by air attack as he was convinced that 'over them will pass either victory or defeat!' That day, every available Allied light bomber was employed in an attempt to destroy the three bridges but lost about 44% of the Allied bomber strength for no result.

Guderian had indicated on 12 May that he wished to enlarge his corps' bridgehead to at least 12.5 miles (20 km), but von Kleist ordered him, on behalf of Hitler, to limit his moves to a maximum of 5 miles (8 km) before halting to consolidate. At 11.45 on 14 May, von Rundstedt confirmed this order, which implied that the German armoured formation and units should now start to dig in. Guderian was able to get von Kleist to agree on a form of words for a permissible 'reconnaissance in force' by threatening to resign and behind-the-scenes intrigues. Guderian thus continued the advance, despite the halt order. In von Manstein’s original plan, as Guderian had suggested, secondary attacks would be carried out to the south-east, in the rear of the 'Ligne Maginot' as this would confuse the French command and occupy ground on which French counter-offensive forces might be able to assemble. This element had been removed by Halder, but Guderian sent the 10^th Panzerdivision and Infanterieregiment 'Grossdeutschland' to the south over the Stonne plateau.

The commander of the French 2ème Armée, Général d’Armée Charles Huntziger, intended to carry out a counterattack at the same spot by the 3ème Division Cuirassée in order to eliminate the German bridgehead. Both sides attacked and counterattacked between 15 and 17 May. Huntziger considered this at least a defensive success and limited his efforts to protecting the flank. Success in the 'Battle of Stonne' and the recapture of Bulson would have enabled the French to defend the high ground overlooking Sedan and bombard the bridgehead with observed artillery fire, even if they could not take it. Stonne changed hands 17 times and fell to the Germans for the last time during the evening of 17 May. Guderian turned the 1^st Panzerdivision and 2^nd Panzerdivision to the west on 14 May, and the two formations advanced swiftly down the Somme river valley toward the English Channel.

On 15 May, Guderian’s motorised infantry fought their way through the reinforcements of the new French 6ème Armée in its assembly area to the west of Sedan, undercutting the southern flank of the French 9ème Armée. The latter collapsed and its men surrendered en masse. The 102ème Division de Forteresse, its flanks unsupported, was surrounded and destroyed on 15 May at the Monthermé bridgehead by Generalleutnant Werner Kempf’s 6^th Panzerdivision and Generalleutnant Adolf Kuntzen’s 8^th Panzerdivision without air support. The French 2ème Armée had also been seriously damaged, and the French 9ème Armée was also giving way because it lacked the time to dig in, as Generalmajor Erwin Rommel’s 7^th Panzerdivision had broken through the French line within 24 hours of the battle’s beginning. The 7^th Panzerdivision then raced forward. Rommel refused to allow the division rest and it advanced by day and night, moving forward some 30 miles (48 km) in just 24 hours.

Rommel lost contact with Hoth, his corps commander, after disobeying orders by not waiting for the French to establish a new line of defence. The 7^th Panzerdivision continued to advance to the north-west toward Avesnes sur Helpe, just ahead of the 1^st Panzerdivision and 2^nd Panzerdivision. The French 5ème Division d’Infanterie Motorisée had bivouacked in the path of the German division with its vehicles neatly aligned along the roadsides, and the 7^th Panzerdivision dashed through them. The slow speed, overloaded crews and lack of battlefield communications undid the French. The 5^th Panzerdivision joined in the fight, and while the French inflicted many losses on the division, they could not cope with the speed of the German mobile units, which closed fast and destroyed the French armour at close range. The remaining elements of the 1ère Division Cuirassée, resting after losing all but 16 of its tanks in Belgium, were also engaged and defeated before retiring with just three operational tanks, while defeating only 10% of the 500 German tanks.

By 17 May, Rommel claimed to have taken 10,000 prisoners while suffering only 36 losses. Guderian was delighted with the speed of the advance and encouraged the XIX Corps (mot.)to head for the coast of the English Channel, continuing until fuel was exhausted. Hitler worried that the German advance was moving too fast, and Halder recorded in his diary on 17 May that the 'Führer is terribly nervous. Frightened by his own success, he is afraid to take any chance and so would pull the reins on us…[he] keeps worrying about the southern flank. He rages and screams that we are on the way to ruin the whole campaign.'

Through deception and different interpretations of the halt orders from Hitler and von Kleist, the front-line commanders ignored Hitler’s attempts to stop the westward advance to Abbeville.

The French high command, slow to react because of its strategy of 'methodical warfare', reeled from the shock of the German offensive and was overtaken by defeatism. On the morning of 15 May, the French prime minister, Paul Reynaud, telephoned the new British prime minister, Churchill, to tell him that 'We have been defeated. We are beaten; we have lost the battle.' Attempting to offer some comfort to Reynaud, Churchill reminded his French counterpart of all the times the Germans had broken through the Allied lines in World War I only to be stopped, but Reynaud was inconsolable. Churchill flew to Paris on 16 May. He immediately recognised the gravity of the situation when he observed that the French government was already burning its archives and was preparing for an evacuation of the capital. In a sombre meeting with the French commanders, Churchill asked Gamelin 'Where is the strategic reserve?', referring to the reserve that had sa