The Gustav Line was a staunch defensive line built by the Germans that spanned from the Tyrrhenian Sea to the Adriatic Sea. The Gustav Line ran along the Garigiliano and Rapido rivers on the west and on the Sangro river on the east side of the Italian peninsula. The line was defended by 15 German divisions fortified with small arms, artillery, pill boxes, machine gun emplacements, minefields and barbed wire. The German divisions had retreated to and fortified this line after the Allied invasion of Italy. In order to reach Rome, the "center of gravity" of the Italian resistance, the Allies had to push through this line to cut the communication lines of the German Army and open the way to Rome.
The Germans called the Gustav Line a "string of pearls anchored by Monte Cassino." Anchoring the Gustav Line, Monte Cassino was identified by the Germans and Allies as key terrain because of the outstanding observation it provided over the entrance to the Liri Valley. From Monte Cassino, one can see every road and river crossing at the mouth of the Liri Valley. Increasing the complexity of the Monte Cassino terrain was the Benedictine monastery built on top of it. The monastery, the source of the Benedictine Order of monks, was built around 529. The Rapido River formed part of the Gustav Line, acting as a natural moat, protecting Monte Cassino.
