The 45th Infantry Division was an infantry division of the British Army, formed just prior to the start of the Second World War. In March 1939, after the re-emergence of Germany as a significant military power and its occupation of Czechoslovakia, the British Army increased the number of divisions in the Territorial Army (TA) by duplicating existing units. The 45th started forming in August 1939 and became active the following month, as a second-line duplicate of the 43rd (Wessex) Infantry Division. The division's battalions were all raised in the West Country.

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.