The Winter Line was a series of German and Italian military fortifications in Italy, constructed during World War II by Organisation Todt and commanded by Albert Kesselring. The series of three lines was designed to defend a western section of Italy, focused around the town of Monte Cassino, through which ran the important Highway 6 which led uninterrupted to Rome. The primary Gustav Line ran across Italy from just north of where the Garigliano River flows into the Tyrrhenian Sea in the west, through the Apennine Mountains to the mouth of the Sangro River on the Adriatic coast in the east. The two subsidiary lines, the Bernhardt Line and the Hitler Line, ran much shorter distances from the Tyrrehnian Sea to just northeast of Cassino where they would merge into the Gustav Line. Relative to the Gustav Line, the Hitler Line stood to the northwest and the Bernhardt Line to the southeast of the primary defenses.

Monte Cassino is a rocky hill about 130 kilometres (80 mi) southeast of Rome, in the Latin Valley, Italy, 2 kilometres west of Cassino and at an elevation of 520 m (1,710 ft). Site of the Roman town of Casinum, it is widely known for its abbey, the first house of the Benedictine Order, having been established by Benedict of Nursia himself around 529. It was for the community of Monte Cassino that the Rule of Saint Benedict was composed.