The 'Battle of San Marino' was fought between British and German-led forces after German forces occupied the neutral Republic of San Marino in northern Italy, and were then attacked by Allied forces (17/20 September 1944). The engagement is also known on occasion as the 'Battle of Monte Pulito'.

The tiny state of San Marino had declared its neutrality earlier in the war, and had remained broadly unaffected by events in Europe until 1943, when Allied forces had advanced a considerable distance up the Italian peninsula. A major German defensive position, the 'Gotisch-Linie', extended across the peninsula a short distance to the south of the Sammarinese border, and late in June the country was bombed by the Royal Air Force, resulting in the deaths of 35 people, in the belief that German forces had taken up positions in San Marino. In 'Olive' (iii), which was launched late in August, a strong Allied force attacked at the very eastern end of the line, aiming to pass through Rimini, just to the east of San Marino, and break out onto the plains to the north of the enclave state. While San Marino lies to the south-west of Rimini, the Allied plan was that the little state should be bypassed entirely. In response to the Allied movements, however, the Germans sent a small force into San Marino to guard their lines of communication and act as artillery observers.

After a few days, the main thrust of the British offensive was halted south of Rimini by strong resistance and severe weather, and the British and Indian flanking forces began to push to the westward, thereby taking the front line toward San Marino. On 17 September Major General A. W. W. Holworthy’s Indian 4^th Division attacked forces of Generalleutnant Harry Hoppe’s 278^th Division holding two hills just across the Sammarinese border. After heavy fighting for control of these hills, the situation stabilised on 19 September, and British-led forces began to push into the city of San Marino itself. The city had been captured by the afternoon of 20 September, and the Indian 4^th Division left the country on 21 September, leaving it under the control of the local defence forces.

Up to this time, San Marino had played little part in World War II. It had a Fascist government, closely aligned with that of Benito Mussolini in Italy, but remained neutral. It was reported to have declared war against the UK in September 1940, although the Sammarinese government later transmitted a message to the British government stating that it had not done so. Early in 1942, the Sammarinese government reiterated it was not at war with the USA, a position confirmed by the US Department of State. The British Foreign Office noted more equivocally in 1943 that the UK had never declared war on San Marino but also had never formally recognised San Marino’s neutrality, and that it felt that military action on Sammarinese territory would be justified if it were being used by Axis forces.

As noted above, the country was bombed on 27 June 1943, killing at least 35 persons. The Sammarinese government declared the same day that no military installations or equipment were located on its territory, and that no belligerent forces had been allowed to enter. Early in July, it announced that prominent signs had been put up at the border crossings by the German command, to instruct German units not to enter the territory, and again reiterated its complete neutrality.

By the late summer of 1944, German forces in Italy had withdrawn toward the 'Gotisch-Linie' chain of well-sited and strongly defended positions stretching across the Italian peninsula. The Allies formulated a plan to break through the defences, pushing north toward Rimini and the plains of northern Italy. This would involve 'Olive' (iii) as a strong thrust up the eastern seaboard by Lieutenant General Sir Oliver Leese’s British 8^th Army: 11 divisions would attack along a narrow front, converging on the 'Rimini gap', an 8-mile (13-km) stretch of plain along the coast around the city, and then move to the north; once through the 'Rimini gap', the British-led forces would deploy outward onto the Romagna plain, and move to the west against Bologna. Meanwhile, Lieutenant General Mark W. Clark’s US 5^th Army was to drive northward along the centre of the peninsula, hopefully converging on Bologna and trapping a large German force in a pincer movement.

The main Allied assault began on 25 August, reaching the Foglia river valley and thus the 'Gotisch-Linie' proper, two days later. The German defences were quickly breached, and the German command attempted to assemble a second defensive line on the Coriano ridge, a hilly spur to the north of the Conca river, and the last major geographic obstacle to the south of Rimini. The Allied offensive reached the river on 3 September, but then ground to a halt as a result of mechanical difficulties with its tanks, strengthening German resistance, and terrain made very difficult by heavy rain. The Allied forces used the opportunity to bring forward reinforcements as they awaited the opportunity to resume the offensive along the coast. On the left flank of the assault, the attack had been halted in the 'Battle of Gemmano', to the south of the Conca river.

At this point, the forces on the 8^th Army’s left wing, comprising Lieutenant General C. F. Keightley’s V Corps, were strung out in a line running due south from the Coriano ridge, facing west toward San Marino a few miles distant. Major General J. Y. Whitfield’s 56^th Division was opposite Croce, with Major General J. L. I. Hawkesworth’s 46^th Division opposite the heavily defended position at Gemmano. The Indian 4^th Division was to the south of the 46^th Division, forming the very left wing of the offensive. When the assault on Coriano was resumed on 12 September, led by two armoured divisions with heavy artillery support, these forces pushed to the west with the object of passing through toward the town of Montescudo, about 2 miles (3.2 km) from the Sammarinese border. The main assault successfully pushed onto the ridge, and the 56^th Division advanced about 1 mi (1.6 km) past Croce, before digging in on the evening of 13 September. That night, the Indian 4^th Division gained a foothold to the south of Gemmano, which was finally captured by the 46^th Division and the Indian 4^th Division on the morning of 15 September, and the British forces prepared to move toward Montescudo and exploit the German confusion.

The 46^th Division took Montescudo on 15 September, and on the next day the 56^th Division entered the town of Mulazzano, directly to the north of Montescudo and equally close to the border. The fighting pressed westward from here, with the 56^th Division on the northern flank and the 46^th Division on the south, but both formations were held back by strong German resistance.

On 13 September, Generaloberst Gottfried-Heinrich von Vietinghoff-Scheel’s 10^th Army had sent a strong force into San Marino to defend it against the Allies, provide them control of one of the major roads in the area, and allow artillery observers to be sited on the mountain peaks. The defending force was drawn from the 278^th Division, an element of General Traugott Herr’s LXXVI Corps, and the Indian 4^th Division was tasked to attack it on 17 September.

The leading division’s leading battalion, the 3/10^th Baluch Regiment, crossed the Marano river on the eastern border on the night of 17/18 September, with the 1/9^th Gurkha Rifles moving through it to attack Points 343 and 366 near Faetano. These small hills, lying just behind the river, were held by two battalions of the 993^rd Grenadierregiment. Point 343 was taken at 05.00, but the force occupying Point 366 had to fall back after running short of ammunition. Point 343 was held through 18 September, though only with the loss of 63 men. By the evening, a tank force had managed to come up and stabilise the position with the aid of artillery support. The 4/11^th Sikh Regiment moved around the Gurkhas to the north, covering the northern flank of the San Marino heights, and the division’s Indian 11^th Brigade, commanded by Brigadier H. C. Partridge, passed through to help encircle the city. During the evening of 19 September, the Indian 11^th Brigade’s 2/The Queen’s Own Cameron Highlanders began to push into the outskirts of the city from the north, but early in the morning of 20 September was held back by defensive positions in the north-west of the city, where the road to the upper part of the city, situated higher on the mountain, began. Tanks moved into the city’s outer parts, while one company of the Cameron Highlanders moved uphill toward the summit in heavy rain. The city had been secured by a time the early in afternoon, with only 24 casualties among the attackers, and 54 German prisoners taken.

On 21 September, the local defence force was enlisted to help mop up German stragglers, and the Indian 4^th Division pressed onward through a heavy gale and left the country.

Allied forces remained in occupation of San Marino for a short period following the German surrender in May 1945. In October 1945, after the end of the war, the Sammarinese government submitted a claim for 732 million lira to the British government for wartime compensation, of which 500 million lira were given as costs associated with the fighting in September and 20 million lira as the costs of the occupation. The British government rejected this claim, arguing that as Germany had breached the Sammarinese neutrality before Allied troops had entered the country, it was not liable. The British did, however, offer an ex gratia payment of £26,000 in regard to the June bombing, and later increased the figure to £80,000.