After the Phoney War, the Battle of France began in earnest on 10 May 1940. To the east, the German Army Group B had invaded the Netherlands and advanced westward. In response, the Supreme Allied Commander—French General Maurice Gamelin—initiated "Plan D" and invaded Belgium to engage the Germans in the Netherlands. The plan relied heavily on the Maginot Line fortifications along the German-French border, but the Germans had already crossed through most of Holland before the French forces arrived. Thus, Gamelin committed the forces under his command, three mechanised armies, the French First and Seventh and the British Expeditionary Force (BEF) to the River Dyle. On 14 May, German Army Group A burst through the Ardennes and advanced rapidly to the west toward Sedan, then turned northward to the English Channel, in what Generalfeldmarschall Erich von Manstein called the "Sickle Cut" (known as "Plan Yellow" or the Manstein Plan), effectively flanking the Allied forces.[3]
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 (BEF) near Armentières, the French 1st 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.
In one of the most widely-debated decisions of the war, the Germans halted their advance on Dunkirk. Contrary to popular belief, what became known as "the Halt Order" did not originate with Adolf Hitler. Gerd von