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How did the British Expeditionary Force become trapped near Dunkirk?

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Nazi leaders in Paris — Battle of France Adolf Hitler (center) in front of the Eiffel Tower in Paris shortly after signing an armistice with France, June 1940.

The British Expeditionary Force (BEF) became trapped near Dunkirk as a result of a surprise German offensive during the Battle of France.

The encirclement was primarily achieved through a bold German strategy—proposed by Erich von Manstein—that involved a concentrated armoured thrust through the Ardennes Forest. While Allied forces were drawn forward into Belgium to counter a diversionary attack, German forces bypassed the Maginot Line and broke through French defenses near the Meuse River.123

German panzer divisions raced westward toward the English Channel, reaching Amiens and Abbeville by May 20, 1940. This maneuver effectively severed all communications between Allied forces in the north and south, cutting off the BEF and other Allied units in Belgium. As the German forces turned north along the coast, they trapped the Allied armies against the sea, leaving Dunkirk as the only remaining port available for evacuation.345