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.

The Army Reserve is the active-duty volunteer reserve force of the British Army. It is separate from the Regular Reserve whose members are ex-Regular personnel who retain a statutory liability for service. The Army Reserve was known as the Territorial Force from 1908 to 1921, the Territorial Army (TA) from 1921 to 1967, the Territorial and Army Volunteer Reserve (TAVR) from 1967 to 1979, and again the Territorial Army (TA) from 1979 to 2014.