British Expeditionary Force (World War II)

The British Expeditionary Force (BEF) was the contingent of the British Army sent to France in 1939 after Britain and France declared war on Nazi Germany on 3 September, beginning the Second World War. The BEF existed from 2 September 1939 when the BEF GHQ was formed until 31 May 1940, when GHQ closed down and its troops reverted to the command of Home Forces. During the 1930s, the British government had planned to deter war by abolishing the Ten Year Rule and rearming from the very low level of readiness of the early 1930s. The bulk of the extra money went to the Royal Navy and the Royal Air Force but plans were made to re-equip a small number of Army and Territorial Army divisions for service overseas.

General Lord Gort was appointed to the command of the BEF on 3 September 1939 and the BEF began moving to France on 4 September 1939. The BEF assembled along the Belgian–French border. The BEF took their post to the left of the French First Army under the command of the French 1st Army Group (fr:Groupe d'armées n° 1) of the North-Eastern Front (Front du Nord-est). Most of the BEF spent the Phoney War (3 September 1939 to 9 May 1940) digging field defences on the border. When the Battle of France (Fall Gelb) began on 10 May 1940, the BEF constituted 10 per cent of the Allied forces on the Western Front.

The BEF participated in the Dyle Plan, a rapid advance into Belgium to the line of the Dyle River, but the 1st Army Group had to retreat rapidly through Belgium and north-western France, after the German breakthrough further south at the Battle of Sedan (12–15 May).