The 'Gustav-Linie' was the German strategic defence line in Italy stretching from a western end on the Tyrrhenian Sea coast at the mouth of the Garigliano river between Gaeta and Mondragone, running up the line of the Garigliano and Rapido rivers past Cassino into the Apennine mountains, descending to the plain on the eastern side of the country in the region of Casoli, and then paralleling the line of the Sangro river just to its north to arrive at an eastern end on the Adriatic Sea coast between Fossacesia and San Vito (5 November 1943/17 May 1944).
The defence of this line, which added formidable built elements to natural obstacles, was entrusted by Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber 'Süd' (from 16 November 1943 the Oberbefehlshaber 'Südwest') and commander from 26 November of the re-formed Heeresgruppe 'C', to Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10th Army, which deployed General Hans-Valentin Hube’s XIV Panzerkorps (three infantry and two Panzergrenadier divisions) on the Tyrrhenian side and General Traugott Herr’s LXXVI Panzerkorps (one parachute, one infantry and two Panzergrenadier divisions) on the Adriatic side. The former was intended to halt the five infantry divisions (two British, two US and one French colonial) of Lieutenant General Sir Richard McCreery’s British X and Major General John P. Lucas’s US VI Corps of Lieutenant General Mark W. Clark’s US 5th Army, and the latter to check the five divisions (two British, one Indian, one Canadian and one New Zealand) of Lieutenant General Sir Oliver Leese’s British XIII Corps and Lieutenant General C. W. Allfrey’s British V Corps of General Sir Bernard Montgomery’s British 8th Army.
Along the whole of the 'Gustav-Linie' defences, the Germans could deploy an eventual 18 divisions (with another three forming) to the Allied total of 13 divisions, which together with the natural and man-made strengths of their positions gave the Germans a decided tactical and operational edge.
The defences of the western end of the 'Gustav-Linie' were stronger than those of the eastern end, and here the Allied forces were faced by immense difficulties of approach over rain-swollen rivers and the 'Barbara-Linie' and 'Bernhard-Linie' preliminary defence positions before they could approach the bastion represented by the German positions in the mountains on each side of the Liri river and around Monte Cassino. To the Allies, the whole German defence complex comprising the 'Barbara-Linie', 'Bernhard-Linie', 'Gustav-Linie' and 'Führer-Senger-Linie', the last a switch line behind the western end of the 'Gustav-Linie', were known to the Allies as the 'Winter Line'.
The 5th Army attacked on 20 November 1943 and suffered heavy losses for the rest of the year before being halted some 5 miles (8 km) from the 'Gustav-Linie' positions to the south of the Rapido. At the other end of the line the 8th Army was enjoying greater success, for Montgomery reinforced his right wing with two divisions pulled out of the left wing and forced the Sangro river on 15 November, then closed in rapidly on the sector of the 'Gustav-Linie' between Lanciano and the sea. Here the V Corps broke through the defence line and pushed forward to take Ortona on 27 December. The offensive continued until 15 January 1944, and by this time the sector of the 'Gustav-Linie' between Casoli and the sea had been penetrated by Lieutenant General Sir Bernard Freyberg’s New Zealand 2nd Division, Major General G. C. Bucknall’s British 5th Division, Major General C. Vokes’s Canadian 1st Division and Major General C. F. Keightley’s British 78th Division, with Major General D. Russell’s Indian 8th Division following close behind them in the centre of this sector.
On the western end of the line the four Cassino battles (including 'Avenger', 'Dickens' and 'Diadem' as the second, third and fourth) raged until May 1944 before the Allied broke through the 'Gustav-Linie' defences between Valvori in the mountains and Minturno on the coast against a reinforced defence now undertaken in the inland sector by General Valentin Feurstein’s LI Gebirgskorps and in the coastal sector by Generalleutnant Fridolin Ritter und Edler von Senger und Etterlin’s XIV Panzerkorps.
The battle began to open as the 5th Army approached the 'Gustav-Linie' positions along the lower reaches of the Garigliano river at the beginning of 1944, though farther inland in was still held up in front of the 'Bernhard-Linie' outlying positions in front of the Rapido river. The 'Shingle' landing at Anzio was about to happen, and General the Hon. Sir Harold Alexander, commanding the Allied 15th Army Group (from 11 January 1944 the Allied Forces in Italy), ordered a full-scale offensive against the German positions on the western end of the 'Gustav-Linie', confidently expecting that this pressure, in combination with that to be exerted by the VI Corps at Anzio in January, would force Kesselring to order a withdrawal from the 'Gustav-Linie' in the western sector.
The Allies struggled to reach the Rapido river and the 'Gustav-Linie' proper, but finally the VI Corps, supported by Général de Corps d’Armée Alphonse Pierre Juin’s Corps Expéditionnaire Français (French Expeditionary Corps), reached the line and made it possible for Alexander’s first offensive to begin on 17 January as the 1st Battle of Monte Cassino with probing attacks that took Major General P. G. S. Gregson-Ellis’s British 5th Division and Major General G. W. R. Templer’s 56th Division of the X Corps through the outlying positions of the 'Gustav-Linie' close to the coast at Minturno and Castelforte respectively. Inland the offensive began on 20 January with Major General Geoffrey T. Keyes’s US II Corps (replacement for the VI Corps pulled out of the line for 'Shingle') and on 24 January with the Corps Expéditionaire Français. This latter suffered very heavy losses but got Général de Division André Dody’s 2ème Division d’Infanterie Marocaine and Général de Division Joseph de Goislard de Monsabert’s 3ème Division d’Infanterie Algérienne through the 'Gustav-Linie', while the Americans enjoyed the mixed fortune of success with Major General Charles W. Ryder’s 34th Division and failure with Major General Fred L. Walker’s 36th Division, which was repulsed with very heavy losses.
The Allied offensive finally slowed to a stop as the Germans suffered no compulsion to withdraw after the initial failure of 'Shingle', and the US II Corps was in turn replaced by Freyberg’s New Zealand Corps, which renewed the offensive between 15 and 18 February as the 2nd Battle of Monte Cassino in 'Avenger' without appreciable success.
Alexander was well aware that his attempt to storm through the 'Gustav-Linie' defences at Cassino had failed, and called a halt to further operations while the situation was reassessed and Allied forces were reinforced and reshuffled for the 3rd Battle of Monte Cassino which began as 'Dickens' (i) on 15 March but yet again failed.
By the time the offensive was renewed on 11 May as the 4th Battle of Monte Cassino in 'Diadem', therefore, the Allied disposition was (from the coast inland) the US II Corps, Corps Expéditionnaire Français, British XIII Corps, Polish II Corps and British X Corps. On 11 May the Allies went over to the offensive along the 20-mile (32-km) sector from the sea to a point inland of Cassino, and while the British X Corps and the Polish II Corps managed to get across the Rapido and through the 'Gustav-Linie' defences before being contained, the US II Corps and the Corps Expéditionaire Français broke through the 'Gustav-Linie' in more open country and pressed ahead. The Americans were checked at Santa Maria Infante, but the French pushed on and looked set to cut the German lines of communication.
Kesselring saw that the German position was now lost, and ordered a withdrawal on 17 May. On the following day the Poles finally stormed into Cassino, and with the Allied breakthrough of the 'Gustav-Linie' and exploitation to the north, the US VI Corps beleaguered at Anzio could finally break out in 'Buffalo' to link with the rest of the 5th Army.
There can be no doubt that the western end of the 'Gustav-Linie' defences gave the Allies the hardest fighting of the whole Italian campaign, and the result was often in doubt because of the enormous strength of the Germans' natural and man-made positions.