If the troops surrounded at Dunkirk had been captured, the United Kingdom would have had no alternative but to conclude an armistice (like France did) and Hitler would have achieved victory in the west. In the event, the German Army was ordered to halt for a vital 48 hours, allowing Allied forces to defend Dunkirk and then escape across the English Channel. The reason for the delay has been much debated. Suffice to say Marshals Goring and Rundstedt advised Hitler to issue the Halt Order for, what seemed at the time, sound military reasons. The order, however, is not the point, its effect is.
If Britain (as well as France) had made peace in 1940, the Soviet Union would have had to face the Nazi invasion of 1941 alone. It would, nonetheless, have still been victorious because of its numerical superiority in soldiers, tanks and aircraft (thanks to Stalin’s Five Year Plans) as well as its vast area. Germany was a piranha trying to swallow a whale: whichever way the dice fell, it could not succeed. In addition Hitler then crassly declared war against the United States in December 1941. The Miracle of Dunkirk was, therefore, not as crucial as some allege!