'Diadem' was the Allied strategic offensive devised during the spring of 1944 by the headquarters of General the Hon. Sir Harold Alexander’s Allied Central Mediterranean Force (from 9 March the Allied Armies in Italy) for the capture of Rome in the early summer of 1944 after the breakthrough being sought at Monte Cassino (11 May/4 June 1944).

In essence the plan called for a rapid penetration of the 'Gustav-Linie' defences at Cassino and a joint exploitation north by Lieutenant General O. W. H. Leese’s British 8^th Army (up the Liri river valley as far as Sora and up the Sacco river valley as far as Valmontone) and by Lieutenant General Mark W. Clark’s US 5^th Army along the coast to link (in the event on 25 May just to the north of Latina) with Major General Lucian K. Truscott’s US VI Corps, which was to break out from its Anzio lodgement in 'Buffalo' and strengthen the 5^th Army for the final push on Rome through the defences of Generaloberst Eberhard von Mackensen’s 14^th Army.

Alexander’s plan for the defeat of Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army holding the 'Gustav-Linie' was to draw the bulk of the 8^th Army from the Adriatic front across the spine of Italy to a position opposite General Valentin Feurstein’s LI Gebirgskorps on the right of the 5^th Army opposite General Fridolin von Senger und Etterlin’s XIV Panzerkorps and attack on a 20-mile (32-km) front between Cassino and the coast of the Tyrrhenian Sea. The 5^th Army (Major General Geoffrey T. Keyes’s US II Corps and Général de Corps d’Armée Alphonse Juin’s Corps Expéditionnaire Français, or French Expeditionary Corps) would be on the left and the 8^th Army (Lieutenant General S. C. Kirkman’s British XIII Corps and Generał dywizji Władysław Anders’s Polish II Corps) on the right.

The arrival of spring meant that the weather and, just as importantly, terrain conditions were both significantly improved, making it feasible to deploy large formations and armour effectively.

The core of the plan for 'Diadem' was for the US II Corps on the left to attack up the coast along the line of Highway 7 in the direction of Rome; for the Corps Expéditionnaire Français on the US II Corps' right to attack through the Monti Aurunci, which constitutes the primary barrier between the coastal plain and the valley of the Liri river; on the centre right of the front for the XIII Corps to attack along the Liri river valley, and on the right for the Polish II Corps (comprising Generał brygady Bronisław Duch’s 3^rd Carpathian Division and Generał brygady Nikodem Sulik’s 5^th Kresowa Division), which had relieved Major General V. Evelegh’s British 78^th Division in the mountains behind Cassino on April 24, to attempt the task which had defeated the Indians in February during the third of the four battles fought for Monte Cassino, isolate the monastery and push round behind it into the Liri river valley to link with the British XIII Corps' thrust and pinch out the Cassino position. It was hoped that, as a much larger formation than its predecessor, Brigadier H. K. Dimoline’s (from 9 March Major General A. Galloway’s and from 25 March Major General A. W. W. Holworthy’s) Indian 4^th Division, the Polish II Corps would be able to saturate the German defences, which would therefore be unable to give supporting fire to each other’s positions.

As noted above, improved weather, ground conditions and supply would also be important factors. Once again, the pinching manoeuvre by the British XIII Corps and the Polish II Corps was seen as the key to success. Once von Vietinghoff-Scheel’s 10^th Army had been defeated, Truscott’s US VI Corps of the 5^th Army, which was currently holding against von Mackensen’s 14^th Army, would break out of its 'Shingle' lodgement at Anzio to cut the Germans' line of retreat in the Alban hills.

The major redisposition of the formations involved in 'Diadem' occupied a period of two months, largely as a result of the fact that all movements had to be carried out on a unit by unit basis so that secrecy, and therefore surprise, could be maintained. Among the deception efforts were the commitment of Major General Fred L. Walker’s US 36^th Division to amphibious assault training, and in 'Nunton' the creation of road signposts and dummy radio traffic giving the impression that an amphibious assault was being planned for the Tyrrhenian Sea coast to the north of Rome. The object of these and other deception elements was to persuade Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber 'Südwest' and commander-in-chief of Heeresgruppe 'C', to hold his reserve formations well back from the 'Gustav-Linie' so that they could be moved to counter any such landing. Allied troop movements in forward areas were confined to the hours of darkness, and armoured units moving from the Adriatic front left dummy tanks and vehicles so the vacated areas appeared unchanged to German aerial reconnaissance.

The various deception measures were successful: as late as the second day of this fourth and final 'Battle of Monte Cassino', Kesselring estimated that the Allies had six divisions facing his four on the Cassino front, whereas in fact the Allies had 13 divisions. At this stage the reserve available to Heeresgruppe 'C' comprised five formations: Generalleutnant Wilhelm Schmalz’s Fallschirmpanzerdivision 'Hermann Göring' which was technically in Oberkommando der Wehrmacht reserve and earmarked for transfer to north-western France, Generalleutnant Walter Fries’s 29^th Panzergrenadierdivision, Generalleutnant Smilo Freiherr von Lüttwitz’s 26^th Panzerdivision and Generalleutnant Ernst-Günther Baade’s 90^th Panzergrenadierdivision, which constituted the mobile elements, and Generalleutnant Werner Göritz’s 92^nd Division.

It is worth noting that 'Diadem' was roughly co-ordinated with the invasion of northern France in 'Overlord', and was supported by the air attacks of the 'Strangle' campaign.

The two-day initial assault in this '4^th Battle of Monte Cassino' (11/18 May) began at 23.00 on 11 May with a massive artillery bombardment of 1,000 and 600 guns on the 8^th and 5^th Armies' fronts respectively, and within 90 minutes the attack was under way in all four sectors. By daylight the US II Corps had made little progress but the Corps Expéditionnaire Français had reached its objectives and was fanning out in the Monti Aurunci toward the formations of the 8^th Army to its right, rolling up the German positions between the two armies.

Only one of the Allies' attacking corps was successful during the first three days of 'Diadem'. On the coast Keyes’s US II Corps was resolutely opposed by Generalleutnant Bernhard Steinmetz’s 94^th Division of von Senger und Etterlin’s XIV Panzerkorps, under the temporary command of Generalleutnant Otto Hartmann as the corps commander was on leave, which withstood the attacks of two new US formations (Major General John B.Coulter’s 85^th Division and Major General John E. Sloan’s 88^th Division) far better than Kesselring and Hartmann had expected.

On the 8^th Army’s front in the Liri river valley, the XIII Corps had completed two strongly opposed crossings of the Rapido river, and a major success at this time was the erection, during the morning, of a bridge across the Rapido by the engineers of the Indian 8^th Division, which made its possible for the armour of Brigadier R. A. Wyman’s Canadian 1^st Armoured Brigade to cross and provide the vital tank element, so sorely missed by the Americans and New Zealanders in the first and second battles respectively, to beat off the inevitable German armoured counterattacks.

The XIII Corps had to fight hard for every yard it gained. Russell’s Indian 8^th Division attacked to the south of Sant' Angelo and secured a precarious bridgehead over the Rapido river and, as a result of the work by its sappers, who had erected two 30-ton tank bridges before the dawn mist dissipated, managed both to enlarge and to consolidate its bridgehead against the counterattacks of the Kampfgruppe 'Bode' (115^th Panzergrenadierregiment, a parachute machine gun battalion, and elements of Generalleutnant Friedrich-Wilhelm Hauck’s 305^th Division from the Adriatic front) and a detachment of Generalleutnant Rudolf Sperl’s 15^th Panzergrenadierdivision in the Liri Valley.

To the north of Sant' Angelo, closer under Monte Cassino, Major General D. Ward’s British 4^th Division was not so lucky: the formation secured two small bridgeheads, but had no bridges behind it as the sites for any such constructions were so closely overlooked by the German observers in the ruins of the monastery on Monte Cassino that the sappers were driven back before they could start their work. It took the entire second night of 'Diadem' to build a bridge in this division’s sector, and this northern bridgehead was tightly contained by detachments of Generalleutnant Richard Heidrich’s 1^st Fallschirmjägerdivision in Cassino and of Generalleutnant Bruno Ortner’s 44^th Reichsgrenadierdivision 'Hoch- und Deutschmeister' and Generalleutnant Max-Günther Schrank’s 5^th Gebirgsdivision despatched from the north by Feurstein’s LI Gebirgskorps to protect its southern flank. By the evening of 13 May the British XIII Corps had linked its two bridgeheads and was secure on the north-western bank of the Rapido river, but was a long way from breaking through the German defence.

The bomb-shattered monastery still dominated the battlefield and, although it was kept under a pall of artillery smoke all day, its screening was not wholly successful and German artillery observers were still able to call damaging fire down onto the forces in the Rapido valley below it. The corps which lost most heavily was the Polish II Corps. Anders decided that the only way to crush the defenders of Monte Cassino was to swamp all the German supporting posts simultaneously so that they could not give each other supporting fire as they had done in previous attacks. He decided not attack the monastery itself, but to cross the ridges behind the monastery on a wide front using his two divisions, and to cut Highway 6 below it, thereby trapping the garrison of Cassino and the monastery between his forces and the British 4^th Division in the Liri river valley.

Unlike the Indians in 'Dickens' (the '3^rd Battle of Cassino'), the Poles had benefited from the time which had been granted them to stock their positions with everything they needed, and had also effected major improvements to the tracks, allowing some armour to get into supporting positions. The Poles could not patrol their sector as thoroughly as they might have wished, for it was essential for them not to reveal their presence. Moreover, a fact which could not be ignored was the terrain, which was just as rough, broken and difficult as before, and the German positions were as just as strong, if not stronger, than they had been. The Germans were using the night of 11 May to relieve the units holding many of their posts in front of the Poles and the German strength was thus, by chance, stronger than usual with more troops readily at hand.

Although the Polish attack started well, the machine guns of the German paratroopers soon began to inflict severe losses, just as they had done to the Americans and Indians in earlier attacks. Even so, both Polish divisions reached their initial objectives. More of the German posts were overrun than had been before this time, but with the arrival of day the Poles found themselves exposed on the open slopes unable either to advance or retreat; unable to receive reinforcements or supplies; and unable to avoid the German fire which steadily reduced their numbers throughout the day. During the afternoon, Anders decided to order his units to withdraw, and by the evening the Poles were back on their start line after losing about half their strength and apparently gaining nothing. They had, however, greatly eased the problems of the British 4^th Division, which would have been more heavily engaged if Heidrich had not faced the effort the Poles had exerted on Monte Cassino.

The only success came in the French sector. Later accounts have suggested that the positioning of Juin’s corps, skilled in mountain warfare, opposite the difficult mountain features overlooking the Ausente river valley was a calculated move designed to profit by what was the obvious key to Cassino: in fact the positioning of the French corps was not calculated, nor was the key obvious. The Corps Expéditionnaire Français was equipped with US weapons and matériel, and for compelling logistical reasons was kept within the US 5^th Army when Alexander regrouped his forces. The US 85^th Division and 88^th Division of the US II Corps were unsuitable for deployment in the mountain sector, whereas Juin’s divisions were accustomed to this type of terrain. Positioning the Corps Expéditionnaire Français in the upper reaches of the Garigliano river was thus fortuitous and the result of keeping it within the 5^th Army’s area of responsibility. In all the meetings held at the headquarters of the 15^th Army Group, and the following Allied Central Mediterranean Force and Allied Armies in Italy which followed it, no special stress was laid on the French contribution. What is abundantly clear is the fact that Alexander was determined to achieve overwhelming superiority at Cassino and not just on the upper reaches of the Garigliano river.

This was the first unexpected event of the battle. Juin and his North African troops were ideally suited for the sector of the front allotted to them. The Germans did not expect a major attack against Generalleutnant Wilhelm Raapke’s 71^st Division. And Hartmann, commanding the XIV Panzerkorps in von Senger und Etterlin’s absence until the latter hastened back on 17 May, was not an effective corps commander. The French attack took Raapke’s 71^st Division by surprise and, using pack transport and moving over terrain believed by the Germans to be impassable, the Corps Expéditionnaire Français broke through and captured Monte Maio, the southern bastion of the 'Gustav-Linie' defences in the Liri river valley, on 13 May.

As this stage of 'Diadem', Kesselring perceived the Allied offensive as a holding operation designed to draw his reserves away from the coast to the north of Rome and away from the 'Shingle' lodgement at Anzio, and the extent of the French advance was not reported to him as the Germans themselves did not appreciate the depth to which the French had driven into their positions. The only mobile division ordered forward out of reserve was Baade’s 90^th Panzergrenadierdivision, and this started to move to the south along Highway 6 in the direction of the Liri river valley on 13 May.

The two days of 14 and 15 May were therefore decisive in the 5^th Army’s sector. The Corps Expéditionnaire Français broke through into the Ausente river valley and took Ausonia. The US II Corps was now gaining momentum and drove Steinmetz’s 94^th Division, whose inland flank had been exposed by the French advance, to the east along the coast road toward Formia and Gaeta. The remnants of this formation’s neighbour, Raapke’s 71^st Division, were also in full retreat before the French and falling back to the north in the direction of Esperia. Thus the two most southerly German divisions were divided by the mass of the essentially trackless Aurunci mountains.

Juin seized his opportunity, and despatched Général de Brigade Augustin Guillaume’s Commandement de Goums Marocains, a force of about 12,000 men, into the gap and across the mountains to cut the German lateral road from Pico to Formia behind the 'Führer-Senger-Linie' switch line, and the German right flank had collapsed. Only the efforts of the widely dispersed detachments of Sperl’s 15^th Panzergrenadierdivision saved the XIV Panzerkorps from complete rout.

In the Liri river valley, the battles to the south in this same corps' sector had little effect on the LI Gebirgskorps, which was still determined not to lose Cassino. Russell’s Indian 8^th Division benefitted from the French success, but Ward’s British 4^th Division was still encountering strong resistance. The British XIII Corps ordered its reserve, Major General C. F. Keightley’s British 78^th Division into the bridgehead to pass through the 4^th Division and cut Highway 6 in conjunction with a renewed Polish attack behind the monastery. The combined British and Polish attack was scheduled for 15 May. The 78^th Division had difficulty crossing the Rapido bridges, which were still under German artillery fire and frequently blocked, so the attack had to be postponed. Leese then decided to move up Lieutenant General E. L. M. Burns’s Canadian I Corps in anticipation of a breakthrough. The Canadians started crossing the Rapido into the sector of the Indian 8^th Division on 15 May, and on the next day the 78^th Division, now supported by Brigadier F. N. Mitchell’s 26^th Armoured Brigade of Evelegh’s British 6^th Armoured Division, attacked in a great wheeling arc to cut Highway 6 at a point well to the west of Cassino. Each phase of this undertaking was a carefully planned armour, infantry and artillery operation of what had by now become the classic type. The 4^th Division co-operated with a similar attack, wheeling inside the 78^th Division’s arc to cut Highway 6 nearer to Cassino. Both attacks made good progress, and Leese authorised Anders to start the second Polish attack above Cassino on the morning of 17 May.

In the period between 12 and 17 May the Poles had reorganised their two divisions in an effort to make good the losses and disorganisation of their initial attack, but nonetheless offered the Germans no respite during this period. Frequent artillery, mortar and air attacks kept the German paratroopers pinned down, and extensive and aggressive patrolling had given the Poles a considerably better knowledge of the terrain and the locations of the German positions than they had possessed before their first attack. The Poles also studied the reasons for their first failure, and could thus plan their second attack to minimise the chance of any repetition.

That said, the battle fought between Heidrich’s 1^st Fallschirmjägerdivision and the Polish 3^rd and 5^th Divisions on the ridges above Monte Cassino throughout 17 May was far from one-sided. The Germans resisted with great determination, but were inferior in numbers and unable to halt the Poles. By good fortune the Poles had been able to take a key position during the night preceding the main attack as the result of a successful patrol action, and in combination with their careful preparations and planning, this allowed them to start taking the German positions which had defied so many units before them. Moreover, the threat posed by the 4^th and 78^th Divisions in the valley below meant that Heidrich had fewer troops available for counterattack, and the Germans were now being swamped, as Alexander had intended, by superior numbers. By dusk on 17 May, Point 593 above the monastery was in the hands of the Poles, whose divisions were now too exhausted and disorganised to attempt another attack on that night. The 78^th Division was stopped just short of Highway 6, but the 4^th Division was slightly more successful and managed to get patrols across it.

It was thought probable that Heidrich would attempt to pull his paratroopers, battered but unbowed, out along the southern slopes of Monte Cassino during the night, so British artillery fire was ordered to harass any troops using the tracks, but the British lacked troops fresh enough to attempt to close the trap during the night. In the morning of the following day, it was discovered that there were still German rearguards holding key features on the side of the mountain to the west of the monastery. The ruins of the monastery and of Cassino town, however, were empty except for a small number of severely wounded Germans who had been left with orderlies to care for their immediate medical needs. At 10.20 on 18 May, a patrol of the 3^rd Division’s 12^th Podolski Lancers entered the monastery and raised the Polish flag over the ruins. The '4^th Battle of Cassino' was over at the cost to the Poles of almost 4,000 casualties.

Farther to the south, meanwhile, the Corps Expéditionnaire Français had been making good progress. Général de Division Diego Brosset’s motorised 1ère Division Française Libre was moving to the north-west along the southern bank of the Liri river, and had reached the 'Führer-Senger-Linie' immediately to the south of Pontecorvo at the time the British occupied Cassino. Général de Division Joseph de Goislard de Monsabert’s 3ème Division d’Infanterie Algérienne had taken Santa Olivia on the southern extremity of the prepared section of that line, and Guillaume’s Commandement de Goums Marocains and Général de Division François Sevez’s 4ème Division Marocaine de Montagne were past the first section of the Monti Aurunci and looking down on the lateral road linking Pico and Formia.

The US II Corps was making progress almost as rapidly. Formia fell to the 85^th Division on 17 May and Gaeta was taken two days later. The 88^th Division, which like the French was fighting through mountains, was now threatening Itri on the road linking Pico and Formia, and took this on 19 May. The battle for the 'Führer-Senger-Linie' was thus almost lost to the Germans before it had started, although not in the sector in which Alexander had expected this to happen. The whole southern sector had been overrun before it could be properly manned and before the Canadian I Corps, entrusted with the task of breaking this switch line, had even come within sight of it.

The 'Führer-Senger-Linie' comprised two parts: the northern section blocked the Liri river valley, was complete and would be difficult to break if properly manned, while the southern sector comprised the 'Dora-Linie' extension was at this time little more than a plan on which a just a small number of rudimentary defences had been started. The 'Führer-Senger-Linie' proper ran from the village of Piedimonte on the western slopes of Monte Cairo through Pontecorvo to Santa Olivia on the southern side of the Liri river. The 'Dora-Linie' southward extension continued the line through the Monti Aurunci to the coast.

Kesselring did not become fully aware of the disaster suffered by the XIV Panzerkorps until 17 May, at about the time that the German radio interception service identified the Canadian I Corps, which was thought to be embarking at Naples and Salerno for the amphibious attack to the north of Rome, was in fact in the Liri river valley. Kesselring could not be certain that the amphibious threat was now over, but knew that if he did not block the French advance round the southern end of the 'Führer-Senger-Linie', the greater part of von Vietinghoff-Scheel’s 10^th Army would be cut off. Kesselring therefore ordered von Lüttwitz’s 26^th Panzerdivision to leave its blocking position in the Alban hills, just to the south-east of Rome, where it was waiting to tackle any attempt by Truscott’s US VI Corps to break out of the 'Shingle' lodgement at Anzio, and to move toward the area of Pico and Pontecorvo as rapidly as it could. He also ordered Hauck’s 305^th Division and Generalleutnant Hellmuth Böhlke’s 334^th Division (the two formations of Hauck’s Korpsgruppe 'Hauck') from their blocking positions on the Adriatic coast, replacing them with the Infanterie-Division 'Böhmen' (soon to become Generalleutnant Hans von Grävenitz’s 237^th Division) from Istria. The 305^th Division was expected to arrive first to join the 26^th Panzerdivision in trying to check the French advance.

When the battle for the the 'Führer-Senger-Linie' began on 18 May, Kesselring had committed three out of the five mobile divisions had had available. But Kesselring had been reluctant to believe that the attack on the 'Gustav-Linie' was anything more than a holding attack, and therefore he had made no effort to create and launch a co-ordinated counterattack of the type characteristic of German tactics at this time. The result was that these potentially decisive divisions entered the battle on a piecemeal basis and were were whittled away in seeking to stem the fresh troops which the Allies were committing. Sperl’s 15^th Panzergrenadierdivision was the first to have been effectively decimated as elements of the formation were expended in trying to bolster Raapke’s 71^st Division and Steinmetz’s 94^th Division, both of which had been defeated. Next to succumb was Baade’s 90^th Panzergrenadierdivision as it was thrown into the Liri cauldron and the desperate fight to stop the French breaking through the Esperia defile. Now von Lüttwitz’s 26^th Panzerdivision and Hauck’s 305^th Division were committed to the battle in the same fruitless way, while Fries’s 29^th Panzergrenadierdivision and Schmalz’s Fallschirmpanzerdivision 'Hermann Göring' remained to the north of Rome, awaiting an invasion which had not even embarked. Few German armies can ever have been so thoroughly deceived and unbalanced by a deception plan, in this case 'Nunton'.

With the fall of Cassino, the Polish II Corps advanced along the southern spurs of Monte Cairo to outflank and if possible also take Piedimonte, the fortified village which was the northern pivot of the 'Führer-Senger-Linie'. Alexander instructed Juin to drive to the north in the direction of Pico to outflank the fortified section of the line from the south. In the centre, the 8^th Army ordered the British XIII Corps toward Aquino and the Canadian I Corps toward Pontecorvo. Both these formations attempted to rush the 'Führer-Senger-Linie', but both were unsuccessful: they had not been fast enough, and Allied intelligence staffs had not appreciated how many PzKpfw V Panther battle tank turrets had been emplaced in the line’s strongpoints.

It was now clear that a set-piece assault would have to be planned and launched. Responsibility for this was given to the Canadian I Corps, which had yet to be committed as a complete formation in the course of the battle. The plan which was now drafted was based on the British XIII Corps maintaining pressure on Aquino while the Canadian I Corps attempted to breach the line at Pontecorvo.

As the Canadians were preparing their assault, which was scheduled for 23 May, Juin’s French troops were closing on Pico, but were slowed by the arrival of the 26^th Panzerdivision and 305^th Division. The French captured Monte Leucio, which dominated the area of Pico and Pontecorvo, but were then expelled by German counterattacks. It took two more days of hard fighting to take Pico, which fell on 22 May. The German defence was now too strong to allow the French to cut in behind the 'Führer-Senger-Linie', so Juin was ordered to advance to the north-west in the direction of Highway 6 at Ceprano. On the other flank, the Polish II Corps reached Piedimonte on 20 May but did not manage to clear the town until 25 May.

The Canadian I Corps attacked early on 23 May and secured a breakthrough only late in the day. Major General G. G. Simonds’s Canadian 1^st Division had brought up from reserve to attack in the centre. On 24 May the Canadians breached the defences and Major General B. M. Hoffmeister’s Canadian 5^th Armoured Division poured through the gap. The Germans suffered heavy losses, including some 500 men taken prisoner. The Canadians suffered as heavily, and large numbers of tanks were also lost, but the 'Führer-Senger-Linie' had been breached and the 8^th Army was thus able to resume its advance up Highway 6.

It is worth noting that the 8^th Army’s greatest difficulty in the Liri river valley was one of its own making. To bring to bear the superiority in infantry, tanks and guns thought necessary, the British XIII Corps and Canadian I Corps had advanced side-by-side on the narrow plain between Monte Cairo and the Liri river, which is 5 miles (8 km) wide at most. This was the 'gateway to Rome' which had played so important a part in Allied thinking in the autumn of 1943, but had now been found to be very far from ideal for an armoured breakthrough even in dry summer weather. The floor of the valley was cut by a number of awkward streams and ditches, many of which had to be bridged to allow tanks and vehicles to move forward: their small size made these bridges chokepoints which resulted in major traffic jams. For this the 8^th Army’s staff was largely responsible, for its had not insisted that the divisions advance on the basis of the minimum possible transport. Using very few vehicles and depending largely on pack transport, the French divisions were able to move more quickly than the vehicle-dependent British divisions. It is clear, therefore, that no one in the British command structure had appreciated that lavish equipment scales, which were appropriate to wide-ranging desert operations and then to operations on the broader fronts typical of the earlier stages of the Italian campaign, were a hindrance to mobility in the type of classic set-piece battle in which overwhelming concentration of force was required.

The breaching of the 'Führer-Senger-Linie' had lost importance in the minds of the opposing army group commanders some days before the attack of the Canadian I Corps.

Kesselring’s concern for his southern flank deepened as the US II Corps and the Corps Expéditionnaire Français debouched across the lateral road linking Pico and Formia on 19 May. The 26^th Panzerdivision appeared on the verge of stabilising the front in the central area of Pico and Pontecorvo area, but the 15^th Panzergrenadierdivision could do nothing to stop the Allied drive in the south. With some reluctance, Kesselring ordered von Mackensen to send the 29^th Panzergrenadierdivision south from Civitavecchia to block the advance of the US II Corps in the potentially strong defensive position along Highway 7 between Fondi and Terracina: this was the last defile before Highway 7 emerges in the flat Pontine Marshes area, leading straight to the southern edge of the Anzio lodgement. If the 29^th Panzergrenadierdivision could stop the Americans, Kesselring believed, there was still a chance of preventing the two Allied forces from linking. von Mackensen did not agree, for he believed that the 29^th Panzergrenadierdivision should be withheld to defeat the break-out he was sure would shortly be launched from the lodgement. When Kesselring’s order was received, the staff of the 14^th Army protested to its counterpart at Heeresgruppe 'C'. Kesselring was visiting the main front at the time, and it was only on his return to headquarters that he discovered that his order had not been implemented.

The disagreement did nothing to help the 29^th Panzergrenadierdivision, which was so delayed in its move that it suffered the same unhappy fate as the other mobile divisions. If it had moved to the south when so ordered by Kesselring on 19 May, it would have had time to prepare its positions before the Americans reached the Terracina defile. As it was, the division came into action off the march without proper reconnaissance, and found that it was already too late to stop the Americans, who were already on the high ground above the defile which should have been the basis of the division’s defensive position. The 29^th Panzergrenadierdivision nonetheless did its best and indeed managed to hold Terracina until the night of 23/24 May before being forced to draw back with the remnants of the 94^th Division and 71^st Division through the Monti Lepini toward Highway 6 to avoid being cut off.

von Mackensen had been right in his belief that a break-out from Anzio was imminent. As early as 18 May Alexander had authorised the movement of Walker’s US 36^th Division, the 5^th Army’s reserve, into the lodgement. The success of the US II Corps in the south meant that the 36^th Division would now not be needed on the main front, and would be of more use strengthening the force which was to break out. The movement of the division would require four days, so the earliest that the break-out could be attempted was 23 May. As it happened, the course of the battle made this date suitable for other reasons. The 8^th Army and the Corps Expéditionnaire Français would be attacking the 'Führer-Senger-Linie' on that day and the US II Corps would be advancing toward Terracina. So there was no difficulty in deciding the date for the break-out, but its axis was more problematical, and resulted in a clash between Alexander and Clark, and between their staffs.

In his instructions for 'Diadem', Alexander had emphasised that the operation’s objective was the destruction of German divisions. The capture of Rome was a secondary consideration and would probably result as a byproduct of the primary aim. On the other hand Clark eyes fixed on Rome rather than the German army, and believed strongly that his 5^th Army had struggled all through the winter of 1943/44 for this prize. In Alexander’s view, the US VI Corps’s break-out from Anzio, with five US and two British divisions, was to be made in the direction of Valmonte to cut Highway 6 and thereby sever the 10^th Army's main line of retreat. Clark objected to this on two grounds: firstly, he would be executing a dangerous flank march across the front of the Germans holding the dominating Alban hills, and secondly, when he reached Valmonte, the 10^th Army could easily escape the trap by withdrawing up the two good roads and many subsidiary tracks leading to the north-west from Highway 6 to Highway 5, the main lateral road connecting Pescara and Rome, which had at one time been Montgomery’s proposed line of advance on Rome. Once on this lateral, the Germans would have little difficulty in reaching the 'Cäsar-Linie' defences. Alexander accepted Clark’s point about the Alban hills, but did not agree with the second point on the grounds that the fewer the lines of retreat left open to the Germans, the greater would be the execution done on the Germans' retreating columns by the Allied air forces.

Alexander seems to have temporised and left Clark with the feeling that it would not take much of a setback to the Valmonte thrust to make him change his mind and allow a direct attack west of the Alban hills on Rome, which was of course Clark’s intention. The US VI Corps had several alternative plans prepared for the various contingencies, but it was finally agreed that the attack should be 'Buffalo' made, as Alexander directed, toward Valmonte. The break-out started, as planned, on 23 May with an attack on Cisterna by Brigadier General John W. O’Driscoll’s US 3^rd Division, Major General Ernest N. Harmon’s US 1^st Armored Division and Brigadier General Robert T. Frederick’s US/Canadian 1^st Special Service Force. It was then intended to pass the fresh US 36^th Division and the US 1^st Armored Division through the breach in the German defences and forward through the gap between the Alban and Lepini hills to Valmonte, giving the Alban hills as wide a berth as possible. Major General William W. Eagles’s US 45^th Division would protect the northern flank by widening the breach toward the Alban hills, while Major General J. L. I. Hawkesworth’s British 1^st Division and Major General P. G. S. Gregson-Ellis’s British 5^th Division held the northern perimeter against any relieving attack by the three divisions of General Alfred Schlemm’s I Fallschirmkorps.

The US 1^st Armored Division and US 3^rd Division gained tactical surprise as they attacked, and tore a wide gap in the defences of Generalmajor Heinz Greiner’s 362^nd Division and Generalleutnant Hans-Georg Hildebrandt’s 715^th Division of General Traugott Herr’s LXXVI Panzerkorps. The 715^th Division broke first and tried to pull back towards the Lepini hills, but Greiner’s 362^nd Division did not and, aided by elements of Generalleutnant Heinrich Trettner’s 4^th Fallschirmjägerdivision from the northern side of the lodgement, held the northern shoulder of the US breach. Heavy fighting went on through 23/24 May, and on 25 May the leading elements of Schmalz’s Fallschirmpanzerdivision 'Hermann Göring' started to appear.

However, Kesselring had been compelled to commit the last of his reserve mobile formations to stop the US break-through, although the airborne Panzer division had been earmarked by OKW for removal to north-western France to oppose 'Overlord'. So desperate had the German position become that this division risked moving south in daylight and in consequence suffered heavy punishment from the air. During 25 May some of the German reinforcements were seen as they headed to the south from Valmonte in the direction of the US breach and were heavily bombed and strafed by Allied warplanes. The US VI Corps claimed some 600 German vehicles destroyed and 400 damaged, and while the exact figure will never be known, as they advanced beyond Cisterna, the Americans found the roads blocked by burned-out German equipment.

By dusk on 25 May a wide breach had been secured. Cisterna, Cori and the northern edge of the Lepini hills were in US hands and the leading elements of Keyes’s US II Corps, advancing from Terracina, made contact with a patrol from the Anzio lodgement. The 5^th Army had thus been reunited after three and a half months. The US 1^st Armored Division’s tank losses had been significant, and the US 3^rd Division lost more than 1,000 men on the first day, but both divisions were still full of fight.

It was at this stage that Clark decided to change direction and aim his main effort along Highway 7 and the road linking Anzio and Albano straight for Rome. To sugar the pill for Alexander, Clark proposed to continue the attack toward Valmonte with the US 3^rd Division, while the US 34^th Division, 36^th Division, 45^th Division and 1^st Armored Division launched an attack directly onto the 'Cäsar-Linie' to the west of the Alban hills on 26 May. Alexander accepted Clark’s revised plan on the assurance that Clark would maintain the thrust toward Valmonte. However, Clark paid only lip service to his assurance to Alexander.

The US VI Corps had started to move along its new axis by 12.00 on 26 May. The US 3^rd Division, continuing the attack on the original axis, seized Artena only 3 miles (4.8 km) to the south of Highway 6 near Valmonte, but here was halted by a German force comprising Schmalz’s Fallschirmpanzerdivision 'Hermann Goring', Böhlke’s 334^th Division from the Adriatic coast, and Göritz’s 92^nd Division. This German force was firmly established in this key position, holding open the 10^th Army's escape route.

Clark’s primary attack made good initial progress but then came up against the main 'Cäsar-Linie' positions and, for the next four days, could make no impression on the German defences. It seemed that Kesselring might at last have managed to stabilise the German front.

The German plan was now to withdraw von Vietinghoff-Scheel’s 10^th Army as quickly as possible into the 'Cäsar-Linie' to the left of von Mackensen’s 14^th Army. To do this, von Senger und Etterlin’s XIV Panzerkorps fought a determined delaying action in the area of Arce and Ceprano, using Heidrich’s 1^st Fallschirmjägerdivision at the former and Baade’s 90^th Panzergrenadierdivision at the latter. So successful was this effort that the 8^th Army failed to break through the German rearguards until the bulk of Feurstein’s LI Gebirgskorps and von Senger und Etterlin’s own XIV Panzerkorps had escaped to the north-west. Now comprising Schrank’s 5^th Gebirgsdivision, Ortner’s 44^th Reichsgrenadierdivision 'Hoch- und Deutschmeister' and Generalleutnant Dr Hans Bölsen’s 114^th Jägerdivision, the LI Gebirgskorps slipped back along Highway 82 pursued by Lieutenant General Sir Bernard Freyberg’s New Zealand 2^nd Division of the British X Corps, which had come round the north of Monte Cairo through Atina to Sora. von Luttwitz’s 26^th Panzerdivision and Hauck’s 305^th Division pulled back up the road from Frosinone to Arsoli road, and Fries’s 29^th Panzergrenadierdivision moved through the Monti Lepini, in the process shepherding the remnants of Raapke’s 71^st Division and Steinmetz’s 94^th Division, and headed for the area to the north of the Fallschirmpanzerdivision 'Hermann Göring' at Valmonte. von Senger und Etterlin yielded the position at Arce and Ceprano on 28 May, despatching the 1^st Fallschirmjägerdivision straight to Subiaco and the 90^th Panzergrenadierdivision to Arsoli on the main lateral road between Rome and Pescara. von Senger und Etterlin then opposed the advance of the British XIII Corps, the Canadian I Corps and the Corps Expéditionnaire Français with little more than light rearguards, using the now-familiar German demolition and mining techniques in which his units were so skilled.

The 8^th Army suffered from severe traffic congestion and made very slow progress in the face of this opposition. Thus there seemed every chance that Kesselring would be able to repeat his performance after the 'Avalanche' landings at Salerno and re-establish a continuous line across Italy, but then an unexpected disaster destroyed his chances of holding to the south of Rome just as order began to appear out of chaos in the German position.

On 29 May Clark regrouped his forces attacking the western end of the 'Cäsar-Linie'. He brought Keyes’s US II Corps forward to assume command of the US 3^rd and 85^th Divisions, which were to continue the attack on Valmonte. The Corps Expéditionnaire Français had followed the 29^th Panzergrenadierdivision through the Monti Lepini and was now nearing the US 3^rd Division at Artena. Truscott’s US VI Corps was now approaching exhaustion, but Clark felt that one more effort might break the Germans, who were equally tired. Then, and in a totally unexpected fashion, as its tried to outflank the German positions in the centre of the Alban hills at Velletri, the US 36^th Division had ordered a programme of vigorous patrols during the night of 30 May. One of these patrols found the high ground behind the town of Monte Artemisio unoccupied. Kesselring had also noted this weakness in von Mackensen’s dispositions and had ordered the gap to be filled immediately. Before the German could do this, the US 36^th Division silently infiltrated first one regiment and then another onto this key feature, and the division’s third regiment passed round behind Velletri and cut the German garrison’s line of retreat.

Thus there had appeared a dangerous gap in the strongest section of the 'Cäsar-Linie'. The Fallschirmpanzerdivision 'Hermann Göring' counterattacked but failed to dislodge the US 36^th Division. Clark ordered an immediate exploitation and a general attack by both of his corps during the next morning. Now commanding both its original divisions, the US 85^th Division and 88^th Division, as well as the US 3^rd Division, the US II Corps was instructed to renew its attack round the northern side of the Alban hills, cutting Highway 6 and taking Valmonte. Truscott’s US VI Corps was instructed to advance round the south-western side of the hills even as the US 36^th Division drove through the centre from its newly won position. US artillery observers on Monte Artemisio could bring fire to bear on the German positions blocking the thrusts of these two corps.

On 1/2 June Clark committed his entire strength to an all-out punch. Some 11 divisions, including the two British divisions on the coast, smashed forward against the tired German formations. The Fallschirmpanzerdivision 'Hermann Göring' and 344^th Division were the first to give way, so letting the US II Corps through on Highway 6. These two divisions 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 Trettner’s 4^th Fallschirmjägerdivision. The rest of the 14^th Army conformed and pulled back over the Tiber river to the west of Rome, destroying the bridges and ferries behind them. 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 the US 88^th Division entered the Piazza Venezia in the centre of Rome. The Corps Expéditionnaire Français, which had joined the right flank of the US II Corps to the south of Valmonte, wheeled closely round the eastern outskirts of the city. As laid down in the original 'Diadem' plan, the 8^th Army cut across the hills to continue its advance up the eastern bank of the Tiber, heading for Terni. It had two fresh armoured divisions available, but it was difficult to use them to effect the final destruction of the German forces. Until the 5^th Army’s three corps, pressing down on the Italian capital, cleared Highway 6 there was no way of moving these powerful formations through the traffic congestion.