1940 May 24th. German advance stopped.

German advance halted

By 24 May the Germans captured the port of Boulogne and surrounded Calais. Captain (later Generaloberst) Heinz Guderian's engineers of the 2nd Panzer Division constructed five bridgeheads over the Canal Line, and only a single Britishbattalion blocked the Germans from seizing the port facilities at Dunkirk.  At this point, at the urging of Rundstedt and Commander in Chief of the Luftwaffe Hermann Göring, Hitler issued the Halt Order: the Panzers were ordered to stop their advance. After the war, several commanders, including Rundstedt and OKH Chief of Staff General Franz Halder, tried to put the blame for this decision exclusively on Hitler. However, according to Rundstedt's official war diary, Rundstedt ordered the halt, concerned about the vulnerability of his flanks and the question of supply to his forward troops. He was also concerned that the marshy ground around Dunkirk would prove unsuitable for the use of tanks, and he wished to save some of the armour for the upcoming advance on Paris. Both these concerns were shared by Hitler, who merely validated the order several hours after the fact. Hitler was urged by Göring to let the Luftwaffe (aided by Army Group B  finish the British off, to the consternation of Halder, who noted in his diary that the ability to effectively use the air force was dependent upon the weather. In addition, the pilots were worn out after two weeks of battle. The order was sent uncoded, and was picked up by the British: "By order of the Fuhrer ... attack northwest of Arras is to be limited to the general line Lens-Bethune-Aire-St Omer-Gravelines. The Canal will not be crossed."

Army Group B and the Luftwaffe were unable to complete their mission due to heavy activity in the area by the Royal Air Force, a lack of motorised transport that slowed the advance of the infantry, and bad weather. On 26 May, Hitler ordered the Panzer groups to continue their advance, but the delay allowed the construction of defences vital to the evacuation. 

The Halt Order has been the subject of much discussion by historians. Guderian considered the failure to order a timely assault on Dunkirk to be one of the major mistakes the Germans made on the Western Front.  Rundstedt called it "one of the great turning points of the war", and Manstein described it as "one of Hitler's most critical mistakes".