The 'Cäsar-Linie' was German defensive line in Italy, running from a point about mid-way between Anzio and Lido di Ostia on the coast of the Tyrrhenian Sea across the Alban Hills and the Apennine mountains via Valmontone, Avezzano and Popoli to Pescara on the coast of the Adriatic Sea (spring/June 1944).
The main Allied target defended by this line, which was held by Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10th Army and Generaloberst Eberhard von Mackensen’s 14th Army of Generalfeldmarschall Albert Kesselring’s Heeresgruppe 'C', in the east and west respectively, was the Italian capital city of Rome.
After the 'Buffalo' breakout of 23 May 1944 by Major General Lucian K. Truscott’s US VI Corps from its Anzio lodgement, and after the rest of Lieutenant General Mark W. Clark’s US 5th Army had begin its drive to the north after breaking through the 'Führer-Senger-Linie' defences that had been designed to halt an Allied breakthrough of the 'Gustav-Linie' defences along the Garigliano and Rapido rivers, Clark decided to change the overall plan for the 5th Army’s advance. Although it had been planned that every effort would be made to cut the lines of withdrawal for the 10th Army and 14th Army, Clark now ordered the 5th Army to change the axis of its advance from the north-east to the north-west and make its primary effort along Highway 7 and the road from Anzio to Albano straight toward Rome.
To placate General the Hon. Sir Harold Alexander, heading the Allied Armies in Italy command, Clark proposed to continue the north-eastward attack toward Valmontone on Highway 6 with Major General John W. O’Daniel’s 3rd Division, while Major General Charles W. Ryder’s 34th Division, Major General Fred L. Walker’s 36th Division, Major General William W. Eagles’s 45th Division and Major General Ernest N. Harmon’s 1st Armored Division opened a direct attack on the 'Cäsar-Linie' defences to the west of the Alban Hills on 26 May. Alexander accepted Clark’s plan on the assurance that he would maintain the thrust to Valmontone, whose seizure would cut off the bulk of the 10th Army and much of the 14th Army.
The VI Corps moved off on its new north-westerly axis by 12.00 on 26 May. The 3rd Division continued on its original axis and seized Artena, a mere 3 miles (4.8 km) to the south of Highway 6 near Valmontone, where it was brought to a sharp halt by a German force consisting of Generalleutnant Wilhelm Schmalz’s 1st Fallschirmpanzerdivision 'Herman Göring', Generalleutnant Hellmuth Böhlke’s 334th Division newly arrived from the Adriatic coast, and Generalleutnant Werner Göritz’s 92nd Division only recently formed in the Rome area. Not notably effective, and composed of a mix of several types of ethnic Germans, the last was nonetheless firmly established in this key position, holding open the 10th Army's line of withdrawal.
Clark’s main attack made good initial progress but then encountered the main positions of the 'Cäsar-Linie'. For the next four days the VI Corps could make no significant impression on the German defences, and it appeared that Kesselring had finally succeeded in stabilising the German front. Kesselring’s plan was to pull back the 10th Army as rapidly as possible into the 'Cäsar-Linie' defences on the left of the 14th Army. To do this, General Fridolin Ritter und Edler von Senger und Etterlin’s XIV Panzerkorps fought a determined delaying action in the area of Arce and Ceprano using Generalmajor Richard Heydrich’s 1st Fallschirmjägerdivision at the former and Generalleutnant Gerhard Graf von Schwerin-Krosigk’s 90th Panzergrenadierdivision at the latter. So successful was von Senger und Etterlin that Lieutenant General Sir Oliver Leese’s British 8th Army could not break through his rearguards until the bulk of General Valentin Feurstein’s LI Gebirgskorps and his own XIV Panzerkorps had escaped to the north-west.
Now comprising Generalleutnant Dr Julius Ringel’s 5th Gebirgsdivision, Generalleutnant Heinrich Deboi’s 44th Division and Generalleutnant Dr Hans Bölsen’s 114th Jägerdivision, the LI Gebirgskorps slipped back along Highway 82 pursued by Lieutenant General Sir Bernard Freyberg’s New Zealand 2nd Division of Lieutenant General Sir Richard McCreery’s British X Corps, this division having passed round the north of Monte Cairo through Atina to Sora. Generalleutnant Smilo Freiherr von Lüttwitz’s 26th Panzerdivision and Generalleutnant Friedrich-Wilhelm Hauck’s 305th Division pulled back up the road from Frosinone to Arsoli, and Generalmajor Fritz Polack’s 29th Panzergrenadierdivision drew back through the Monti Lepini in company with the remnants of Generalleutnant Wilhelm Raapke’s 71st Division and Generalleutnant Bernhard Steinmetz’s 94th Division, and headed for the area to the north of the 1st Fallschirmpanzerdivision at Valmontone. von Senger und Etterlin yielded the position round Arce and Ceprano on 28 May, sending the 1st Fallschirmjägerdivision back to Subiaco and the 90th Panzergrenadierdivision to Arsoli on the main lateral road between Rome and Pescara.
von Senger und Etterlin then employed the Germans' well proved and highly effective rearguard combination of small parties, mining and demolitions to check the rate at which Lieutenant General S. C. Kirkman’s British XIII Corps, Lieutenant General E. L. M. Burns’s Canadian I Corps and Général de Corps d’Armée Alphonse Pierre Juin’s Corps Expéditionaire Français (French Expeditionary Corps) could advance. At this time the 8th Army suffered from a surfeit of motor transport on a decidedly limited road network, and made very only slow progress against von Senger und Etterlin’s excellent defence.
Thus there appeared to be every likelihood that Kesselring would be able to repeat his performance after the 'Avalanche' landing at Salerno by re-creating a continuous line across the Italian peninsula, but then an unexpected change destroyed his chances of halting the Allies south of Rome. On 29 May, Clark regrouped his forces at the western end of the 'Cäsar-Linie' defences, bringing Major General Geoffrey T. Keyes’s II Corps up to take control of the 3rd Division and Major General John B. Coulter’s 85th Division, which were to continue the attack on Valmontone; the Corps Expéditionaire Français had followed the 29th Panzergrenadierdivision through the Monti Lepini and was now approaching the 3rd Division at Artena. The VI Corps was on the verge of exhaustion at this time, but Clark felt that one more effort might break the confidence of the Germans, who were themselves equally exhausted.
Then the unexpected occurred. The 36th Division, trying to outflank the German positions in the centre of the Alban Hills at Velletri, had ordered vigorous patrol activity during the night of 30/31 May, and one of the patrols found that the high ground behind the town, on the Monte Artemisio, was unoccupied. Kesselring had also noted this weakness in von Mackensen’s dispositions and had already ordered the gap filled without delay. But before this could be done the 36th Division silently infiltrated two of its regiments onto this key feature, while the division’s third regiment struck round and then behind Velletri, cutting the German garrison’s line of retreat.
A dangerous gap thus appeared in the strongest part of the 'Cäsar-Linie' defences. The 1st Fallschirmpanzerdivision counterattacked but failed to shift the 36th Division, and Clark ordered immediate exploitation and a general attack by his two corps on the morning of the following day. The II Corps now had both of its original formations, the 85th Division and Major General John E. Sloan’s 88th Division, as well as the 3rd Division, and was ordered to renew its attack round the northern side of the Alban Hills, cutting Highway 6 and taking Valmontone on the way. The VI Corps was to advance round the south-western side of the hills, while 36th Division drove through the centre from its newly won position. Observers placed by the 36th Division on Monte Artemisio could bring artillery fire down on the German positions blocking both corps' thrusts.
On 1/2 June Clark committed all his strength into the fray: 11 divisions, including the two British formations on the coast, drove irresistibly forward against the shattered German formations. The 1st Fallschirmpanzerdivision and 344th Division gave way first, opening the way for the II Corps to reach Highway 6, as they pulled back during the night of 2/3 June to the Aniene river in the area to the east of Rome, covered by rearguards provided by Generalleutnant Heinrich Trettner’s 4th Fallschirmjägerdivision. The rest of 14th Army of necessity conformed, and thus retired over the Tiber river to the west of Rome, destroying the bridges and ferries in their wake.
The bridges in Rome were left intact as Kesselring had declared Rome an open city. At 19.15 on 4 June the leading elements of 88th Division entered the Piazza Venezia in the centre of Rome. Clark and his 5th Army had won a well-deserved victory, marred only by his petty behaviour in trying to exclude the troops of every other nation from sharing the honours with him.
The Corps Expéditionaire Français, which had moved up on the II Corps' right flank south of Valmonte, wheeled round close in eastern outskirts of the city. The 8th Army, as laid down in the original 'Diadem' plan, cut across the hills to continue its advance up the eastern bank of the Tiber, heading for Terni in the western foothills of the Apennine mountains. It had two fresh armoured divisions available, but the terrain made it difficult to use these to any real advantage in trying to encompass the final destruction of the German army. Until the 5th Army’s three corps, pressing down on the Italian capital, cleared themselves from Highway 6 there was no way of speeding these powerful formations past Rome, for all roads in this area certainly led to the Italian capital.
The 14th Army fell back to the 'Trasimenisch-Linie' and realigned itself with the 10th Army before withdrawing to the 'Gotisch-Linie' defences.