Battle of Dunkirk (Operation Dynamo, May–June 1940)

Despite warnings from senior officers, Adolf Hitler ordered the attack on the Benelux countries and France ("Fall Gelb") on 10 May 1940. In the north, Army Group B under Colonel General Fedor von Bock advanced through Belgium and the Netherlands.

Following the advance of German Army Group A under Gerd von Rundstedt across the Ardennes, which came as a surprise to the Allies, and the forcing of the crossing of the Meuse at Sedan (→ Battle of Sedan), tank units under General Ewald von Kleist reached the scene of the Battle of the Somme in the First World War on 19 May. At the same time, Army Group B in the north continued its advance through Belgium. Due to the German breakthrough in the south, the commander-in-chief of the Allied Northern Group Gaston Billotte, to whom the British and Belgian armies were subordinate, ordered a withdrawal from the Dyle line to the Scheldt on 16 May. Hitler and the generals of the Wehrmacht were surprised by the rapidity of the advance of their units. With the advance of the XIX Army Corps of Panzer Group Kleist under Heinz Guderian, it became apparent on 18 May that the main thrust of Army Group A's attack was neither the Maginot Line to the southeast nor Paris to the south: Guderian rolled west, toward the Atlantic coast.

Since in the south the French 3rd Army Group largely confined itself to a defense of the Somme line, it thereby exposed the southwestern flank of the Allied northern Army Group. This made it clear that the British Expeditionary Force (BEF) under Lord Gort, the Belgian Army, and the French 1st Army and 7th Army could be separated from the main French force in the south. The way was open for the German armoured divisions to roll up the rear areas of the Allied Northern Group and capture the Channel ports of Calais and Boulogne. On 19 May, the Royal Navy, on the instructions of the British War Cabinet under Winston Churchill and following a suggestion by Lord Gort, began preparations for a rescue operation. Vice-Admiral Bertram Ramsay was entrusted with the planning, and the evacuation of 300,000 soldiers was anticipated during the planning phase.

Attempts to close the gap between the Somme in the south and the Scarpe in the north, through which the German armored units were advancing, were as unsuccessful as a British-led counterattack at Arras on 21 May, which used up the last armored reserves of the Northern Army. Already on 20 May, the 2nd Panzer Division of the XIX Army Corps had reached the Channel coast at Abbeville. This left the Allied Northern Group of some 1,200,000 men (63 divisions: 29 French, 22 Belgian and 12 British) trapped between the Somme and the sea by Army Group A to the south and Army Group B to the east. On the German side, the decision was now made to turn north in order to tighten the encirclement ring and take the Channel ports.

Lord Gort had a decision to make: either join the French allies in battle and put Britain's main force at risk, or try to escape by sea. Although in doing so he went against the interests of the French army leadership above him, he suggested by telegram to the British Minister of War, Anthony Eden, that an evacuation be attempted. Gort nevertheless led the French commander-in-chief, Maxime Weygand, to believe for several days that British troops would join in a double offensive to reunite the Northern Group with the main French forces, which would in turn have cut off the German armored divisions from their communications.

Two million Belgian and eight million French civilians were fleeing the German Wehrmacht and hindering the movement of the Allied armies.

On 22 May Guderian's tanks launched the attack in the direction of Calais and by 24 May were only 18 kilometres from Dunkirk. Unexpectedly, von Rundstedt, confirmed by Hitler during a visit to the front on the same day, ordered the tanks to halt. Such halting orders had occurred before in the course of the Western campaign, most recently on 17 May. They were intended as a pause for the top Panzer echelon, which often rushed ahead without adequate escort, to consolidate with the remaining units. There was also a fear that a coordinated action by the British in the north and the French in the south might include the tank top. That the last British battle tanks had long since been parked at Arras was unknown to von Rundstedt. Field Marshal Hermann Göring also announced that he would destroy the troops by air attacks alone. This suggestion suited von Rundstedt, as he wanted to save the tanks for the upcoming Battle of France ("Fall Rot") and give the troops, exhausted by the rapid advance, a rest.

The reasons for the halt order of 24 May are still controversial today. Some historians attribute it to Hitler's sheer eccentricity, claiming that as Führer he wanted to assert himself as the supreme authority over the army command. That he would sacrifice a certain military triumph for this reason, however, is unlikely. Other attempts at explanations or theories (for example: the trapped British troops could serve as a pledge for possible peace negotiations with the British) are considered similarly unlikely.

To von Rundstedt's astonishment, on an order from Generaloberst Walther von Brauchitsch, the army's supreme commander, the tanks in the south and southwest of the encirclement ring were placed under the command of Army Group B in order to place the troops involved in the encirclement of the enemy under a common high command. This Army Group, approaching from the east and pushing the Belgian Army northward, had until then only the 9th Panzer Division at its disposal. Army Group A was now to concentrate with its other armies (2nd, 12th, and 16th) on securing the Somme-Aisne Line, which was a perfectly sensible move on the part of the OKH, but von Brauchitsch had issued this order without Hitler's knowledge or approval. Hitler immediately reversed this order, which was to take effect at 8 p.m. that same evening: Army Group B would be able to cope in this section without tanks. Hitler also snubbed von Brauchitsch by transferring further operational command to Army Group A, thus temporarily disempowering the OKH. This added to the confusion about the direction of the thrust of the German armored units lying west of the Aa River.