The 'Battle of San Pietro Infine', more commonly known as the 'Battle of San Pietro', was a major engagement in Italy between Allied forces attacking from the south and German forces holding strongly fortified positions in the 'Gustav-Linie' in and around the town of San Pietro Infine, just to the south of Monte Cassino about mid-way between Naples and Rome (8/17 December 1943).

The Allied victory in this battle was crucial in the ultimate drive to the north on the western side of Italy. The battle was the first in which men of the Royal Italian army fought as co-belligerents of the Allies following the armistice of September 1943 in which, among other things, the Italians defected from the Axis. The original town of San Pietro Infine was destroyed in the battle.

The Allied invasion of Italy from the south followed the Allied successes in North Africa. General Sir Bernard Montgomery’s British 8^th Army had advanced from the east in 'Guillotine' and 'Fire Eater' following the '2^nd Battle of El Alamein' and the British-American invasion of French North-West Africa by Lieutenant General K. A. N. Anderson’s Allied 1^st Army in 'Torch' had led to the surrender of almost 250,000 Axis troops in May 1943.

The Germans and Italians retreated to the island of Sicily, and on the night of 9/10 July 1943, an Allied armada of 2,590 vessels launched one of the largest combined operations of World War II as 'Husky' (i). The invasion was launched by the US 7^th Army, under the command of Lieutenant General George S. Patton, and the British 8^th Army, still under the command of Montgomery. Both armies were under the control of the Allied 15^th Army Group, commanded by General Sir Harold Alexander. Over the next five weeks, 500,000 Allied soldiers, sailors and airmen fought German and Italian forces for control of the island. Although the Allied powers were victorious, the Axis partners managed to evacuate more than 100,000 men and 10,000 vehicles from Sicily across the Strait of Messina during the first 17 days of August. The Allies then invaded the Italian mainland early in September 1943 at Salerno in 'Avalanche', in Calabria in 'Baytown' (i) and at Taranto in 'Slapstick'.

On 8 September, before the main invasion at Salerno by Lieutenant General Mark W. Clark’s US 5^th Army, which included significant British elements, Italy’s surrender to the Allies was announced. Italian units ceased to fight, and the Royal Italian navy’s main warships sailed to Allied ports to surrender. This effected a great change in Germany’s defensive strategy, and the Germans now regarded their former allies as enemies and in 'Achse' (ii) moved to disarm Italian units and occupy important defensive positions. 'Avalanche' was ultimately successful, although the Allies suffered heavy losses, and the Allies subsequently captured nearby Naples on 1 October. German forces then withdrew to the north, toward Rome, and dug in along a series of well-fortified lines. By a time late in 1943, the fighting had reached the 'Gustav-Linie', also known as the 'Winter Line'.

The German commander-in-chief in Italy, Generalfeldmarschall Albert Kesselring, had marked out the 'Gustav-Linie' as three parallel defensive systems to the south of Rome. These defensives lines were the 'Reinhard-Linie', 'Gustav-Linie' and 'Hitler-Linie', which were constructed with an interval of some 11 miles (18 km) between them and taking maximum advantage of the point at which the Italian peninsula is at its narrowest: the three lines served as a formidable series of obstacles in the path of the Allied march to the north through Italy. The 'Reinhard-Linie' (or 'Bernhard-Linie') was the most southerly of the three and was the German fallback position from the 'Barbara-Linie' and 'Volturno-Linie' forward defences farther to the south as Germans retreated as slowly as possible up the peninsula. The 'Reinhard-Linie' was in reality a southern bulge in the stronger 'Gustav-Linie' to the north. On the eastern side, the 'Reinhard-Linie' extended from the Sangro river to the Adriatic Sea, along which length it was identical to the 'Gustav-Linie'. On the western side, the line bulged to the south from Cassino to incorporate the mountains overlooking the approaches to the Liri river valley and then extended to the west to the mouth of the Garigliano river. The line passed directly through the town of San Pietro Infine, blocking the Mignano gap, the pass through which the railway and Route 6, the primary routes up the centre of Italy from Naples to Rome, ran toward Cassino and the entrance to the Liri river valley.

The Germans occupied San Pietro in September 1943 to prepare the defences. They evacuated all non-essential Italians from the town, conscripted able-bodied men to help construct the defences, and requisitioned available vehicles and beasts of burden. They established a defensive apparatus in the whole area, in particular on Mt Sambúcaro (generally shown as Sammucro on Allied maps) and Mt Lungo, which overlooked the Mignano gap. These were strategically important positions, for they provided control of the long stretch of Route 6, important for the advance of the Allies. The 5^th Army began to attack the 'Reinhard/Bernhardt-Linie' on 5 November, and the attacks continued into December.

The 'Battle of San Pietro' was preceded by Allied attacks on the Camino hill massif at the entrance to the Mignano gap, so named for the small town on the road at that point. The entire massif is about 6.2 miles (10 km) long and 3.7 miles (6 km) wide. After that, the main Allied effort was directed against the German defences on Mt Sambúcaro and Mt Lungo, which dominate the narrow valley on the north-east and south-west respectively. The US assault on Mt Lungo was aided, in the first instance of the war, by the Italian 1^st Motorised Group, part of the recently reconstituted Italian army now fighting on the side of the Allies.

The direct attack on the German positions in and around San Pietro was launched on 8 December by Major General Geoffrey T. Keyes’s US II Corps of the 5^th Army, and the positions were defended by two battalion-sized elements of the 15^th Panzergrenadierregiment of Generalleutnant Walter Fries’s 29^th Panzergrenadierdivision and one battalion of Generalleutnant Wilhelm Raapke’s 71^st Division, all part of General Fridolin von Senger und Etterlin’s XIV Panzerkorps within General Joachim Lemelsen’s 10^th Army.

After a week of intense attacks and counterattacks, the 143^rd Infantry of Major General Fred L. Walker’s US 36^th Division, the US 3^rd Ranger Battalion and the US 504^th Parachute Regimental Combat Team had gained control of the heights of the Sambúcaro massif. The 36^th Division then planned a further effort for 15 December: the 143^rd Infantry, assisted by the 504^th Parachute Regimental Combat Team, was to continue to push to the west along the shoulders of Sambúcaro and take San Vittore del Lazio, while to the south of Route 6 the 142^nd Infantry, supported by the Italian 1^st Motorised Group, was to take Mt Lungo. In the centre, the 141^st Infantry was to attack San Pietro itself. The 36^th Division’s main attack began at 12.00 on 15 December. In an effort to break the German defences in the town, two platoons of the 753^rd Tank Battalion attacked with 16 Sherman medium tank and tank destroyers. The armoured attack failed in the face of mines and anti-tank fire, and only four of the 16 armoured fighting vehicles survived. After four successive Allied attacks and German counterattacks, the Germans pulled back from San Pietro since the dominating ground on each flank, Mt Lungo and the Sambúcaro peaks, was now in the hands of the II Corps. The Germans launched a counterattack on 16 December to cover their withdrawal to positions farther to the north at Cedro hill, Mt Porchia, San Vittore and the western spurs of Sambúcaro.

The 'Battle of San Pietro' had been part of the overall campaign to breach the 'Bernhardt/Reinhard-Linie', some 6.2 miles (10 km) deep at that point. It took six weeks of heavy fighting from a time early in November to a time late in December, to overcome the German defences. During that time, the 5^th Army sustained 16,000 casualties. The road and railway routes through the Mignano gap to the Liri river valley had become known as 'Death Valley' to the men of the attacking forces. The battle had wholly destroyed the town of San Pietro Infine through a combination of close combat, both Allied and German mortar and artillery, and German scorched-earth policy.

By a time in the middle of January 1944, the 5^th Army had reached the formidable defenses of the 'Gustav-Linie' and on §7 January embarked on the '1^st Battle of Monte Cassino'.