'Honker' was the British portion of the Allied 'Diadem' advance, after the breakthrough of the 'Gustav-Linie' defences at Cassino, by Lieutenant General Sir Oliver Leese’s 8^th Army (May/June 1944).

Thus while Lieutenant General Mark W. Clark’s US 5^th Army was to link with Lieutenant General Lucian K. Truscott’s US VI Corps breaking out from the 'Shingle' lodgement at Anzio in 'Buffalo', and then to strike north through Generaloberst Eberhard Mackensen’s 14^th Army to sever the lines of retreat for Generaloberst Heinrich-Gottfried von Vietinghoff-Scheel’s 10^th Army, the 8^th Army was to advance up the Liri and Sacco river valleys as far as Sora and Valmontone respectively.

It took Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber Südwest' and commander of Heeresgruppe 'C', some time to realise the extent of the Allied concentration between Cassino and the sea in the first stages of 'Diadem'. On 12 May he believed that there were only six Allied divisions on this front, including a single division of Général de Corps d’Armée Alphonse Juin’s Corps Expéditionnaire Français (French Expeditionary Corps) forward in the Garigliano river bridgehead. There was no trace of Lieutenant General E. L. M. Burns’s Canadian I Corps, and Kesselring was convinced the Canadian formations had joined Major General Fred L. Walker’s US 36^th Division, which was known to have undertaken assault landing exercises. Thus his appreciation of the situation persuaded Kesselring that the Allies were planning to execute an amphibious landing behind the left flank of the German front. Two days later the German commander-in-chief in Italy demanded identification data from every sector 'as it was intolerable that troops could be in fighting contact for two days without knowing whom they were fighting'.

In fact the Canadian formation was not identified until 16 May, and during the evening of the same day Kesselring discussed the situation with von Vietinghoff-Scheel and agreed to a German general withdrawal to the 'Führer-Senger-Linie' defences.

Generalleutnant Richard Heidrich refused to allow his 1^st Fallschirmjägerdivision to abandon Cassino until he had received a personal order from Kesselring. On the following day General Fridolin Ritter und Edler von Senger und Etterlin arrived back at the headquarters of his XIV Panzerkorps at Pico to find von Vietinghoff-Scheel and his staff also established there after being bombed out of their own headquarters near Frosinone. von Senger und Etterlin at once set off to Itri, where a specially constituted machine gun battalion of Generalleutnant Heinrich Deboi’s 44^th Reichsgrenadier Division 'Hoch- und Deutschmeister', ordered forward by Kesselring, was attempting to stem the Allied tide. He found the situation far worse than reported, with the machine gunners in retreat and the Allies in hot pursuit.

On 18 May Kesselring and von Vietinghoff-Scheel met to discuss the situation, and von Senger und Etterlin was asked to give his appreciation. With Generalmajor Rudolf Sperl’s 15^th Panzergrenadierdivision divided and much of its strength already lost, there was no chance of mounting a properly formulated counter-attack on the XIV Panzerkorps' front and von Senger und Etterlin had to recommend an immediate orderly withdrawal to prevent his corps from being cut off.

As the architect of what was now called the 'Hitler-Riegel', which he had designed specifically to guard against a penetration of the 'Gustav-Linie' near the coast, von Senger und Etterlin was very outspoken about the situation. With General Valentin Feurstein’s LI Gebirgskorps still holding in the Cassino sector and his own corps split down the centre and desperately struggling to maintain contact on the left, von Senger und Etterlin saw little chance that the layback position which he had been so carefully prepared would ever being properly manned. But Kesselring would accept no suggestion of a significant retreat as he was concerned only with the creation of an intermediate line as far east of Anzio as possible. To achieve this he had already committed Generalmajor Ernst-Günther Baade’s 90^th Panzergrenadierdivision to hold the Esperia defile, where it had reached in a number of separate detachments that had been roughly handled by the French, and had also brought up Generalleutnant Smilo Freiherr von Lüttwitz’s 26^th Panzerdivision to block the sector to the south of Pico.

Part of Generalleutnant Friedrich-Wilhelm Hauck’s 305^th Division had reached the valley of the Liri river, and Kesselring now sent for the remainder of the division and most of Generalmajor Hellmuth Böhlke’s 334^th Division from the Adriatic coast, where it was to be replaced by Generalmajor Harry Hoppe’s 278^th Division from Istria.

von Vietinghoff-Scheel undoubtedly appreciated the sense of von Senger und Etterlin’s reasoning and felt that the opportunity for an orderly withdrawal to the 'Hitler-Riegel' defences had been lost. Although that section of the line down as far as Pontecorvo was at that very moment being occupied, von Vietinghoff-Scheel was extremely concerned about the situation to the south of Pico for, in the event that the French broke through in this sector, they might well reach Ceprano or Frosinone with results disastrous to the Germans.

However, Kesselring had decided that whether the Germans fought on the 'Hitler-Riegel' or had to pull back, the support of the right wing had to be the German forces' primary consideration. Determined to restore the situation on the crumbling right flank, on 19 May Kesselring ordered the 29^th Panzergrenadierdivision forward to the area of Fondi.

Even as Kesselring was attempting to regain a grip on the rapidly evolving operational and tactical situation, the Allies were making ground and the battle was becoming more fluid. On 18 May a small armoured force of the Derbyshire Yeomanry, of Major General V. Evelegh’s 6^th Armoured Division, just failed in a coup-de-main attempt to take Aquino, which was the key northern part of the 'Hitler-Riegel' defences in the Liri river valley, and the Canadian I Corps was coming up fast on its left. After the capture of Santa Oliva the French now overlooked Pontecorvo, while farther toward the coast Major General Geoffrey T. Keyes’s US II Corps took Itri and Gaeta on 19 May.

General the Hon. Sir Harold Alexander, commanding the Allied Armies in Italy (ex-Allied 15^th Army Group) had meanwhile decided to increase the pressure. The US 36^th Division was sent into the Anzio beach-head and the 8^th Army were ordered to attack up the valley of the Liri river before the Germans could settle into the 'Hitler-Riegel' defences. Enveloping attacks were ordered by the French on Pico and by the Poles on Piedmonte. All of these Allied these attacks would start on 19 May. On the right, Generał dywizji Władisław Anders’s Polish II Corps took Villa Santa Lucia, but the 1^st Fallschirmjägerdivision held out in Piedmonte and on Monte Cairo for seven days of bitter and costly fighting. On the other flank, the 26^th Panzerdivision and 90^th Panzergrenadierdivision were equally determined to hold their ground, so Monte Leucio was the scene of desperate fighting for two days before being recaptured by the French on 22 May. Pico eventually fell to Général de Division Augustin Guillaume’s 3ème Division d’Infanterie Algérienne on the same day. In the valley, meanwhile, Major General C. F. Keightley’s 78^th Division, with Major General C. Vokes’s Canadian 1^st Division on its left, had reached the main defences of the 'Hitler-Riegel'.

In many respects these defences were more formidable still than those of the 'Gustav-Linie' in the valleys of the Garigliano and Rapido rivers. Although the defences were only weakly manned, it immediately became evident that only a set-piece attack could succeed in breaking through. To give time for preparations and also for Major General D. Russell’s Indian 8^th Division and Evelegh’s 6^th Armoured Division to be moved up over now-congested roads, Alexander ordered that this co-ordinated attack should start on 23 May. The Canadian II Corps was to attack to the north of Pontecorvo, while Lieutenant General S. C. Kirkman’s XIII Corps closed up ready to break out to the north of Aquino.

As the 8^th Army was squaring up to the north-eastern part of the Hitler Line, the US II Corps continued to make good progress farther to the south-west. The transfer of 36^th Division to the Anzio beach-head had taken place between 18 and 22 May so as not to draw attention to the move. Lacking sufficient numbers of landing craft to accelerate the rate of the Anzio lodgement’s reinforcement, Clark now decided to make a determined effort to reach the lodgement overland, taking advantage of the collapse of 10^th Army's right flank, for once Highway 7 was open the supply problems at Anzio would be eased.

Major General John B. Coulter’s US 85^th Division and Major General John E. Sloan’s US 88^th Division, currently the only two formations of the US II Corps, were directed on Terracina and Fondi with orders to exploit down Highway 7. These attacks to the north-west of Itri, which fell on 19 May, were to be pressed forward immediately.

Kesselring expected Fries’s 29^th Panzergrenadierdivision to reach Fondi on the morning of 20 May, but when he got back to headquarters that evening he was told that von Mackensen had protested against the removal of his last mobile division and that the 29^th Panzergrenadierdivision had not yet started its move to the south. Kesselring now ordered von Mackensen to take full responsibility for the threatened area on a line from Sperlonga on the coast through Fondi to Frosinone and Valmontone, but it was too late. Fondi had been taken on 20 May, and by 23 May a US advance guard was approaching Priverno.

Elements of the 29^th Panzergrenadierdivision started to reach Terracina on 22 May and were able to delay its capture for two days. This belated German attempt was both too weak and too late, however, and the Germans abandoned Terracina during the night of 23/24 May.

Within 24 hours armoured patrols from the two wings of the 5^th Army had advanced to meet each other on the coastal road at Lake Fogliano.

On 21 May Alexander issued orders for the 'Buffalo' break-out of Truscott’s US VI Corps from the Anzio lodgement to be launched on 23 May. The attack was timed for 06.30, just after the Canadian assault went in against the 'Hitler-Riegel' defences. Major General Ernest N. Harmon’s US 1^st Armored Division and Brigadier General John W. O’Daniel’s US 3^rd Division, plus Brigadier General Robert T. Frederick’s 1^st Special Service Force, would capture the area around Cisterna as a base and the 36^th Division would then pass through and advance to a line running across the valley below Velletri supported by the 1^st Armored Division. Then reinforced by the US 3^rd Division, it was to advance on Valmontone.

To counter any attempted break-out from the Anzio lodgement, von Mackensen had five divisions, reinforced by the arrival of Böhlke’s 334^th Division from the Adriatic coast, but with the 26^th Panzerdivision and the 90^th Panzergrenadierdivision transferred to the 10^th Army, his armour was reduced to the assault guns of Oberst Hans Hecker’s 3^rd Panzergrenadierdivision and a handful of tanks from the two independent Panzergrenadier battalions. For more than four months von Mackensen had been responsible for investing the Anzio lodgement, and these defensive operations had been remarkably successful. Now that the Generalmajor Wilhelm Schmalz’s Panzerdivision 'Hermann Göring' was on its way to the south from Oberkommando der Wehrmacht reserve in northern Italy, he felt that his sector of the 'Cäsar-Linie' defences could be held without much difficulty.

Deprived of air reconnaissance by the Allies' overall air superiority, however, neither Kesselring nor von Mackensen had any real estimate of the Allied build-up in the Anzio lodgement. Constantly harassed from the air and forced to move mainly by night, by 23 May the leading elements of the Panzerdivision 'Hermann Göring' had progressed only as far as Viterbo.

The battle of the Liri valley was about to reach its climax. In the 8^th Army sector opposite the 'Hitler-Riegel' defences, 400 pieces of artillery were tasked with softening the German positions, and on the night 19/20 May as many as 1,000 shells per hour were falling on known strongpoints. This softening effort continued at varying rates of fire and timings until the attack of Vokes’s Canadian 1^st Division on 23 May. By this date more than 800 guns were in position to support the assault with a barrage on a 3,000-yard (2745-m) front and to a depth of the same distance.

By 12.00 on 24 May Pontecorvo had been captured, each side sustaining heavy losses. Continued resistance at Aquino and delays in getting Major General B. M. Hoffmeister’s Canadian 5^th Armoured Division through the breach in the line, however, prevented the XIII Corps from advancing until the following day. Meanwhile a Canadian armoured brigade, led by the tanks of Lord Strathcona’s Horse with the lorried infantry battalions of the Westminster Regiment, seized a crossing over the Milfa river on the afternoon of 24 May and held it overnight against counterattacks by the 90^th Panzergrenadierdivision. This penetration struck the junction point between the two German corps, and from captured orders it was subsequently discovered that it was not before the Canadian had crossed that the 1^st Fallschirmjägerdivision received orders to hold this river line.

On the other side of the valley the Corps Expéditionaire Français was making ground in a north-westerly direction toward the Sacco river and Ceccano. At Anzio, 'Buffalo' had penetrated, on 24 May, 2 miles (3.2 km) to the north of Cisterna, although the town itself had not yet fallen.

The moment of decision had now come for Kesselring: the left wing of the 14^th Army seemed likely to disintegrate and would, in any event, soon be under attack by the US formations moving inland from the coast. The strong positions in the Alban Hills could only act as a pivot for the 10^th Army's withdrawal if Valmontone was held as a point of contact between the two armies and Highways 5 and 6 were kept open as long as possible. The Panzerdivision 'Hermann Göring''s concentration at Valmontone had to be speeded and the troops in the mountains around Belmonte pulled back. If the 'Cäsar-Linie' defences were to be occupied, a general withdrawal by the 10^th Army must start immediately.

This withdrawal was to be covered by the XIV Panzerkorps, which would take command of all mobile troops in the valley. Over the next two days von Senger und Etterlin ordered his shattered infantry units to move to the north to an assembly point at Frosinone and sent the 29^th Panzergrenadierdivision to halt the drive of the Corps Expéditionaire Français toward Highway 6. Behind this armoured screen von Senger und Etterlin gradually withdrew his armour north.

In the mountains opposite the X Corps, the Germans started their demolition programme during the night of 24/25 May, and by 26 May both Lieutenant General Sir Bernard Freyberg’s New Zealand 2^nd Division and Generale di Brigata Umberto Utili’s Corpo Italiano di Liberazione (Italian Corps of Liberation) had taken up the advance on the general axis linking Atina and Avezzano via Sora. Minefields, extensive demolitions and rearguards in the extremely difficult mountainous country, however, made the Allied progress very slow.

On 24 May Truscott was holding the 36^th Division ready to pass through toward Cori and Artena, but he pointed out to Clark, who had established a command post in the lodgement and now agreed with Truscott’s appreciation, that von Mackensen might be able to concentrate at Valmontone not only the Panzerdivision 'Hermann Göring' but also the 3^rd Panzergrenadierdivision and Generalmajor Heinrich Trettner’s 4^th Fallschirmjägerdivision.

On the next day the 3^rd Division took Cisterna and reached Cori. As the attack was going well Truscott held the 36^th Division in reserve. A strong armoured force of the 1^st Armored Division, supported by infantry, was by this time well through the valley past Velletri to within less than 4 miles (6.5 km) of Artena. The 1^st Special Service Force was also making good progress through the mountains to protect the right flank of this thrust. Truscott’s appreciation was that his troops would be astride Highway 6 at Valmontone by the following morning, and his corps had already taken 9,000 prisoners. When he got back to his command post late in the afternoon he found a staff officer from the headquarters of the 5^th Army waiting to tell him that Clark’s orders were that the attack to the north-west would be mounted as soon as possible. Truscott was astounded: there was no sign of any German withdrawal from the western part of the bridgehead, his attack was going well against light opposition, and now, within a few hours of its success, he was ordered to turn and launch a head-on attack against the strongest German positions. Truscott said he must first speak to Clark personally, but was told that Clark had already left the lodgement and could not be reached even by radio.

This order turned the main effort of the VI Corps away from the Valmontone gap, and thereby prevented the destruction of the 10^th Army. Truscott now had to extricate the 1^st Armored Division, which was replaced by the 36^th Division, and move it laterally behind his front to reach the front one more between Major General Charles W. Ryder’s US 34^th Division and Major General William W. Eagles’s US 45^th Division. These three divisions would now be directed on Campoleone and Lanuvio on a 5-mile (8-km) front. The divisional commanders received their orders at 23.00 on 25 May and the attack started a mere 12 hours later after a remarkable piece of staff work. On the following day Major General Alfred M. Gruenther, Clark’s chief-of-staff, had to explain Clark’s unilateral decision to Alexander.

The revised US offensive initially proceeded well, but the three German formations facing it (3^rd Panzergrenadierdivision, Generalmajor Hellmuth Pfeiffer’s 65^th Division and 4^th Fallschirmjägerdivision) were falling back on well-prepared positions in the strongest part of the 'Cäsar-Linie'. After four days of hard fighting with heavy losses on each side, the US forces took Campoleone, but on the right the 34^th Division was still 1 mile (1.6 km) short of Lanuvio. With the support of a small tank force and the 1^st Special Service Force, the 3^rd Division took Artena during the afternoon of 27 May but was unable to reach Highway 6, which lay 2.5 miles (4 km) beyond the town. Here the Panzerdivision 'Hermann Göring' was concentrating and had been joined by elements of Generalmajor Werner Goeritz’s inexperienced 92^nd Division, just arrived from points to the north of Rome. Kesselring had expected the Panzerdivision 'Hermann Göring' to reach Valmontone by 24 May at the latest, but this formation’s progress toward the front had been under constant air attack and the division’s main body did not arrive until 27/28 May. Kesselring in fact had hoped that the division would be brought into action forward of Artena in conjunction with the remnants of Generalmajor Heinrich Greiner’s 362^nd Division and Generalmajor Kurt Hoffmann’s 715^th Division.

On the 8^th Army’s front Kesselring had one last opportunity to check the British advance. About Ceprano, the Liri river flows from the north parallel with Highway 82, while the valley of the Sacco river continues through Ceccano toward Valmontone. Arce could be approached only through a defile, and defensive positions here and on the two river lines were designed to protect the retreat of the remnants of the 10^th Army along Highways 82 and 6.

On 26 May the Indian 8^th Division occupied Roccasecca, but the 6^th Armoured Division was delayed at Arce. On 27 May the 78^th Division was despatched across country toward Ceprano, ready to take up the advance along Highway 6 as soon as the Canadians had forced a crossing. But it was clear that the German positions could not hold out much longer and that the withdrawal of the 10^th Army by Highway 6 and through the Simbruini mountains must be accelerated. All depended now on the [ne]14^th Army. The Corps Expéditionnaire Français, advancing in two columns to the north-west, was approaching Ceccano on 28 May, by which time Ceprano had fallen and a bridge had been constructed below the town.

Although von Senger und Etterlin was becoming concerned about withdrawing his corps from Frosinone, he was distinctly more worried about the gap developing between the two armies, and on 26 May suggested to von Vietinghoff-Scheel that the mobile divisions should be sent back as quickly as possible to hold Valmontone, which he considered the 'point of greatest threat to all German divisions standing to the south of the line Valmontone-Sora'. Against overwhelming Allied air superiority movement along the mountain roads to the east of Highway 6 had become almost certain death, and he feared that the main escape route through Subiaco to Tivoli would be cut by a drive to the north across the head of the Sacco river valley.

The last chance of re-establishing contact with the 14^th Army after 26 May, and of gaining time for the occupation of the 'Cäsar-Linie' defences, was a concentration of this essential point of Valmontone, where von Senger und Etterlin thought he could commit a division each day and possibly counterattack toward the Alban Hills with a view to preventing an Allied breakthrough to Rome. Kesselring was still relying on von Mackensen to hold the hinge position around the Alban Hills, however, and left the 10^th Army to extricate itself as rapidly as it could.

On 28 May the 6^th Armoured Division took the positions held by the 1^st Fallschirmjägerdivision on Monte Grande, to the south of Arce: the denial of this position to the Allies had been vital to cover the LI Gebirgskorps' withdrawal toward Avezzano. The Canadians took Pofi on 29 May and entered Frosinone two days later, while the XIII Corps advanced to Arpino and began to swing north. The Corps Expéditionaire Français was still making good progress and Juin was worried not so much by the slowing of the 8^th Army’s advance as by the situation developing on his own front, where he was about to run into a bottleneck and anticipated considerable congestion on Highway 6, where his own formation, the Canadian I Corps and possibly part of the XIII Corps were likely to converge on Colleferro. On 28 May he suggested to Clark that the 8^th Army’s left flank should be on the axis from Subiaco to Terni via Rieti, leaving him with a sector bounded to the left by the axis from Prossedi to Tivoli via Gavignano. As Juin pointed out, the Corps Expéditionaire Français would then be well placed to take Valmontone from the east if von Mackensen continued to stand in front of Rome, and would also be able slightly later to occupy the headquarters of the Tiber river in the mountains. Despite his general preoccupation with the battle south of Albano, Clark agreed to the proposal.

Alexander still wanted Highway 6 as one of the primary axes for the advance of the 8^th Army, however, and Juin was therefore instructed to keep his corps to the south of the Secco river, while Clark concentrated his 85^th and 88^th Divisions at Anzio and sent them with the US II Corps headquarters to take over the Artena sector, where the 3^rd Division and the 1^st Special Service Force were still held down. In the sector between Velletri and Lanuvio, Clark now had the 36^th, 34^th and 45^th Divisions and the 1^st Armored Division, with Major General W. R. C. Penney’s British 1^st Division to protect the flank of his attack, while Generał brygady Nikodem Sulik’s Polish 5^th 'Kresowa' Division held the coastal sector. The attack by both the US II and US VI Corps was ordered for 1 June.

Walker’s 36^th Division meanwhile had been pushing forward patrols to the east of Velletri and had found a gap in the German positions on Monte Artemisio. Truscott immediately sent his corps' engineers to help develop a tank track, and before the main attack opened on 1 June the 36^th Division had reached the crest of the Colli Laziali (Alban Hills) and cut the road west of Velletri. von Mackensen’s failure to close this gap resulted in his subsequent replacement. This brilliant operation by the 36^th Division gave Truscott the key to the German positions astride the Alban Hills. After a bitter fight Lanuvio fell on 3 June. The Polish 5^th Division beat off a heavy counterattack, and the Germans were now pushed back against the Tiber river along the whole front.

The US II Corps had been equally successful, taking Valmontone on 2 June and pushing forward to Palestrina. The defeat of 14^th Army was complete. Racing along Highway 6, the US II Corps turned down the left bank of the Tiber to enter Rome on 4 June, while the 1^st Armored Division of the VI Corps entered the city along Highway 7. While Clark was being driven through the streets of Rome to take the formal surrender, the advanced guard of the Corps Expéditionaire Français was working through the mountains close to Tivoli and Major General W. H. E. Poole’s South African 6^th Armoured Division was leading the Canadian advance into Paliano.

On the other axis of advance, the XIII Corps was halfway to Subiaco and the X Corps was to the south-west of Avezzano.

On the Adriatic flank the Germans had no option but to conform to the retreat of the two defeated armies, now pulling back to north from Rome on both sides of the Tiber. Leaving 20,000 prisoners and probably more than 10,000 dead and wounded, the 10^th Army and 14^th Army pressed to the north as best they could. The railways were still being cut by Allied bombings and, forced on to the roads, the German transport columns came under relentless air attack: for the period between 12 and 31 May Major General John K. Cannon’s Allied Tactical Air Force claimed 2,556 German vehicles destroyed and 2,236 damaged, and the figures for June were some 33% greater than these.