'Sichelschnitt' was the German nickname rather than formal codename for the definitive strategic offensive through the Netherlands, Belgium and Luxembourg into north-eastern France initially planned as 'Gelb' (10 May/5 June 1940).
In this undertaking German armoured units drove through the thickly forested Ardennes hill region past the northern end of France’s fixed defences, the 'Ligne Maginot', breaking though the Allied front at Sedan and driving through to Noyelles on the south coast of the English Channel at the mouth of the Somme river. This divided the Allied armies in Belgium and northern France into two parts, many of the survivors in the northern part then being evacuated to England in 'Dynamo', and the southern block being defeated in following 'Rot' (iii).
Germany’s defeat of Poland in 'Weiss' (i) during September 1939 was followed the so-called 'Phoney War' in which no major operations were undertaken by either side. Adolf Hitler originally ordered the planning of 'Gelb' for an invasion of the Low Countries and France as early as 12 November, but was convinced by the military command to postpone the invasion until the following year in order to secure better weather and to provide the time needed for the development of newly raised divisions to the operational standard of their longer-established counterparts in the German order of battle.
The overall aim was the defeat of the western European nations as the essential precursor for Germany' conquest of territory in the east without the possibly adverse factor of a two-front war.
During April 1940, the Germans launched their 'Weserübung' invasion of Norway for strategic reasons, in the process seizing Denmark to provide security and interim airfields, and in response the British, French and Free Poles launched a fruitless campaign to aid the Norwegians in ousting the Germans. Neither the British nor the French had anticipated a German success of such stunning speed and totality in Poland, and the German victory, based operationally and tactically on the first outing of their new Blitzkrieg concept of mobile warfare with fast-moving armoured formations relying on tactical support by aircraft rather than artillery, came as a considerable shock to some at least of the senior British and French generals. Even so, the Allies believed that they would be able to check and contain any German offensive into France, for they confidently expected the Germans to adopt a strategy based on the 'Schlieffen plan' of World War I, which had mandated a great right-wing thrust designed to pass along the southern coast of the English Channel before wheeling to the south to pass round the west of Paris. The Allies therefore believed that they could halt such an operation and bring about a stalemate which, even without an Eastern Front to threaten the German rear, would allow the war to be won by a tight blockade of Germany, as in World War I.
Such a view was more prevalent in the UK than in France, which had suffered greater losses during World War I in terms of men and material devastation. The French leadership, in particular Edouard Daladier, prime minister since 1938, also brought into the equation of the considerable gap between France’s human and economic resources by comparison with those of Germany. The supreme commander of the French army, Général d’Armée Maurice Gustave Gamelin, was akin to his political masters in expecting a German campaign based ultimately on the 'Schlieffen Plan', which was in fact just what the Germans were planning in 'Gelb'. Gamelin believed that the Germans would launch what was in effect only a modernised version of the 'Schlieffen Plan' and felt that it would be preferable to tackle such a German undertaking defensively, as the French military staff believed its country was not equipped militarily or economically, at least in the short term, to launch a decisive offensive. In these circumstances, Gamelin believed, it would be better for the French army to check the Germans and then wait until 1941 before seeking to make a full exploitation of the Allies' combined manpower and economic superiority over the Germans.
To confront the expected German plan which, he rightly believed, would be based on an offensive through the Low Countries to turn the northern end of the 'Ligne Maginot', Gamelin schemed the 'Dyle plan' to launch the most capable formations of the French army, along with the divisions of Field Marshal the Lord Gort’s British Expeditionary Force, to the north and east in order to hold the Germans in the area of the Dyle river to the east of Brussels, the capital of Belgium, until a decisive victory could be achieved with the support of the united French, British, Belgian and, possibly, Dutch armies.
In the north-eastern part of their country, therefore, the French created Général Alphonse Joseph Georges’s Théâtre des Opérations du Nord-Est with three army groups under command. These were the 1er, 2ème and 3ème Groupes d’Armées, of which the first was intended for mobile operations within the context of the 'Dyle plan', and the second and third for static service holding the 'Ligne Maginot' defences. At the northern end of the French deployment, Général d’Armée Gaston Henri Gustave Billotte’s 1er Groupe d’Armées (22 divisions including two light armoured divisions) comprised Général d’Armée Georges Maurice Jean Blanchard’s 1ère Armée (Général de Corps d’Armée René Jacques Adolphe Prioux’s Corps de Cavalerie, Général de Corps d’Armée Léon Benoit de Fornel de la Laurencie’s III Corps d’Armée, Général de Corps d’Armée Pierre Louis André Boris’s IV Corps d’Armée and Général de Corps d’Armée Darius Paul Bloch-Dassault’s V Corps d’Armée [motorisé]); Général d’Armée Charles Léon Clément Huntziger’s 2ème Armée (Général de Corps d’Armée Pierre Paul Jacques Gransard’s X Corps d’Armée and Général de Corps d’Armée Eugène Jules Rochard’s XVIII Corps d’Armée); Général d’Armée Henri Honoré Giraud’s 7ème Armée (Général de Division Théodore Marcel Sciard’s I Corps d’Armée and Général de Division Marie Bertrand Alfred Fagalde’s XVI Corps d’Armée); and Général d’Armée André Georges Corap’s 9ème Armée (Général de Corps d’Armée Jean Gabriel Bouffet’s II Corps d’Armée [motorisé], Général de Corps d’Armée Julien François René Martin’s XI Corps d’Armée and Général de Division Emmanuel Urbain Libaud’s XLI Corps d’Armée de Forteresse). Gort’s British Expeditionary Corps (10 divisions) was independent of but co-operated with the 1er Groupe d’Armées.
Général d’Armée André Gaston Prételat’s 2ème Groupe d’Armées (35 divisions) in the centre comprised Général d’Armée Charles Marie Condé's 3ème Armée (Général de Division Henry Freydenberg’s Corps d’Armée Colonial, Général de Corps d’Armée Lucien Loiseau’s VI Corps d’Armée, Général de Corps d’Armée François Marie Jacques Fougère’s XXIV Corps d’Armée and Général de Division Désiré Louis Sivot’s XLII Corps d’Armée de Forteresse); Général d’Armée Edouard Jean Réquin’s 4ème Armée (Général de Corps d’Armée Auguste Marie Emile Laure’s IX Corps d’Armée and Général de Corps d’Armée Louis Eugène Hubert’s XX Corps d’Armée); and Général d’Armée Victor Bourret’s 5ème Armée (Général de Corps d’Armée Aubert Achille Jules Frère’s VIII Corps d’Armée, Général d’Armée Henri Fernand Dentz’s XII Corps d’Armée, Général de Corps d’Armée Onésime Paul Noël’s XVII Corps d’Armée and Général de Corps d’Armée Fernand Joseph Louis Lescanne’s XLIII Corps d’Armée de Forteresse).
Général d’Armée Antoine Marie Benoit Besson’s 3ème Groupe d’Armées (14 divisions) in the south comprised Général d’Armée Jeanny Jules Marcel Garchery’s 8ème Armée (Général de Division Pierre Louis Célestin Michel Champon’s [from 11 May Général de Division Marie Joseph de la Porte du Theil’s] VII Corps d’Armée, Général de Corps d’Armée Georges Henri Jean Baptiste Misserey’s XIII Corps d’Armée and Général de Brigade Julien Maurice Tencé's XLIV Corps d’Armée de Forteresse); and the Réserve de Groupe d’Armée (Général de Corps d’Armée Marius Daille’s XLV Corps d’Armée de Forteresse).
As noted above, in its form as originally conceived 'Gelb' was indeed close to Gamelin’s expectations, but then the crash in Belgium of a liaison aeroplane carrying two German officers with a copy of the current version of the 'Gelb' invasion plan forced Hitler to demand the creation of a new and, he hoped, more radical, alternative. The final plan for 'Gelb', which came to be known generally if unofficially in its key element as 'Sichelschnitt', was based on the thinking of Generalleutnant Erich von Manstein, at the time the chief-of-staff of Generaloberst Gerd von Rundstedt, commanding Heeresgruppe 'A'. Initially rejected by the Oberkommando des Heeres, von Manstein’s plan proposed what was in effect a mirror image of 'Gelb' with a deep penetration, well to the south of the original primary axis of advance, and exploiting the speed and mobility of Panzer divisions to split the Allied forces and allow each of the resulting two portions to be encircled at destroyed separately.
von Manstein’s plan had the virtue of being wholly improbable, from the defenders' militarily orthodox point of view, as it was based on a primary axis through the Ardennes region of southern Belgium, which is an area of steep hills and deep river ravines covered with thick forest, and therefore deemed to be impassable, for practical military purposes, by mechanised forces. 'Sichelschnitt' also had the very real advantage of being unknown to the Allies as no copies were carried between headquarters, and also of being dramatic, a fact which seems to have appealed strongly to Hitler.
'Sichelschnitt' was based on a breakthrough of the weak Allied centre using a great local superiority of forces, after which the mechanised forces would drive to the north and west to the English Channel coast and so trap the Belgian, the British and the best of the French armies in a pocket with their backs to the sea, and only then would the German major forces advance on Paris.
The plan also derived benefit from an Allied response similar to that which they had prepared for 'Gelb', namely the fact that a large part of the French and British strength was drawn to the north to defend Picardy and Belgium. To help ensure this result, the Generaloberst Fedor von Bock’s Heeresgruppe 'B' was still to attack the Netherlands and Belgium, and so draw Allied forces to the east and therefore deeper into the developing encirclement, as well as obtaining bases for a later attack on the UK.
After the original 'Gelb' plans had been captured and assessed, the Allies' senior military and political leaderships were initially jubilant that they had apparently secured a major advantage even before the firing of the first shot. However, Gamelin and Gort were not so sanguine as they realised that the Germans must inevitably develop a new plan which would not be what the Allies had initially expected. Gamelin became ever more convinced that the Germans would try to attempt a breakthrough by concentrating their mechanised forces. Such a force, Gamelin rightly appreciated, could not break through the 'Ligne Maginot' defences on the Allied right wing between Longuyon in the north and the Franco-Swiss border in the south, or overcome the Allied strength on the concentration of forces on the Allied left wing, and this left only the centre. But most of the centre was covered by the Meuse river, and the wisdom of the time had it that mechanised forces were of little avail in defeating fortified river positions. The Meuse river makes a sharp turn to the north-east after its confluence with the Sambre river just upstream of Namur, however, so creating a gap between itself and the Dyle river. This Gembloux gap, between Namur and Wavre, was ideal for mechanised warfare and was thus a dangerous sector of weakness.
Gamelin therefore decided to concentrate half of his armoured reserves in that sector. Of course the Germans might try to overcome the Meuse river position by the use of infantry, but that could be achieved only with the aid of massive artillery support, and the build-up of any such capability would provide Gamelin with ample warning of what might be expected.
The German ground forces for 'Sichelschnitt' were allocated to three army groups: von Rundstedt’s Heeresgruppe 'A' in the centre comprised 45.5 divisions including seven Panzer and three motorised infantry divisions, and was to deliver the decisive blow through the Allied defences in the Ardennes, using the two divisions of General Hermann Hoth’s XV Corps (mot.), the two divisions of General Georg-Hans Reinhardt’s XLI Corps (mot.) and the three divisions of General Heinz Guderian’s XIX Corps (mot.), to create the salient and then the breakthrough in the Allied line.
von Bock’s Heeresgruppe 'B', comprising 29.5 divisions including three Panzer divisions in General Rudolf Schmidt’s XXXIX Corps (mot.) and General Erich Hoepner’s XVI Corps (mot.), was to break through in the Low Countries and push the northern formations of the Allied armies into a pocket.
Generaloberst Wilhelm Ritter von Leeb’s Heeresgruppe 'C', comprising 19 infantry divisions, was to prevent any flanking movement from the east, and to launch holding attacks against the 'Ligne Maginot' and the upper reaches of the Rhine river adjacent to the Swiss border.
'Sichelschnitt' was scheduled for implementation on the morning of 10 May, with a few preparatory moves taking place during the preceding night.
During the night of 9/10 May, German forces successfully and almost bloodlessly occupied Luxembourg, and in the morning of 10 May Heeresgruppe 'B' launched what could be described as a feint offensive into the Netherlands and Belgium, though its objectives were very real. Paratroops of Generalleutnant Kurt Student’s (from 16 May Generalmajor Richard Putzier’s) 7th Fliegerdivision and gliderborne troops of Generalleutnant Hans Graf von Sponeck’s 22nd Luftlande-Division executed surprise landings at the Hague, on the road to Rotterdam and against the Belgian Fort Eben-Emael on the operation’s first day with the goal of facilitating the advance of Heeresgruppe 'B'.
The Allies reacted immediately but erroneously, sending forces north to combat a plan which, as noted above, they expected to resemble the earlier 'Schlieffen' plan. This move to the north in the 'Dyle plan' committed the Allies' best forces, diminished their fighting power through loss of readiness, and their mobility through loss of fuel. That evening French troops crossed the Dutch border.
The French and British air command was less effective than its leaders had anticipated, and the Luftwaffe quickly obtained air superiority, depriving the Allies of any effective reconnaissance capability and disrupting the Allies' fastest means of communication and co-ordination.
While the advancing Germans secured all the strategically vital bridges in and toward Rotterdam, which penetrated the 'Vesting Holland', between Rotterdam and Amsterdam, and bypassed the fortifications of the old 'Waterlijn', an attempt to seize Den Haag, the seat of the Dutch government, ended in complete failure. The Germans took the airfields surrounding the city (Ypenburg, Ockenburg and Valkenburg) on 10 May, although only at the expense of heavy casualties, but lost them on the same day to furious counterattacks launched by the infantrymen of the two Dutch reserve divisions. The Dutch killed or took prisoner 1,745 airborne troops, and later shipped 1,200 prisoners to England. The French marched to the north to establish a connection with the Dutch army, which came under attack from German paratroops but, simply not understanding German intentions, failed to block German armoured reinforcements, in the form of Generalmajor Dr Alfred Ritter von Hubicki’s 9th Panzerdivision, from reaching Rotterdam on 13 May.
The Dutch, their poorly equipped army under General Henri Gerard Winkelman still largely intact, surrendered on the following day after the Germans had 'terror bombed' the port city of Rotterdam. The Dutch troops in Zeeland continued the fight, however, while Queen Wilhelmina established a government-in-exile in the UK, and the forces of the Dutch colonies adhered to this government.
Fort Eben-Emael, hub of the Belgian defensive line, had been seized by German gliderborne troops on 10 May in 'Granit', allowing the German forces to cross the bridges over the Albert Canal, although the arrival of the British Expeditionary Force managed to save the Belgians for a time. Gamelin’s plan in the north was achieved when the British army reached the Dyle river. Then the expected major tank battle took place in the Gembloux gap between the 2ème and 3ème Divisions Légères Mécaniques (light mechanised divisions) of Prioux’s Corps de Cavalerie in Blanchard’s 1ème Armée and Generalleutnant Georg Stumme’s 3rd Panzerdivision and Generalleutnant Generalmajor Joachim Stever’s 4th Panzerdivision of the XVI Corps (mot.), each side losing about 100 vehicles. The German offensive in Belgium seemed stalled, but this was only a feint.
In the centre of the German offensive, Heeresgruppe 'A' had driven through the French infantry regiments and light cavalry divisions defending the Ardennes, and secured the decisive moment on 14/15 May when they won crossings over the Meuse river between Dinant in the north (XV Corps [mot.] with Generalmajor Max von Hartlieb-Walsporn’s 5th Panzerdivision and Generalmajor Erwin Rommel’s 7th Panzerdivision) and Sedan in the south (XIX Corps [mot.] with Generalleutnant Friedrich Kirchner’s 1st Panzerdivision, Generalmajor Rudolf Veiel’s 2nd Panzerdivision and Generalleutnant Ferdinand Schaal’s 10th Panzerdivision) with Monthermé in the centre (XLI Corps [mot.] with Generalmajor Werner Kempf’s 6th Panzerdivision and Generalleutnant Adolf-Friedrich Kuntzen’s 8th Panzerdivision).
For the Allies, the situation at Sedan was most alarming. Instead of slowly massing a force of breakthrough artillery, which could have afforded the Allies time to plan and execute a riposte, the Germans had used the full might of their bomber forces to punch a narrow breach through the French line with a mix of level bombing and dive-bombing. The units of Général de Brigade Henri Jean Lafontaine’s 55ème Division holding this part of the front were routed after many hours of the heaviest air bombardment the world had yet witnessed. Even so, the German infantry units which drove back the French lost as much as 75% of their effective strength. During the night some units in the last French defence line at Bulson were panicked by the false rumour German tanks were already behind their positions.
On the morning of 14 May two French tank battalions counterattacked, but the French armour was repulsed by the first German tanks to cross the river, at 07.20, on pontoon bridges. The news of the crossing spread with extraordinary speed to the men of Général de Brigade Joseph Antoine Jacques Louis Baudet’s neighbouring 71ère Division, whose units panicked and thereby triggered a general rout. That afternoon every available Allied light bomber was employed in an effort to destroy the German bridges but, despite the highest action losses in the entire history of the British and French air forces, failed in this task.
Huntziger, commander of the 2ème Armée, immediately took effective measures to prevent a further weakening of his position. The 3ème Division Cuirassée de Réserve and one motorised division blocked further German advances around his flank. Guderian was not concerned with Huntziger’s flank and, leaving the 10th Panzerdivision to protect the German bridgehead against attacks by the 3ème DCR, moved his 1st Panzerdivision and 2nd Panzerdivision sharply to the west on 15 May, undercutting the flank of Corap’s 9ème Armée by 25 miles (40 km) and forcing the 102ème Division de Forteresse to leave the positions which had blocked the XVI Corps (mot.) at Monthermé. While the 2ème Armée had been seriously mauled and had rendered itself impotent, the 9ème Armée now began to disintegrate, for in Belgium its divisions, not having had the time to dig themselves in, had been pushed back from the river by the unrelenting pressure of German infantry, allowing the 7th Panzerdivision to break free. Général de Brigade Marie Germain Christian Bruneau’s 1ère DCR was sent to block the 7th Panzerdivision's progress, but moving with great speed the German formation surprised the 1ère DCR as its tanks were refuelling on 15 May and dispersed it, despite some losses caused by the heavy French tanks.
This initial stage of the Battle of France has often been described as the first real instance of Blitzkrieg (lightning war): that is the defeat of an enemy by means of a strategic envelopment executed by mechanised forces leading to his operational collapse, and von Manstein certainly had had a strategic envelopment in mind. However, the three dozen infantry divisions following the armoured divisions of the motorised corps were not there merely to consolidate the armour’s gains, and in the Battle of France it was in fact the other way round. In the eyes of the German high command, the Panzer divisions and their parent motorised corps (not formally designated as Panzerkorps until the middle of 1942) had now fulfilled the tasks which had been laid down for them: the Panzer divisions' motorised infantry had secured the river crossings, and their tank regiments had overcome a dominant position. According to the high command, this was the moment for the Panzer divisions to consolidate, allowing the infantry divisions to position themselves for the real battle, possibly in the form of a Kesselschlacht (cauldron battle) should the Allies remain in the north, perhaps an encounter battle should the Allies seek to escape south.
In each case an enormous mass of German divisions, both armoured and infantry, would co-operate to annihilate their opponent in accord with established doctrine. The Panzer divisions were not to bring about the collapse of the Allies by themselves, however, and according to German army doctrine should halt and await the arrival of the infantry divisions.
On 16 May, however, both Guderian and Rommel, in an act of open insubordination against their superiors, disobeyed their explicit orders and moved their divisions to the west as far and as fast as they could manage. Guderian reached Marle, 50 miles (80 km) from Sedan, while Rommel crossed the Sambre river at Le Cateau, 60 miles (100 km) from his bridgehead at Dinant. While nobody knew Rommel’s location, as he had advanced so far that he was out of radio range, an enraged General Ewald von Kleist (commanding the Gruppe 'von Kleist' whose two subordinate formations were the XLI Corps [mot.] and XIX Corps [mot.]) flew to a meeting with Guderian on the morning of 17 May, and after a heated argument relieved him of all duties; von Rundstedt did not approve, however, and refused to confirm von Kleist’s order.
It is difficult to explain the actions of these two commanders. Rommel was forced to commit suicide in 1944 and never revealed his motivation. Interrogated after the end of the war, Guderian claimed that he had acted on his own initiative, essentially inventing Blitzkrieg. Some have since considered this to be an empty boast, denying any fundamental divide within the German operational doctrine of the time, downplaying the conflict as a mere difference of opinion about timing and pointing out that Guderian’s claim is inconsistent with his professed role of being the prophet of Blitzkrieg even before the war. Guderian’s pre-war writings in fact explicitly rejected strategic envelopment by mechanised forces alone as a generally sufficient means to cause operational collapse.
The Panzer divisions now slowed the pace of their advances considerably, but had put themselves in a very vulnerable position. In the terms of conventional warfare they were dangerously stretched between the spearheads in the van and their support forces in the rear, with only the most limited lines of communication and supply, and were exhausted, short of fuel and low in numbers as many of their tanks had broken down. There now was a dangerous gap between them and the infantry. A determined attack by a fresh large mechanised force could have cut them off and wiped them out.
The French high command was still in total disarray after the shock of the German forces' sudden offensive, however, and was already suffering from a sense of defeatism: on the morning of 15 May Paul Reynaud, the French prime minister, telephoned his just-appointed British counterpart, Winston Churchill, and said 'We have been defeated. We are beaten. We have lost the battle.' Seeking to console Reynaud, Churchill reminded the French prime minister of the times the Germans had broken through the Allied lines in World War I only to be stopped. Reynaud was not to be consoled, however. Churchill flew to Paris on 16 May, and immediately perceived the gravity of the situation when he observed that officials of the French government were already burning the governmental archives and preparing for to leave the capital for Bordeaux in the west of the country. In a sombre meeting with the French commanders, Churchill asked Gamelin 'Where is the strategic reserve?', meaning a reserve of the type which had saved Paris in World War I. 'There is none,' Gamelin replied, and Churchill later described this as the single most shocking moment of his life. Churchill asked Gamelin when and where the general proposed to launch a counterattack against the flanks of the German bulge. Gamelin replied that he could not make any such effort for 'inferiority of numbers, inferiority of equipment, inferiority of methods'.
Gamelin was right, and most of the reserve divisions had already been committed. The only armoured division still in reserve, Général de Brigade Albert Charles Emile Bruché's 2ème DCR, attacked on 16 May. The French infantry armoured divisions (divisions cuirassées de réserve) were in fact specialised breakthrough formations, optimised for attacks on fortified positions. They could be quite useful for defence, if dug in, but had very limited utility for an encounter battle: they could not execute combined infantry/tank tactics as they simply had no important motorised infantry component; they had poor tactical mobility as their most important equipment, the Char B1bis heavy tank in which half of the French tank budget had been invested, had to refuel twice a day. So the 2ème DCR was divided into a covering screen, its small sub-units fighting bravely but to no strategic effect.
Some of the best French units in the north had as yet seen little fighting as they had been moved into what was in effect a military vacuum. Had they been kept in reserve they could have been used for a decisive counter stroke. They had lost much fighting power simply by being moved to the north, and a hurried move to the south once again would cost them even more. The most powerful Allied division, the 1ère Division Légère Mécanique ('light' in this context meaning mobile), deployed near Dunkirk on 10 May, had moved its forward units some 135 miles (220 km) to the north-east, beyond the Dutch city of 's-Hertogenbosch, in 32 hours. Finding that the Dutch had already retreated to the north, these forward units had then been withdrawn and the division was now moving to the south. When it finally gained contact with the Germans, the division had serviceable only three of its original complement of 80 SOMUA S-35 tanks, most of the others having succumbed to mechanical problems.
Nevertheless, a radical decision to retreat to the south, avoiding contact, could probably have saved most of the mechanised and motorised divisions, including those of the BEF. This would have required that some 30 infantry divisions be left to their fate, however. The loss of Belgium alone would be an enormous political blow. Moreover, the Allies were uncertain about the intentions of the Germans, who were now threatening in four directions: to the north in a direct assault on the Allied main force; to the west in a move to cut off the Allied main force; to the south for the occupation of Paris; and even to the east for an assault on the rear of the 'Ligne Maginot' defences.
The French decided to create a new reserve including a reconstituted 7ème Armée, under the command of Général de Corps d’Armée Robert Auguste Touchon and using every unit which could safely be withdrawn from the forces holding the 'Ligne Maginot', to block the way to Paris. Colonel Charles André Joseph Marie de Gaulle, in command of the hastily formed 4ère Division Cuirassée de Réserve, attempted to launch an attack from the south and achieved a measure of success that would later accord him considerable fame and a promotion to général de brigade. de Gaulle’s attacks on 17 and 19 May did not materially alter the overall situation, however. While the Allies did little either to threaten them or to extricate themselves from the danger they posed, the Panzer divisions used 17/18 May to recuperate, replenish and undertake essential maintenance.
On 18 May Rommel persuaded the French to abandon Cambrai merely by feinting an armoured attack. On 19 May German high command grew considerably in confidence, in part because the Allies seemed incapable of coping with events. There appeared to be no serious threat from the south: General Franz Halder, chief-of-staff of the Oberkommando des Heeres, even toyed with the notion of attacking Paris immediately to knock France out of the war in one blow. The Allied troops in the north were retreating toward the Escaut river, their right flank giving way to the 3rd Panzerdivision and 4th Panzerdivision, and the Germans realised that it would be foolish to remain inactive for any longer, allowing the Allies to reorganise their defence or even to escape. Now it was time to bring the Allies into even more serious trouble by cutting them off. On the following day the Panzer divisions started to move once more, smashing through Major General B. C. T. Paget’s British 18th Division and Major General W. N. Herbert’s British 23rd Division, which were weak Territorial Army formations, occupying Amiens and securing the westernmost bridge over the Somme river at Abbeville almost on the English Channel coast, so isolating the British, French, Belgian and Dutch forces in the north.
During the evening of 20 May a reconnaissance unit of the 2nd Panzerdivision reached Noyelles, where the Somme river flows into the English Channel, and thereby cutting the Allied armies into two parts.
On the same day Reynaud replaced Gamelin with Général d’Armée Maxime Weygand as commander-in-chief of the French army. Weygand immediately attempted to devise new tactics to contain the Germans, but still more pressing was his need to produce a new and effective Allied strategy to contain the German offensive. This resulted in the formulation of the so-called 'Weygand plan', which was posited on the concept of pinching off the German armoured spearhead by combined attacks from the north and the south between Arras and Albert. This seemed feasible on paper as the corridor along which von Kleist’s two corps had moved to the coast was a mere 25 miles (40 km) wide at this point, and as Weygand seemed to possess sufficient strength to execute the plan: in the north there were three DLMs and the divisions of the BEF, and in the south de Gaulle’s 4ème DCR. These formations had an organic strength of about 1,200 tanks and the Panzer divisions were once again apparently vulnerable as the mechanical condition of their tanks was deteriorating rapidly. In reality, though, the condition of the Allied divisions was far worse: in the south and the north they could in reality muster only handfuls of tanks.
Weygand flew to Ypres on 21 May, and sought to convince the commanders of the British and Belgian forces of the soundness of his plan. On the same day, an armoured detachment of the British Expeditionary Force under the command of Major General H. E. Franklyn, commanding the 5th Division, had already attempted at least to delay the German offensive, and perhaps even to cut off the leading elements of the German army. The resulting Battle of Arras demonstrated the capability of the heavily armoured British Matilda infantry tanks, which were impervious to the shot fired by the Germans' standard 37-mm anti-tank gun, and this limited raid with 58 tanks overran two German regiments. The panic that resulted temporarily delayed the German offensive before the arrival of reinforcements allowed the Germans to drive the British back to Vimy Ridge on the following day.
Although this attack was not part of any co-ordinated Allied attempt to destroy the Panzer divisions, the German high command was considerably more concerned than Rommel had been. For a moment the high command feared that the German spearhead had been deliberately ambushed and that 1,000 Allied tanks were about to smash the army’s elite armoured formations. By the next day the high command had regained its composure, however, and now ordered Guderian’s XIX Corps (mot.) to press forward to the Channel ports of Boulogne and Calais behind the British and Allied forces in the northern pocket. On the same day the French tried to attack in the area to the south-east of Arras, with some infantry and tanks, but by now the German infantry had begun to catch up and the attack was halted, with difficulty, by Generalleutnant Franz Böhme’s 32nd Division. Only on 24 May could the first attack from the south be launched when the 7ème DIC, supported by a handful of tanks, failed to retake Amiens. This was a weak effort, but on 27 May the same fate befell a supposedly more capable formation, Major General R. Evans’s British 1st Armoured Division, hastily brought over from England, as it attacked Abbeville in force but was beaten back with crippling losses. On the following day de Gaulle tried again with the same result.
By now even complete success could not have saved the Allied forces in the northern pocket. In the early hours of 23 May Gort ordered a British retreat from Arras. He had no faith in the 'Weygand plan' or Weygand’s proposal for the Allies to hold a pocket on the Flemish coast as the so-called 'Réduit de Flandres' (Flanders Redoubt). The ports needed to supply such a foothold were already threatened: on 23 May the 2nd Panzerdivision and 10th Panzerdivision assaulted Boulogne and Calais respectively. The British garrison in Boulogne surrendered on 25 May, although 4,368 troops were evacuated. Though strengthened by the arrival of the 3rd Royal Tank Regiment (equipped with cruiser tanks) and Brigadier C. Nicholson’s 30th Brigade, Calais fell to the Germans on 27 May.
While the 1st Panzerdivision was ready to attack Dunkirk on 25 May, on the previous day Hitler had ordered this formation to halt. This remains one of the most controversial decisions of the entire war. Generalfeldmarschall Hermann Göring, the commander-in-chief of the Luftwaffe, had convinced Hitler that his air formations could prevent an evacuation, and von Rundstedt had also warned the German leader that any further effort by the Panzer divisions would lead to a much prolonged refitting period before any further offensive operations could be undertaken. Moreover, operations against and in urban areas was not part of the normal task for armoured units under any operational doctrine.
The British now launched 'Dynamo' and 'Aerial' as twin efforts to evacuate the British, Belgian and French forces trapped against the seas in north-western Belgium and north-eastern France (between de Panne in Belgium and Dunkirk in France) and along the rest of the French coast, starting on 26 May. The Allied position was further complicated by the surrender of Belgium, on the orders of King Léopold II, who was also the Belgian commander-in-chief, on the following day but postponed until 28 May. Confusion still reigned, however, as after the evacuation at Dunkirk, and while Paris was enduring its short-lived siege before being declared an open city, Major General A. G. L. McNaughton’s Canadian 1st Division and Major General J. S. Drew’s British 52nd Division were landed at Brest in Brittany and moved 200 miles (320 km) inland toward Paris before the discovered that the French capital had fallen and France had capitulated. The two divisions then retreated and re-embarked for England.
At the same time that the Canadian 1st Division landed in Brest, the Canadian-manned No. 242 Squadron of the RAF flew its Hawker Hurricane fighters to Nantes, some 100 miles (160 km) south-east, and from there provided air cover.