The 12th (Eastern) Infantry Division was an infantry division of the British Army, which fought briefly in the Battle of France during the Second World War. In March 1939, after the re-emergence of Germany as a European power and its occupation of Czechoslovakia, the British Army increased the number of divisions within the Territorial Army by duplicating existing units. The 12th (Eastern) Infantry Division was formed in October 1939, as a second-line duplicate of the 44th Infantry Division.

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.