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The British Expeditionary Force (BEF) was the British force in Europe from 1939–1940 during the Second World War. Commanded by General Lord Gort, the BEF constituted one-tenth of the defending Allied force.

The British Expeditionary Force was started in 1938 in readiness for a perceived threat of war after Germany annexed Austria in March 1938 and the claims on the Sudetenland, which led to the invasion of Czechoslovakia in March 1939. After the French and British had promised to defend Poland, the German invasion of that country began and war was declared on 3 September 1939.

The BEF was sent to France in September 1939 and deployed mainly along the Belgian—French border during the so-called Phoney War leading up to May 1940. The BEF did not commence hostilities until the invasion of France on 10 May 1940. After the commencement of battle, they were driven back through Belgium and north-western France, forcing their eventual evacuation from several ports along the French northern coastline in Operations Dynamo, Ariel and Cycle. The most notable evacuation was from the Dunkirk region and from this the phrase Dunkirk Spirit was coined.