The Gustav Line was the most rearward of the three German defensive lines on the Italian peninsula south of Rome. Built along the Garigliano and Rapido rivers it was fortified with gun pits, concrete bunkers, turreted machine-gun emplacements, barbed wire and minefields. The Gustav Line was held by 15 divisions of the German Army under the command of General Albert Kesselring.

Several attempts were made to breach the western flank of the Gustav Line during January 1944 when General John Lucas and US 5th Army landed at Anzio. However, it was not until May 1944 that Operation Diadem under the leadership of General Harold Alexander that Allied forces managed to breakthrough the Gustav Line.