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.
In August 1939, the 45th Division began to take form and became active the next month, serving as a duplicate of the 43rd (Wessex) Infantry Division. Its battalions were recruited from the western regions of England.