Battle of Dunkirk
The Battle of Dunkirk occurred in May 1940 between a British and French force against a German armored army that had advanced to the Channel coast at Calais in France thus cutting off the British and French soldiers. In this attack over 330,000 Allied troops were evacuated by 900 British ships to England. It was considered to be a pivotal moment in World War II and the first of many Nazi blunders. Hitler had his armies stop instead of overrunning Dunkirk so his airforce (the Luftwaffe) could finish the job and share in the glory of the victory.
The results of a British surrender are unknown, but could quite possibly have changed the course of the war.