The Battle of Dunkirk was the defense and evacuation of the British and Allied forces that had been separated from the main body of the French defenses by the German advance.

After the seven months of the Phoney War, the Battle of France began in earnest on 10 May 1940. To the east, the German Army Group B invaded and subdued the Netherlands and advanced westwards through Belgium. On the 14 May, Army Group A burst through the Ardennes region and advanced rapidly to the west toward Sedan, then turned northwards to the English Channel, in what Generalfeldmarschall Erich von Manstein called the "sickle cut" (known as the Manstein Plan).

A series of Allied counter-attacks, including the Battle of Arras, failed to sever the German spearhead, which reached the coast on 20 May, separating the British Expeditionary Force near Armentières, the French First Army, and the Belgian army further to the north from the majority of French troops south of the German penetration. After reaching the Channel, the Germans swung north along the coast, threatening to capture the ports and trap the British and French forces before they could evacuate to Britain.