Plan for a New Offensive
The strategy decided upon by the Allied leaders, an amphibious landing behind the Gustav Line, had been under consideration from the time when German intentions in Italy became clear. By late October 1943 it was evident that the Germans intended to compel the Allied forces to light a slow, costly battle up the peninsula. To meet this situation, Allied staffs began to consider a plan for landing behind the enemy lines, with the purpose of turning the German Bank, gaining a passage to the routes to Rome, and threatening the enemy lines of communication and supply. On the Eighth Army front, a small-scale amphibious landing at Termoli on 2-3 October 1943 furnished a pattern for such an attack.
On 8 November 1943 General Alexander ordered the Fifth Army to plan an amphibious landing on the west coast. The target date was set at 20 December. The landing, to be made by a single division, was to be the third phase of an over-all operation in Italy. In the first phase the Eighth Army was to carry out an offensive which would put it astride Highway No.5, running from Pescara on the Adriatic coast through Popoli and Collarmele toward Rome. The second phase would be a Fifth Army drive up the Liri and Sacco Valleys to capture Frosinone. Dependent on the progress of the first two phases, a landing south of Rome directed toward Colli Laziali (the Albanese Mountains) would be made, to link up with the forces from the south. Because of tenacious German opposition and difficult terrain, the Eighth and Fifth Armies in the Winter Line campaign could not reach their assigned objectives. This situation, together with the lack of available landing craft, made the plan for an immediate amphibious end-run impracticable, and the project was abandoned on 20 December 1943.
The slow progress of the Allied advance led to the revival of the plan for an amphibious operation south of Rome along the lines previously contemplated. At Tunis on Christmas Day the chief Allied military leaders drafted new plans for an amphibious landing below Rome with increased forces and the necessary shipping. Two divisions, plus airborne troops and some armor over twice the force originally planned-were to make the initial assault between 20 and 31 January, but as near 20 January as possible to allow a few days latitude if bad weather should force postponement. The amphibious operation was again to be coordinated with a drive from the south , which would begin earlier.
Main Fifth Army, reinforced by two fresh divisions from the quiescent Eighth Army front, was to strike at the German Tenth Army across the Garigljano and Rapido Rivers, breach the Gustav Line, and drive up the Liri Valley. This offensive was planned in sufficient strength to draw in most of the available German reserves. While the enemy was fully occupied in defending the Gustav Line, the surprise landing would be made in his rear at the twin resort towns of Anzio and Nettuno, about thirty miles south of Rome. Once established, the assault force was to thrust inland toward the volcanic heights of Colli Laziali. The capture of Colli Laziali would block vital enemy supply routes and threaten to cut off the German troops holding the Gustav Line. The Allied leaders believed that the Germans lacked sufficient strength to meet attacks on two fronts and that they would be forced to rush troops northward to meet the grave threat to their rear. Thus weakened, the Germans could be forced to withdraw up the Liri Valley from their Gustav Line positions. Eighth Army, though depleted of two divisions which were to go to the Fifth Army front, was to make a show of force along its front in order to contain the maximum number of enemy forces. If possible, Eighth Army would reach Highway No.5 and develop a threat toward Rome through Popoli by 20 January. Main Fifth Army was to follow up the anticipated enemy withdrawal as quickly as possible, link up with the beachhead force, and drive on Rome.
The area chosen for the amphibious landing was a stretch of the narrow Roman coastal plain extending north from Terracina across the Tiber River. Southeast of Anzio this plain is covered by the famous Pontine Marshes; northwest toward the Tiber it is a region of rolling, often wooded, farm country. The 3,100-foot hill mass of Colli Laziali lies about twenty miles inland from Anzio and guards the southern approaches to Rome. Highway No.7 skirts the west side of Colli Laziali; on the southeast the mountains fall away into the low Velletri Gap leading inland toward Highway No.6 at Valmontone. The main west-coast railways parallel these highways. On the east side of the Velletri Gap rise the peaks of the Lepini Mountains which stretch along the inner edge of the Pontine Marshes toward Terracina.
An area roughly seven miles deep by fifteen miles wide around Anzio was to form the initial Allied beachhead. Its 26-mile perimeter was considered the maximum which could be held by the initial assault force and yet include the best natural features for defense. In the sector northwest of Anzio the beachhead was bounded by the Maletta River. Here the low coastal plain was cut up by a series of rough-hewn stream, gullies, the largest of them formed by the Maletta and the Incastro Rivers running southwest from the higher ground inland toward the sea. These gullies, though their small streams were easily fordable, were often fifty feet deep and offered difficult obstacles to armor. In the central beachhead sector, east of the first overpass on the Anzio-Albano road, the line ran 6,000 yards across a broad stretch of almost level open fields to meet the west branch of the Mussolini Canal below the village of Padiglione. This stretch of open country leading inland along the Albano road formed the best avenue of approach into or out of the beachhead and was to be the scene of major Allied and German attacks.
Between Cisterna and Littoria the plain merged with the northern edge of the Pontine Marshes, a low, flat region of irrigated fields interlaced with an intricate network of drainage ditches. The treeless, level expanse offered scant cover for troops, and during the rainy season the fields were impassable to most heavy equipment. From Padiglione east the entire right flank of the initial beachhead line was protected by the Mussolini Canal, which drains the northern edge of the Pontine Marshes. The line ran east along the west branch of the canal to its intersection with the main branch and from there down the main branch to the sea. The canal and the Pontine Marshes made the beachhead right flank facing Littoria a poor avenue of attack; this flank could be held with a minimum of forces.
Most of the beachhead area was within an elaborate reclamation and resettlement project. The low, swampy, malarial bogland of the Pontine Marshes had been converted into an area of cultivated fields, carefully drained and irrigated by an extensive series of canals and pumping stations. Only in the area immediately north of Anzio and Nettuno had the scrub timber, bog, and rolling grazing land been left untouched. At regular intervals along the network of paved and gravel roads crisscrossing the farmlands were the standardized 2-story podere, or farmhouses, built for the new settlers. Such places as the new community center of Aprilia, called the "Factory" by Allied troops, and the provincial capital of Littoria, were modernistic model towns. The twin towns of Anzio (ancient Antium) and Nettuno in the center of the beachhead were popular seaside resorts before the war.