The 'Grüne-Linie' was the German defence line, 170 miles (275 km) long, in the Apennine mountains of central Italy to cover the valley of the Po river, and also known as the 'Gotisch-Linie' defences until in June 1944 Adolf Hitler ordered a change of name lest a major defence line with so quintessentially a German name should be broken by the Allied armies (June 1944/March 1945).

Using more than 15,000 slave labourers, the Germans created the 'Grüne-Linie' with more than 2,000 well-fortified machine gun nests, casemates, bunkers, observation posts, and artillery fighting positions to repel any attempt to breach it.

Following the almost simultaneous 'Diadem' breakthrough on the 'Gustav-Linie' and 'Buffalo' break-out from the Anzio lodgement in the spring of 1944, the 11 nations representing the Allies in Italy finally had a chance to trap the Germans in a pincer movement and to realise some of Prime Minister Winston Churchill’s strategic goals for the long, costly campaign against the Axis 'underbelly'. This would have required Lieutenant General Mark W. Clark’s US 5^th Army to commit most of his Anzio forces to a drive to the east from Cisterna, and to execute the envelopment envisaged in the original planning for the Anzio landing, and thereby turn the western flank of Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army and sever its line of retreat from Cassino to the north. Instead, fearing that General Sir Bernard Montgomery’s British 8^th Army might beat him to Rome, Clark diverted a large part of his Anzio force in that direction in an attempt to ensure that he and the 5^th Army would have the honour of liberating the 'Eternal City'.

As a result, most of the forces controlled by Generalfeldmarschall Albert Kesselring slipped the noose and fell back to the north, fighting delaying actions as they did so, notably late in June on the 'Trasimeno-Linie' running from a point just to the south of Ancona on the east coast, past the southern shores of Lake Trasimeno near Perugia and on to the west coast to the south of Grosseto, and in July on the 'Arno-Linie' running from the west coast up the line of the Arno river and into the Apennine mountains to the north of Arezzo. This gave the Germans the time they needed to consolidate the 'Gotisch-Linie' as a belt of fortifications 10 miles (16 km) deep and extending from a point to the south of La Spezia on the west coast to the Foglia river valley, through the natural defensive wall of the Apennine mountains, which ran unbroken nearly from coast to coast some 50 mi (80 km) deep and with high crests and peaks rising to 7,000 ft (2135 m), to the Adriatic Sea between Pesaro and Ravenna on the east coast.

The emplacements included numerous concrete-reinforced gun pits and trenches, and 2,376 machine-gun nests with interlocking fields of fire, 479 anti-tank, mortar and assault gun positions, 131,235 yards (120000 m) of barbed wire entanglements, and many miles of anti-tank ditches. This last redoubt proved the German determination to continue fighting.

Nevertheless, it was fortunate for the Allies that at this stage of the war the Italian partisan forces had become highly effective in disrupting the German preparations, especially in the higher parts of the mountains. By September 1944, German generals were no longer able to move freely in the area behind their main lines because of partisan activity.

Construction of the defences was also hampered by the deliberately poor quality of the concrete provided by local Italian mills, and captured partisans forced into the construction gangs supplemented the natural lethargy of forced labour with clever sabotage. Nevertheless, before the Allied attack Kesselring had declared himself satisfied with the work, especially on the Adriatic side.

Thus comprising a number of major strongpoints connected by extensive field fortifications, this defensive line ran north-west/south-east across Italy from the area of La Spezia and Viareggio on the Tyrrhenian Sea along a line some 22 miles (35 km) north of the Arno river in its western sector, past Pistoia to Pesaro on the Adriatic Sea.

The Germans had high expectations for this formidably constructed and powerfully defended strategic defence line, along which they could muster a total of 26 divisions against 20 Allied divisions, but in the event the Allies managed to penetrate it without undue difficulty and casualties in 'Olive'.

First to punch through was Lieutenant General Sir Oliver Leese’s British 8^th Army, which had some 10 divisions allocated to Lieutenant General E. L. M. Burns’s Canadian I Corps, Generał dywizji Władysław Anders’s Polish II Corps, Lieutenant General C. F. Keightley’s British V Corps and Lieutenant General Sir Richard McCreery’s British X Corps, with one more division in army reserve. These forces approached the 'Grüne-Linie' positions in August 1944, and Leese considered that his formations would find it easier to punch through in the inland sector than along the coast, and he was proved right when the Canadian I Corps and British V Corps broke through between Osteria Nuova and Montecalvo on 29 August, advancing through the junction of General Valentin Feurstein’s LI Gebirgskorps and General Traugott Herr’s LXXVI Panzerkorps of von Vietinghoff-Scheel’s 10^th Army, advancing rapidly to the dire battles of the Gemmano and Coriano Ridges in the middle of September.

Clark’s US 5^th Army (nine divisions in Lieutenant General S. C. Kirkman’s British XIII Corps, Major General Geoffrey T. Keyes’s US II Corps and Major General Willis D. Crittenberger’s US IV Corps) had a harder time pushing through the 'Grüne-Linie' positions in the sector held by General Alfred Schlemm’s I Fallschirmkorps and General Fridolin Ritter und Edler von Senger und Etterlin’s XIV Panzerkorps of General Joachim Lemelsen’s 14^th Army, and then wore itself out in desperate attempts to reach Bologna before the arrival of winter and the end of the useful campaigning season.

In the end the line stabilised temporarily on 15 January 1945 with the 8^th Army firmly through the 'Grüne-Linie' positions and halted along the line of the Senio river, and the 5^th Army checked along a less well defined line from Firenzuola in the Apennines via Vergato on the southern side of the Po river valley to the Tyrrhenian coast between Livorno and La Spezia. A small portion of the 'Grüne-Linie' positions along the Tyrrhenian coast was still in German hands at this time, which both sides used to recuperate before the arrival of spring and the next campaigning season. In the command sector, Clark succeeded Field Marshal the Hon. Sir Harold Alexander as commander-in-chief of the Allied 15^th Army Group on the latter’s elevation to Supreme Allied Commander in the Mediterranean Theatre, while Lieutenant General Lucian K. Truscott and McCreery assumed command of the 5^th and 8^th Armies respectively.