Hitler surrounded the English army on the atlantic coast and could have captured/massacred them. Instead, he let them go, to show a gesture of good faith to Churchill, who then insisted that the war continued.
Hitler wasn't trying to give a gesture of good will. He was stalled by the French army who valiantly fought the Germans to a stand still and made sure the British could retreat in good order.

The fact that the British could retreat was because they still ruled the Channel and at least the fear of their fleet kept the Germans at bay. That the British people helped the British army retreat speaks to how tenacious the people were. An flotilla of small personal boats and yachts helped rescue the men from the beaches.

One of my great uncles was at both Dunkirk and Normandy. So I've had second hand accounts of it growing up from my mother.