The 'Senger-Linie' was a German defence line on the western side of south central Italy, previously known as the 'Führer-Linie' (spring 1944).
The line was held by General Fridolin Ritter und Edler von Senger und Etterlin’s XIV Panzerkorps (Generalleutnant Eberhard Rodt’s 15th Panzergrenadierdivision, Generalleutnant Wilhelm Raapke’s 71st Division and Generalleutnant Bernhard Steinmetz’s 94th Division) within Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10th Army of Generalfeldmarschall Albert Kesselring’s Heeresgruppe 'C'.
The line had been designed as a fall-back position behind the 'Gustav-Linie' defences, the main German defence line of this area, and covered the entrance to the Liri river valley. Thus lying to the north of the 'Gustav-Linie', the 'Senger-Linie' extended approximately parallel with, but between 10 and 15 miles (16 and 24 km) to the north of, the 'Gustav-Linie'. The 'Senger-Linie' defences were based on the Tyrrhenian Sea at Terracina, and then extended inland between the Monti Ausoni and Monti Aurunci into the valley of the Liri river (from Pico to Piedimonte San Germano via Pontecorvo on the Liri). The line then ran just to the north of Cassino before ending at Terelle on the western slopes of the Apennine mountains.
The 'Senger-Linie' was schemed as the last German barrier to any Allied advance up the valley of the Liri river toward Rome.
The breaking of this line constituted the second of the three phases of the 'Diadem' plan devised by General the Hon. Sir Harold Alexander’s Allied 15th Army Group (from 11 January Allied Forces in Italy, from 18 January Allied Central Mediterranean Force, and from 9 March Allied Armies in Italy): the first phase was the defeat of the Germans at Cassino and the breaching of the 'Gustav-Linie', and the third phase a rapid exploitation of success in the second phase to reach Valmontone, in conjunction with a break-out from the Anzio beach-head by Major General Lucian K. Truscott’s (up to 28 February Major General John P. Lucas’s) US VI Corps, and so cut off the 10th Army but, as conceived, was not intended to take Rome, which had political significance but in itself no genuine military value.
With Lieutenant General Sir Oliver Leese’s British 8th Army (Lieutenant General S. C. Kirkman’s XIII Corps, Generał dywizji Władisław Anders’s Polish II Corps and Lieutenant General R. L. McCreery’s X Corps) co-operating in the north-eastern sector of the offensive to effect the final reduction of the German strongpoint at Cassino (held by Generalmajor Richard Heidrich’s 1st Fallschirmjägerdivision), Lieutenant General Mark W. Clark’s US 5th Army was to play the major role in the offensive, deploying Major General Geoffrey T. Keyes’s US II Corps (Major General John B. Coulter’s 85th Division and Major General John E. Sloan’s 88th Division) on the coastal sector, and Général de Corps d’Armée Alphonse Pierre Juin’s Corps Expéditionnaire Français (French Expeditionary Corps with Général de Division Diego Brosset’s 1ère Division Française Libre, Général de Division André Dody’s 2ère Division d’Infanterie Marocaine, General de Division Joseph de Goislard de Monsabert’s 3ère Division d’Infanterie Algérienne, and Général de Division François Sevez’s 4ère Division Marocaine de Montagne) on the right.
The plan schemed by Clark was a coastal advance by the US II Corps while the Corps Expéditionnaire Français advanced to take Monte Maio just over the 'Gustav-Linie' and then wheeled right via Ausonia and Castelnuovo into the Liri river valley at Pontecorvo. Juin objected to this simplistic tactical scheme on the grounds that his formations would be caught in the valley of the Liri by German fire from the mountains on each flank, and that it would be better to press on from Monte Maio into the Monti Aurunci using the mountain warfare capabilities of his French colonial troops, thus avoiding the main strength of the German defence in the Liri valley, to break through a flank of the 'Senger-Linie' defences.
Clark readily agreed to this modification, and the offensive began on 11 May 1944. Despite the Allies' overall superiority of 12 divisions to six, only the French made real progress through the area held by the 71st Division. By 17 May the 1st Division Française Libre had reached Monte d’Oro and the 2ère Division d’Infanterie Marocaine had taken Esperia.
Kesselring, the Oberbefehlshaber 'Südwest' as well as commander of Heeresgruppe 'C', saw that the 'Gustav-Linie' had been breached in the south-west, and thus ordered an immediate withdrawal, Cassino finally falling to the Poles on 18 May. Meanwhile Général de Brigade Augustin Guillaume’s force of goumiers marocains was pressing through the defences of the 94th Division and advancing into the Monte Aurunci to take its dominating peaks of Monte Petrella, Monte Revole and Monte Faggeta.
In the Liri river valley Lieutenant General E. L. M. Burns’s Canadian I Corps had replaced the XIII Corps and took Pontecorvo in the centre of the 'Senger-Linie' on 19 May, and on the same day the 1st Division Française Libre took Pico slightly farther to the west. The 'Senger-Linie' was thus breached, and by 23 May the Germans were in severe difficulties as the VI Corps started its 'Buffalo' break-out from Anzio to meet the II Corps on 25 May on the shores of Lake Fogliano to the south-east of the lodgement. The Corps Expéditionaire Français and Canadian I Corps were pushing forward at the same time, and the first two phases of 'Diadem' had been completed successfully.
Then on 25 May Clark altered Alexander’s overall plan, as had been presaged by Clark’s orders to Lucas before the launch of 'Shingle', ordering the VI Corps to make not to the north-east for Valmontone but straight to the north for the Alban Hills as a preliminary move toward Rome, and so was lost a perfect opportunity to cut off the entire XIV Panzerkorps, and also elements of General Traugott Herr’s LXXVI Panzerkorps which had been containing the Anzio lodgement.