A summary of the WWII events this day:

German armored columns, led by General Guderian (a tank expert), severed all communication between the British Expeditionary Force (BEF) in the north and the main French army in the south. He also cut off the BEF from its supplies in the west. The Germans now faced the sea, England in sight.

• German Army Group A, with 38 infantry and 7 armored divisions, was the main column under the command of Gerd von Rundstedt. This group was to march through the Ardennes.
• German Army Group B, with 26 infantry and 3 armored divisions, was to invade the Low Countries under the command of Fedor von Bock. Though strong, this force was considered diversionary.
• German Army Group C, with 19 infantry divisions, attacked the Maginot Line under the direction of Wilhelm von Leeb to pin down the French forces there.
• After the disaterous Battle of France, French General Gamelin was replaced by General Maxime Weygand.

From Walter Lord’s book, The Miracle of Dunkirk, British Army Chief Edmund Ironside confronts BEF Commander Gort on strategy:

Army Chief Ironside arrived at BEF Commander Gort’s Command Post at 6am. With the War Cabinet’s order backing him up, he told Gort that his only chance was to turn the army around and head south for Amiens [instead of north to Dunkirk]. If Gort agreed, he’d issue the necessary orders at once.

But Gort didn’t agree. For some moments, he silently pondered the matter, then explained that the BEF was too tightly locked in combat with the Germans to the east. It simply couldn’t turn around and go the other way. If he tried, the enemy would immediately pounce on his rear and tear him to bits.

Then, asked Ironside, would Gort at least spare his two reserve divisions for a push south which might link up with a French force pushing north? Gort thought this might be possible, but first they must coordinate the effort with General Billotte, the overall [Allied/French] commander for the area.

Taking Pownall [Gort Chief of Staff] in tow, Ironside now rushed down to French headquarters at Lens. He found Billotte with General Blanchard of the French First Army—both in a state of near collapse. Trembling and shouting at each other, neither had any plans at all. It was too much for the volcanic Ironside. Seizing Billotte by the buttons on his tunic, he literally tried to shake some spirit into the man.

Ultimately it was agreed that some French light mechanized units would join Gort’s two reserve divisions in an attack tomorrow south of Arras. They would then meet up with other French forces pushing north. A command change at the very highest level should help: the placid Gamelin had finally been replaced by General Maxime Weygand. He was 73, but said to be full of fire and spirit.

Ironside now returned to London, convinced that once the two forces joined, the way would finally be opened for the BEF to turn around and head south—still his pet solution for everything. Gort remained unconvinced, but he was a good soldier and would try.