- Theatre: Mediterranean
- Dates: 17 January to 18 May 1944
- Location: Central Italy
- Outcome: German forces were dislodged from the Gustav Line, opening the way to Rome.
- Note: The abbey of Monte Cassino, which was destroyed by Allied bombing, was founded in AD 524. The abbey's treasures were removed to Rome earlier in the war
- Allies: General Oliver Leese's 8th Army (consisting of 5th Corps, 10th Corps and 13th Corps); General Mark Clark's US 5th Army (consisting of 2nd Corps and the British 10th Corps, supported by 6th Corps)
- Germany: General Heinrich von Vietinghoff's German 10th Army; General Eberhard von Mackensen's 14th Army
By the end of 1943, the Allied advance northwards into Italy had forced the Germans back to the fourth and best fortified of their defensive lines. The Gustav Line ran through the mountainous regions of Abruzzi and Campania, south west of Rome.
At the centre of the line, blocking the route to Rome, was the town of Cassino, dominated by the mountain of Monte Cassino with its 1,400 year old Benedictine abbey. The abbey had been evacuated by the Germans following the Allied landings; both the Germans and the Allies had assured the Vatican that it would not be put to military use or attacked.
In December 1943 General Bernard Montgomery returned to Britain in preparation for the Normandy landings; his place as commander of the 8th Army was taken by General Oliver Leese.
In January 1944 General Harold Alexander, the overall commander of US and British armies in the area, launched a two-part assault on the Gustav Line. On 17 January Clark's 10th Corps, including American, British and French Moroccan troops, broke through the Gustav Line west of Monte Cassino.
The attack was to be supported by 6th Corps, which was to establish and then move out from a beachhead at Anzio, 95km (60 miles) behind the Gustav Line. The line was breached in several places, but the crucial valley headed by Monte Cassino remained under German control, and 6th Corps did not succeed in breaking out from Anzio.
On 15 February, the 8th Army made a renewed assault on the mountain using Indian and New Zealand troops. The attack was repulsed, but it had been preceded by a heavy bomber attack which all but destroyed the abbey. As well as being unjustified in military terms, the bombing was counter productive: the Germans did not use the abbey until after it had been bombed, when they started using its ruins for shelter. A second assault on 15 March was preceded by aerial bombardment of the town of Cassino; the result was a stalemate.
Alexander launched Operation Diadem, a final co-ordinated assault on the Gustav Line, on 11 May. While 5th Army made a flanking attack to the south, aiming to converge with a breakout from Anzio by the 6th Corps, the 8th Army made a frontal assault on the line at Cassino, using British, Canadian and Indian troops.
In addition, Cassino was outflanked by French Moroccan 8th Army troops in the west and a Polish division in the north. Kesselring ordered German withdrawal on 16 May; the Poles entered Cassino two days later.
With the breakout from Anzio finally achieved on 23 May, the reunited armies pursued the Germans north. Rather than attempting to encircle the retreating German forces, Clark directed the 5th Army onward towards Rome. The Caesar Line, Kesselring's last defensive line south of Rome, was breached on 2 June; two days later the city fell to the 5th Army.
Marked by outstanding military achievement in appalling conditions, the battles of Monte Cassino opened the road to Rome and the beginning of the end for the German occupation of Italy. But the human and material cost was high, and the Allies had not succeeded in seriously disrupting the Germans' staged withdrawal to prepared defensive positions.
After the war, the Allies insisted that the bombing of the abbey had been justified, and that they had solid evidence that the buildings had been used as a part of German defences. A 1949 report concluded that no such evidence existed, but it was kept from the public until 30 years later.