The German campaign plan Fall Gelb (Case Yellow) had evolved into a decoy operation in the Netherlands and Belgium, with the main effort through the Ardennes.

Instead of consolidating bridgeheads on the west bank of the Meuse, the Germans began an advance down the Somme river valley towards the English Channel.

The Allies made some early gains and panicked a number of German units but after an advance of up to 6.2 mi (10 km), they were forced to withdraw after dark to avoid encirclement.

The attack was a failure but had a disproportionate effect on Hitler and Oberkommando der Wehrmacht (OKW, German armed forces high command).

The Allies used the pause to reinforce the Channel Ports, prevent their rapid capture and fortify the western approaches to Dunkirk before the Germans arrived, making the evacuation of the British and French forces in Operation Dynamo possible.

Army Group A (Generaloberst Gerd von Rundstedt) defeated the French at the Battle of Sedan from 12 to 15 May and crossed the Meuse.

The German counter-attacks were supported by Fliegerkorps VIII (Generaloberst Wolfram von Richthofen) and the French lost 32 tanks and armoured vehicles.

[2] German spearheads broke through the Peronne–Cambrai gap and threatened Boulogne and Calais, cutting the lines of communication of the Allied armies of Groupe d'armées 1 (General Gaston Billotte) in the North-East Theatre of Operations (Général d'armée Alphonse Joseph Georges), separating them from the main French armies south of the Somme.

[5] On the evening of 19 May, the French Prime Minister Paul Reynaud had sacked Gamelin and replaced him with General Maxime Weygand.

Rommel ordered Panzer-Regiment 25 to probe towards Acq 6.2 mi (10 km) forward with two motorised infantry regiments to follow later, which left most of the division without tanks.

D'Astier was also unable to contact the First Army headquarters or the HQ of the Air Component RAF of the BEF, which was out of action, as it was moving back to bases in England.

[14] The attack had been intended to be made by two French and two British infantry divisions but Frankforce comprised only about 15,000 men, after the detachment of brigades to the defence of Arras, the river to the east and into reserve.

A Right Column of the 7th RTR, 8th DLI, 365th Battery, 92nd Field Regiment RA, 260th Battery, 65th (Norfolk Yeomanry) Anti-Tank Regiment RA, a platoon of the 151st Brigade Anti-Tank Company and a motor-cycle scout platoon of the 4th Battalion, Northumberland Fusiliers and a Left Column of the 4th RTR, 6th DLI, 368th Battery, 92nd Field Regiment RA, 206th Battery, 52nd Anti-Tank Regiment RA, a platoon of the 151st Brigade Anti-Tank Company, one company and a scout platoon of the 4th Northumberland Fusiliers, were to cross the Arras–Doullens road at 2:00 p.m., which meant that the infantry would have to make an 8 mi (13 km) approach march to reach the jumping-off point, over roads filled with traffic and refugees.

Berneville to the south was also captured and a party of the 7th RTR and 8th DLI pressed on to the Arras–Doullens road, where they met part of Schützen-Regiment 7 and troops of the SS-Totenkopf Division.

[15] Junkers Ju 87 Stukas belonging to I, and III Gruppen, Sturzkampfgeschwader 2 dive-bombed British forces at Arras.

Another 2 mi (3.2 km) further on at Achicourt, six Matildas overran a line of anti-tank guns and then the column kept going to Agny and Beaurains, before a party reached Wancourt on the Cojeul.

[24] At around 4:00 p.m. (German time) II Battalion, Schützen-Regiment 7 was attacked by about 40 British tanks, which were engaged by artillery on Hill 111, roughly 0.62 mi (1 km) north-west of Wailly.

[25] The maximum depth of the British advance was 10 mi (16 km); 400 German prisoners had been taken, many tanks and much equipment had been destroyed but two Matilda IIs had been knocked out.

The garrison in Duisans also retired after dark, in the Bren carriers of the 9th DLI, covered by the anti-tank guns of the brigade reserve in Marœuil.

The Left Column troops in Agny and Beaurains were bombed and then attacked by tanks as they retreated, one party missing the road (and eventually reaching Boulogne).

[27] Attacking to the south-west was rejected and OKH ordered Panzergruppe von Kleist to capture Boulogne and Calais, about 50 mi (80 km) away.

XIX Corps attacked again at 8:00 a.m. on 22 May but was still hampered by a "near paralysis" in the German command, Rundstedt ordering that the instructions from OKH were to be ignored and the advance on Boulogne and Calais would have to wait.

The 6th Panzer Division signalled that "a strong enemy force was making a breakthrough" which caused alarm Panzergruppe Kleist.

[35] Frieser wrote that the Franco–British counter-attack at Arras, had a disproportionate effect on the Germans because the higher commanders were apprehensive about flank security.

Colonel-General Günther von Kluge, the 4th Army commander, ordered the tanks to halt, with the support of Rundstedt.

On 22 May, when the attack had been repulsed, Rundstedt ordered that the situation at Arras must be restored before Panzergruppe von Kleist moved on Boulogne and Calais.

Most of the BEF and the French First Army were still 62 mi (100 km) from the coast but despite delays, British troops were sent from England to Boulogne and Calais just in time to forestall the XIX Corps panzer divisions on 22 May.