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 10^th 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 5^th 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 8^th 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 5^th 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 8^th 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 2^nd Division, Major General G. C. Bucknall’s British 5^th Division, Major General C. Vokes’s Canadian 1^st Division and Major General C. F. Keightley’s British 78^th Division, with Major General D. Russell’s Indian 8^th 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.