Alexander’s objective in the great 'Diadem' operation was the destruction of Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army of Generalfeldmarschall Albert Kesselring’s Heeresgruppe 'C' by means of a double pincer envelopment: the first would open the Liri river valley to the Allies, and the second would begin, once the Allies had pushed into the area to the north of Frosinone, by the break-out of the VI Corps from its Anzio lodgement and its subsequent advance to meet them.

Commanding the Corps Expéditionnaire Français, which had taken over from the X Corps in the Garigliano river bridgehead, Juin was not happy with the task allocated to his formation, namely the seizure of Monte Maio, as this was the same kind of narrow turning movement which had led to the very high casualties which his men had suffered on the Colle Belvedere and on Monte Cassino. On 4 April, therefore, Juin set out his ideas on the manoeuvre in a memorandum to Clark: after Monte Maio had been captured, the Corps Expéditionaire Français should not turn right but 'infiltrate under cover of surprise into the massif dominated by [Monte] Petrella and seize the key points…and thence, by an outflanking movement, open the way to frontal advances mounted concurrently to secure Highway 7 and the road from Esperia up to and including the road running parallel to the front of Arce…the aim being to bring to bear on the Arce sector a force of considerable size so as to be able to break out in strength behind the [German] rear and advance toward Rome'.

Clark was soon won over to Juin’s plan, which had the great advantage of including in the outflanking movement of the Corps Expéditionnaire Français the 'Führer-Senger-Linie' (otherwise the 'Senger defile') blocking the Liri river valley at Pontecorvo. On the other hand there still remained the formidable obstacle of the Monti Aurunci, which reach a height of 5,030 ft (1533 m) at Monte Petrella and with other heights at Monte Revole and Monte Faggeta. It might be assumed that the Germans had not occupied the heights in great strength, and that surprise could be achieved by a clever exploitation of the massif’s natural features. All depended on the speed with which an early success could be exploited, for it was to be expected that Kesselring would respond would respond with his customary vigour and speed.

Nonetheless, Juin felt that he had the advantage in his Moroccan mountain troops supported by 4,000 mules for the timely delivery of the ammunition and other supplies they needed. Though Clark agreed to the French plan, he could not get Leese to accept its corollary, the Atina plan, and though he used a corps against Cassino where Lieutenant General Sir Bernard Freyberg’s New Zealand Corps had used a division, the pincer was still too short and there were heavy losses.

At this time Kesselring had 23 divisions, but most of these formations were exhausted and short of ammunition, whereas the Allied divisions had an abundance of all the matériel they needed. Another serious disadvantage faced by Heeresgruppe 'C' was the Allies' total aerial and maritime superiority. Thus Kesselring had to be concerned not only about an amphibious landing at Civitavecchia or Livorno farther to the north on Italy’s west coast, but also about whether or not Allied air power would cut the lines of communication serving General Fridolin Ritter und Edler von Senger und Etterlin’s XIV Panzerkorps at Frosinone. However, Kesselring’s most pressing concern was the composition, point of attack and subsequent axis of advance of the Corps Expéditionnaire Français.

Thus the headquarters of the 10^th Army and its subordinate formations were instructed to report straight to the headquarters of Heeresgruppe 'C' as soon as they had identified the Corps Expéditionnaire Français in its sector of the front. Nevertheless, the Corps Expéditionnaire Français had camouflaged itself so well when it moved into position in the foothills of the Monti Aurunci that Kesselring realised that it was there only when the Monte Maio action had been completed.

A decoy movement by Alexander as 'Nunton' made Kesselring think that the frontal attack would be combined with a landing in the area of Civitavecchia, and would start on 14 May.

At the start of the campaign the 10^th Army was deployed with, between the Tyrrhenian Sea and the Liri river, the XIV Panzerkorps (Generalleutnant Bernhard Steinmetz’s 94^th Division and Generalleutnant Wilhelm Raapke’s 71^st Division); from the Liri to the Meta General Valentin Feurstein’s LI Gebirgskorps (Generalmajor Ernst-Günther Baade’s Gruppe 'Baade', Generalleutnant Richard Heidrich’s 1^st Fallschirmjägerdivision, Generalleutnant Heinrich Deboi’s 44^th Division, and Generalleutnant Dr Julius Ringel’s 5^th Gebirgsdivision); from the Meta to the Adriatic Sea Generalleutnant Friedrich Wilhelm Hauck’s Gruppe 'Hauck' (Hauck’s 305^th Division, Generalleutnant Hellmuth Böhlke’s 334^th Division, and Generalleutnant Alexander Bouquin’s 114^th Jägerdivision); and in army reserve, behind the LI Gebirgskorps, Baade’s 15^th Panzergrenadierdivision.

Thus the initial clash pitted 12 Allied divisions (two Polish, four British, four French, and two US) against six German divisions. The Germans' current numerical inferiority was compounded by the fact that at the time of the Allied attack’s start both von Vietinghoff-Scheel and von Senger und Etterlin were absent from the front on leave, and, despite Kesselring’s order, the 94^th Division had placed no men on the Petrella massif.

At 23.00 on 11 May, 2,400 Allied guns started a simultaneous barrage on a front of about 25 miles (40 km), and one hour later the Allied infantry moved forward. When dawn broke both Leese and Clark had to admit that in spite of the surprise effect the night attack had not brought the expected success. Generał dywizji Władisław Anders’s Polish II Corps (Generał brygady Bronislaw Duch’s 3^rd 'Kresowa' Division and Generał brygady Nikodem Sulik’s 5^th 'Carpathian' Division) had failed on the slopes of Monte Cassino and, for all the fighting spirit shown by these men, their losses were very heavy. In the Liri river valley the XIII Corps had managed to push two of its divisions across the Rapido river, but without fully puncturing the defence of the LI Gebirgskorps, and in this area the fighting qualities of the 1^st Fallschirmjägerdivision were again particularly evident.

Though the Corps Expéditionnaire Français had been strengthened by the arrival of Général de Division François Adolphe Laurent Sevez’s 4ème Division Marocaine de Montagne and Général de Brigade Diégo Charles Joseph Brosset’s 1ème Division Motorisée (1^st Free French Division), its task was made more difficult by the fact that the opposition, in the form of the 71^st Division, had been expecting the attack and was therefore fully prepared. During this night operation, which they undertook to facilitate the XIII Corps' crossing of the Rapido river, the French stumbled on to minefields and were attacked by flamethrowers. By the end of 12 May there were fears that the French might be exhausted and that Kesselring would have time to occupy the whole of the Petrella massif. Juin very rapidly recast his artillery attack to concentrate everything on the Monte Maio bastion, and this bold change broke the 71^st Division's resistance, making it possible during the afternoon of 13 May for Général de Division André Dody’s 2ème Division d’Infanterie Marocaine to reach the summit of the 3,000-ft (915-m) hill. On its right Brosset’s 1ème Division Motorisée had cleared the bend in the Garigliano river. On its left Général de Division Joseph de Goislard de Monsabert’s 3ème Division d’Infanterie Algérienne had captured Castelforte and was moving toward Ausonia.

Farther to the west, the US II Corps was well on its way to Formia, and on discovering that the Germans were starting to pull back and so opening a gap between the II Corps and his own formation, Juin committed his mountain troops into this gap. The French order of battle now included Sevez’s 4^th Division Marocaine de Montagne and the mountain groupes of Général de Division Augustin Léon Guillaume’s Groupe de Goums Marocains. Relying on man-packing of weapons such as machine guns and mortars, these scaled the steep slopes of Monte Petrella to reach the top on 15 May. From this they immediately advanced on the Revole massif, some 4,150 ft (1265 m) high.

Passing behind the mountain forces, meanwhile, the 3ème Division d’Infanterie Algérienne took Ausonia and reached Esperia, thus capitalising on the success of the 1ème Division Motorisée , which had captured San Giorgio on the right bank of the Liri river. It is possible that if the 8^th Army had at the same time undertaken a wide enveloping movement of the type suggested by Juin, it would have outflanked the Pontecorvo position and placed the XIV Panzerkorps in a position of a probable catastrophe, which could then have been extended to the rest of the 10^th Army. It was only on 17 May that the Polish II Corps' renewed offensive found the abbey on Monte Cassino deserted.

It was again only on 19 May that Major General C. F. Keightley’s 78^th Division of the XIII Corps attacked the 'Senger defile' in the area of Aquino, but failed to break through. This lack of liaison between the British and the French delayed the latter’s exploitation toward Pico and the Monti Ausoni.

Committing all that he could, Kesselring sent elements of Generalleutnant Gerhard Graf von Schwerin-Krosigk’s 90^th Panzergrenadierdivision, the 305^th Division, and General Smilo Freiherr von Lüttwitz’s 26^th Panzerdivision to stop them. He also deployed Generalmajor Walter Fries’s 29^th Panzergrenadierdivision against the US II Corps, which had advanced through Formia and Itri, and by 22 May was threatening Terracina.

However, this major German reaction was made possible only by the removal of these formations from another frontal sector, that of Generaloberst Eberhard von Mackensen’s 14^th Army checking the VI Corps at Anzio. Reinforced to the equivalent of eight divisions by the transfer of Major General Fred L. Walker’s US 36^th Division to the Anzio lodgement, the VI Corps thus encountered no real problem in breaking the resistance of the 14^th Army when it launched 'Buffalo' on 23 May. Two days later the leading elements of the US II Corps and VI Corps met on the shores of Lake Fogliano. On the same day the Corps Expéditionaire Français was spreading out over the Monti Ausoni and the Canadian I Corps (Major General C. Vokes’s 1^st Division and Major General B. M. Hoffmeister’s 5^th Armoured Division), which had just relieved the XIII Corps, was forcing its way through the Pontecorvo defile.

Kesselring tried once more to protect Rome by establishing a new position on the line between the Colli Laziali (Alban Hills) and Monti Lepini to secure von Vietinghoff-Scheel’s right, and to achieve this he withdrew from the area of Livorno his last reserve, Generalleutnant Wilhelm Schmalz’s 1^st Fallschirmpanzerdivision 'Hermann Göring', for immediate despatch to Valmontone. The Allied bombing attacks, which on 26 May alone were responsible for the destruction of 665 of the 14^th Army's vehicles, were a major hindrance to these troop movements. In accordance with Alexander’s orders, Valmontone was now the primary objective of the VI Corps. Had this formation, now in Cisterna, thus driven its main strength along the axis between Corti and Artena, it would have had every chance of cutting off the 10^th Army's rearward move to cover Rome. The latter’s rearguard was still at Ceprano, some 40 miles (65 km) or more to the south-east of Valmontone, and the Germans would thus be driven back against the Abruzzi mountains, which were virtually impassable, and entirely cut off.

But Clark ordered the US VI Corps to attack to the north-west with Major General Charles W. Ryder’s 34^th Division, Major General William W. Eagles’s 45^th Division and Major General Vernon E. Prichard’s 1^st Armored Division along the axis between Velletri and the Colli Laziali, sending only Major General John W. O’Daniel’s slightly reinforced 3^rd Division to the north along the Valmontone axis. Taken in the afternoon of 25 May, this decision elicited only a slight reaction from Alexander, who remained confident that Clark’s main objective was still Valmontone. But the mirage of Rome had seized Clark and, to a lesser extent, Alexander, and the chance to cut off and destroy the 10^th Army was lost. On 31 May the 36^th Division found a gap in the 14^th Army's defences, turned the Velletri position and scaled the Colli Laziali.

Furious at this setback, Kesselring recalled von Mackensen and replaced him with General Joachim Lemelsen. But Kesselring now had to order the evacuation of Rome, which he proclaimed an 'open city'. On 4 June Major General John E. Sloan’s US 88^th Division was the first Allied formation to enter Rome.

On 11 May Kesselring had possessed 23 divisions, but these had now been reduced to remnants: the 44^th, 71^st and 94^th Divisions, as well as Generalleutnant Heinrich Greiner’s 362^nd Division and Generalleutnant Kurt Hoffmann’s 715^th Division, had been effectively destroyed, while his Panzer and Panzergrenadier divisions had lost most of their equipment. Among the reinforcements which Adolf Hitler had ordered to be sent through the Brenner Pass from Austria into northern Italy there were badly trained formations such as Generalmajor Professor Dr Oskar Ritter von Niedermayer’s (from 21 May Generalleutnant Ralph von Heygendorff’s) 162^nd Division, recruited from Turkoman contingents, Generalmajor Wilhelm Crisolli’s 20^th Luftwaffe Felddivision, and SS-Gruppenführer und Generalleutnant der Waffen-SS Max Simon’s 16^th SS Panzergrenadierdivision 'Reichsführer-SS'. These all went to pieces as soon as they entered battle.

During the same period the US forces lost 18,000 killed, missing and wounded and the British forces 10,500, while the casualties of the French, Canadians and Poles were 7,260, 3,742 and 3,700 respectively. These losses were not enough to check the advance of the Allied Armies in Italy, which was soon to be boosted in strength by the arrival of three more French formations, Général de Division Raoul Albin Louis Salan’s 9ème Division d’Infanterie Coloniale, Général de Division Jean Louis Alain Touzet du Vigier’s 1ème Division Blindée and Général de Division Henri Jacques Jean François de Vernejoul’s 5ème Division Blindée.

With their forces now well through the 'Führer-Senger-Linie' defences, the Allies could well have planned a bold offensive along the axis linking Rome and Ancona via Terni, and so brought to an end all German resistance south of the Apennine mountains.

Prime Minister Winston Churchill believed that he could persuade the Americans to cancel the 'Anvil' landing in the south of France in favour of a strengthened Allied drive via the Tarvis col and the Ljubljana gap to Vienna. This was not to be, and between June 11 and July 22, three US and five French divisions dropped out of the Italian fighting in preparation for 'Anvil', although the 9ème Division d’Infanterie Coloniale did take Elba on 17/19 July in 'Brassard'.

The lull in Allied operations presented Kesselring with the opportunity, which he seized with both hands, to re-establish the German forces in the Apennine mountains with new primary positions in the 'Arno-Linie' and 'Gotisch-Linie' defences, and also to cope with the transfer to the forces of Generalfeldmarschall Gerd von Rundstedt, the Oberbefehlshaber 'West', of his 3^rd Panzergrenadierdivision and 15^th Panzergrenadierdivision, and also the diversion to the Eastern Front of his 1^st Fallschirmpanzerdivision 'Hermann Göring'.