On Friday 24th May 1940 Hitler paid a visit to Army Group A headquarters and endorsed the order of the previous day.
The German forces had captured the port of Boulogne and had now surrounded Calais. The engineers of the 2nd Panzer Division had built five bridges over the Canal Line and only one British battalion barred the way to Dunkirk.
Göring urged Hitler to let the Luftwaffe (aided by Army Group B) finish off the British, to the consternation of Franz Halder, who noted in his diary that the Luftwaffe was dependent upon the weather and air crews were worn out after two weeks of battle. Rundstedt issued another order. That was sent un-coded and was picked up by the RAF at 12:42: “By order of the Fuhrer … attack north-west of Arras is to be limited to the general line Lens-Bethune-Aire-St Omer-Gravelines. The Canal will not be crossed.”
Later that day, Hitler issued Directive 13, which called for the Luftwaffe to defeat the trapped Allied forces and stop their escape.