The Gustav Line was a line of German military fortifications which was built by Organization Todt in southern Italy during World War II. It ran across Italy from north of where the Garigliano River flows into the Tyrrhenian Sea in the west, to the mouth of the Sangro River on the Adriatic coast in the east. The Gustav Line was fortified with concrete bunkers, gun pits, machine-gun pillboxes, minefields, anti-tank obstacles, and barbed wire, and was held by 15 divisions of the German Army, commanded by General Albert Kesselring.
The center of the Gustav Line included Monte Cassino, where there was an old abbey on top. There were two subsidiary lines on the western side of the Apennines: the Bernhardt Line in front of the main Gustav positions and the Hitler Line some 5 miles to the rear. Allied forces finally managed to break through the Gustav Line in May 1944 through Operation Diadem, but at a high cost.