On 16 September 1940, the 45th Infantry Division was federalized into the Active duty force.The division's two combat commands, the 89th and 90th Brigades were not activated, as the Army favored smaller and more versatile regimental commands for the new conflict.

The Division was based around the 157th Infantry Regiment, the 179th Infantry Regiment, and the 180th Infantry Regiment. Also assigned to the division were the 158th, 160th, 171st, and 189th Field Artillery Battalions, the 46th Signal Company, the 700th Ordinance Company, the 45th Quartermaster Company, the 45th Reconnaissance Troop, the 120th Engineer Combat Battalion, and the 120th Medical Battalion.