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.
Forming in August 1939, the 45th Division came into existence the next month, functioning as a supplementary duplicate of the 43rd (Wessex) Infantry Division. Its battalions were all recruited from the Western area.