The whole story of Dunkirk raises many questions, and some odd answers.
German ground forces could have easily over-run the defenders, who would have no choice but to surrender en-masse, leaving England without a large part of the army.
Why didn't they? Some say it was cockyness on the part of the Germans, who figured the English would concede defeat if they were evacuate.
Whatever the reason, and it may even have been divine intervention as some suggest, they were wrong.
Another thing that always troubled me was the absence of The Home Fleet, with Battleships, Cruisers, and Desrtroyers covering the operation, and hammering the Germans at the same time.