Operation Undertone

'Undertone' was the US and French offensive by Lieutenant General Alexander McC. Patch’s 7th Army and Général de l’Armée Jean Joseph Marie Gabriel de Lattre de Tassigny’s French 1ère Armée in the area to the south of the Mosel river and Lieutenant General George S. Patton’s 3rd Army in the area to the north of this same river with the object of clearing the 'Saar-Palatinate triangle' as the third stage of the Allies' advance to the Rhine river (15/24 March 1945).

In the belief that the Allied operations to clear the western bank of the Rhine river, to the north of the point at which the Mosel river flows into it, would soon be completed, on 13 February General Dwight D. Eisenhower informed his two US army group commanders (Lieutenant General Omar N. Bradley of the 12th Army Group and Lieutenant General Jacob L. Devers of the 6th Army Group) to start planning a joint offensive which was to be launched on 15 March for the clearance of the Saar-Palatinate triangle. The planned offensive was to begin only after Field Marshal Sir Bernard Montgomery’s Allied 21st Army Group had reached the Rhine river farther to the north. The offensive was intended to draw German formations from the north and also to provide an alternative line of attack across the Rhine river should the primary Allied drive in the north fail.

The offensive, the planners of the Supreme Headquarters Allied Expeditionary Force decided, was to be made by mainly the 7th Army, which was to be strengthened by one armoured and three infantry divisions transferred from the 3rd Army. During the first week of March, Devers approved the 'Undertone' plan developed by the 7th Army. In this plan three corps were to attack abreast of each other from Saarbrücken toward a point to the south-east of Hagenau. Within this basic scheme a narrow strip along the Rhine river leading to the extreme north-eastern corner of Alsace at Lauterbourg was to be cleared by a division of the 1ère Armée under command of the 7th Army as the US Army’s main effort was delivered in the centre up the Kaiserslautern corridor. Eisenhower approved the plan, and noted that the offensive’s task was not merely to clear the Saar-Palatinate triangle but also to establish 6th Army Group bridgeheads across the Rhine river between Mainz and Mannheim, where the Main and Neckau rivers debouch into the Rhine from the east. Eisenhower additionally noted that the 12th Army Group, in the form of the 3rd Army, would be limited to diversionary attacks across the Mosel river to provide northern flank protection for the 7th Army.

Eisenhower approved the plan on 8 March, the day on which Patton obtained Bradley’s approval for the 3rd Army’s plan for a major attack across the Mosel river. Bradley in turn submitted the 3rd Army plan to Eisenhower on the basis that as the Germans had given no indication that they were prepared to pull back from the 'Siegfried-Linie' defences in front of the 7th Army, which might thus be faced with the prospect of a long and costly campaign, Bradley suggested that the 3rd Army should cross the Mosel near Koblenz and advance to the south along the western bank of the Rhine river with a view to cutting the lines of communication for SS-Oberstgruppenführer und Generaloberst der Waffen-SS Paul Hausser’s Heeresgruppe 'G' and at the same time break out from its Saar-Mosel river bridgehead near Trier to sweep into the 'Siegfried-Linie' defences from the rear.

Eisenhower approved the plan without qualification. This was the result of complex politico-military factors, and meant that the limited supporting role originally entrusted on the 7th Army’s northern flank to Patton’s 3rd Army was now developed into a major portion of the offensive, with the 7th Army’s offensive in the south now designed to breach the 'Siegfried-Linie' and so pin the German forces in the sector while Patton’s forces flooded forward over the northern half of the Saar-Palatinate triangle, so securing the line of the Rhine river between Koblenz and Ludwigshafen after the energetic US commander had (in the space of a mere five days) shifted the weight of his army to the south from Brohl and Koblenz on the Rhine river to Cochem and Nette on the Mosel river.

The main weight of 'Undertone' was thus reallocated to the 3rd Army, which deployed Major General Troy H. Middleton’s two-division VIII Corps to keep watch on Koblenz, Major General Manton S. Eddy’s XII Corps (Major General Stafford Le Roy Irwin’s 5th Division, Major General William R. Schmidt’s 76th Division, Major General Thomas D. Finley’s 89th Division, Major General Louis A. Craig’s 9th Division, Major General William M. Hoge’s 4th Armored Division and Brigadier General Charles S. Kilburn’s 11th Armored Division) in the centre against the left wing of General Hans-Gustav Felber’s 7th Army with Bingen and Bad-Kreuznach as its objectives, and Major General Walton H. Walker’s XX Corps (Major General Willard S. Paul’s 26th Division, Major General Horace L. McBride’s 80th Division, Major General Harry J. Malony’s 94th Division, and Major General William H. Morris’s 10th Armored Division) on the right against the right wing of General Hermann Foertsch’s 1st Army with Kaiserslautern as its objective behind the sector of the 'Siegfried-Linie' held by the rest of the 1st Army against the 7th Army. Using 22 divisions in three corps, this latter was to pin these German forces as the XX Corps swept into their rear.

'Undertone' was launched on 12 March, and by 14 March the XII Corps had secured a bridgehead over the Mosel river at Treis with its 5th and 90th Divisions, permitting Eddy to loose the 4th and 11th Armored Divisions toward Worms, via Bad Kreuznach and Kirn respectively, through the sketchy defences of General Gustav Höhne’s LXXXIX Corps and Generalleutnant Ralph Graf d’Oriola’s (later Generalleutnant Max Bork’s) XIII Corps.

The only serious problem encountered was Generalmajor Meinrad von Lauchert’s (from 20 March Generalmajor Oskar Munzel’s) 2nd Panzerdivision, which troubled the 4th Armored Division near Bad Kreuznach. The XX Corps had also broken through General Walther Hahm’s LXXXII Corps in its sector to the south of Trier with the 26th, 80th and 94th Divisions, allowing Walker to send forward his 10th Armored Division (supplemented from 17 March by Major General Roderick R. Allen’s 12th Armored Division from the 7th Army) in the direction of Kaiserslautern and Ludwigshafen.

By 19 March the 4th Armored Division was just 12 miles (19 km) to the south-west of Mainz, and the 10th and 12th Armored Divisions were 15 miles (24 km) distant from Kaiserslautern. Two days later the 90th Division was taking Mainz, the 4th Armored Division was in Worms, the 11th Armored Division was advancing to the south of Worms, the 12th Armored Division was in Ludwigshafen, and the 10th Armored Division was approaching Landau. With the capable aid of Major General Otto P. Weyland’s XIX Tactical Air Command, the 3rd Army had thus driven through the 'bad tank country' of the Hunsdruck area with commendable speed and very light losses.

While Devers had been reluctant at first to add his own endorsement to the 3rd Army’s proposed operation to the south of the Mosel river for fear that the convergent offensives might lead to an entanglement of the 3rd and 7th Armies, he had finally signalled his approval of the revised scheme. Devers and Bradley had then reached agreement on a new boundary which gave the 3rd Army a good road to the north-east from Saarlautern to the headwaters of the Nahe river, about 35 miles (55 km) to the north-east of Saarlautern and from here down the Nahe river valley to the Rhine river at Bingen. The revised inter-army boundary gave the 3rd Army responsibility for clearing the north-western third of the Saar-Palatinate triangle. Bradley and Devers also authorised Patton and Patch to deal directly with each other rather than through their respective army group headquarters.

Faced by the fortifications of the 'Siegfried-Linie', Patch planned a set-piece attack, preceded by an extensive bombing. Before it could launch its attack, however, the 7th Army had to build up supplies, make adjustments to the corps and divisional boundaries, move a number of formations, and bring new divisions up to their jumping-off points, but while he therefore felt that the 15 March start date was no longer feasible Patch nonetheless complied.

A major salient jutting into the Allied line south of the Mosel river, the Saar-Palatinate triangle was held by Hausser’s Heeresgruppe 'G' at the express instructions of Adolf Hitler, who refused to take the advice of his advisers that the army group should be pulled back from the 'Siegfried-Linie' to the better defensive line offered by the Rhine river. Germany had invested great resources in the construction of the 'Siegfried-Linie' defences, and Hitler would hear of no withdrawal from their supposedly impregnable capability. Hausser had originally controlled three armies, but of these General Hans von Obstfelder’s 19th Army had then been placed under the direct control of the Oberkommando der Wehrmacht after its evacuation of the Colmar pocket. Hausser thus controlled, in the south, Felber’s 7th Army (Höhne’s LXXXIX Corps, von Orilia’s XIII Corps and General Dr Franz Beyer’s LXXX Corps, and commanded temporarily by Obstfelder) and, in the south, Foertsch’s 1st Army (Hahm’s LXXXII Corps, General Smilo Freiherr von Lüttwitz’s LXXXV Corps, SS-Gruppenführer und Generalleutnant der Waffen-SS Max Simon’s XIII SS Corps and General Erich Petersen’s XC Corps). Though formidable on paper, these formations were all drastically understrength and their men poorly trained, and the German forces in the sector (from south to north the 19th Army, 1st Army and 7th Army) had only 13 divisions with which to face one French and 21 US divisions. The best of the German divisions were Generalleutnant Willibald Utz’s 2nd Gebirgsdivision and SS-Gruppenführer und Generalleutnant der Waffen-SS Karl Heinrich Brenner’s 6th SS Gebirgsdivision 'Nord'.

On 17 March, along the Moselle between Trier and Koblenz, Felber’s 7th Army was in a dangerous position not so much from a direct attack but from the flanking thrust against the right wing of Foertsch’s 1st Army by Walker’s XX Corps. The 7th Army's collapse was thus only a matter of time. Soon the 1st Army would also be in a very difficult position as, two days earlier on 15 March, the 7th Army had gone over to the offensive against the 1st Army along a 70-mile (115-km) front from the area of Saarlautern along a south-easterly axis toward the Rhine river. Even if the US offensive did not penetrate the 'Siegfried-Linie' it would in all probability pin the 1st Army against the fortifications while the 3rd Army hit it from the rear.

In its order of battle the 7th Army at this time included both experienced and inexperienced formations. Major General Edward H. Brooks’s VI Corps, for example, had fought for some time in the Italian campaign, and its three divisions (Major General John W. O’Daniel’s 3rd Division, Brigadier General Robert I. Stack’s 36th Division and Major General Robert T. Frederick’s 45th Division) were equally experienced. Major General Wade H. Haislip’s XV Corps had joined the 7th Army after fighting across France with the 3rd Army. The 7th Army’s third major formation, Major General Frank W. Milburn’s XXI Corps was relatively new, however, having joined the army only in January.

As the 7th Army launched its offensive, all were concerned to see how long and how well the Germans maintained their forward defence before falling back to the 'Siegfried-Linie'. Only Milburn’s XXI Corps, constituting the 7th Army’s northern wing near Saarbrücken, was comparatively close to the 'Siegfried-Linie', while the other formations were as much as 20 miles (32 km) from it. Making the 7th Army’s primary thrust in the centre, Haislip’s XV Corps faced an apparently difficult obstacle in the town of Bitche. Surrounded by fortresses of the French Maginot Line, Bitche had been taken from the Germans in December after a hard struggle, only to be relinquished in the withdrawal forced by the German counter-offensive. On the 7th Army’s southern wing, Brooks’s VI Corps, farthest of all from the 'Siegfried-Linie', had first to get across the Moder river, and one of the corps' divisions faced the added difficulty of attacking astride the rugged Lower Vosges mountains.

Two German corps and part of a third were in the path of the US offensive. At Saarbrücken, the left wing of von Lüttwitz’s LXXXV Corps faced only part of Milburn’s XXI Corps. Having recently lost Generalleutnant Kurt Freiherr von Mühlen’s 559th Volksgrenadierdivision to the 7th Army, the LXXXV Corps had only two divisions, of which one was holding 'Siegfried-Linie' positions to the north-west of Saarbrücken. To the south-east of this town, facing Haislip’s XV Corps, was Simon’s XIII SS Corps of three divisions. Extending the line to the Rhine river was Petersen’s XC Corps with two Volksgrenadier divisions and remnants of one infantry training division. Although the Germans were more concerned with the possibility of a US breakthrough in the XC Corps' sector into the Wissembourg gap than with a breakthrough of Simon’s XIII SS Corps into the Kaiserslautern corridor, the redispositions of the preceding weeks to salvage reinforcements for the 7th Army had in fact strengthened the XIII SS Corps. In addition to Generalmajor Alexander Möckel’s (from 24 March Generalleutnant Schmidt’s) 16th Volksgrenadierdivision and Generalmajor Karl Britzelmayr’s 19th Volksgrenadierdivision, the XIII SS Corps had SS-Oberführer Fritz Klingenberg’s (soon SS-Oberführer Georg Bochmann’s) 17th SS Panzergrenadierdivision 'Götz von Berlichingen', now a shadow of its former self but nonetheless fielding considerably more tanks and other armoured vehicles than were to be found in the entire adjacent corps.

The primary US effort was therefore directed against the stronger German formations, though at this stage of the war strength in regard to German divisions was but a relative term.

As the 7th Army started its offensive before the break of day on 15 March, the way in which the Germans would fight back seems to become clear. There were only two places in which the resistance was determined. One was on the left wing, where Major General Louis E. Hibbs’s 63rd Division sought to bypass Saarbrücken on the east and sever the German escape routes from the town, and the fact that the 63rd Division was soon in contact with the 'Siegfried-Linie' defences explained why it hit determined opposition there. The other was on the extreme right wing, where Général de Division Augustin Léon Guillaume’s attached 3ème Division d’Infanterie Algérienne was to clear the expanse of flat land between Hagenau and the Rhine river. There an urban area closely backing the Moder river defensive line and flat ground affording superb fields of fire for dug-in automatic weapons meant more difficult fighting. In the other parts of the front there were also engagements, some of them hard-fought and costly, but these were usually of short duration. The Germans had laid large numbers of anti-personnel and anti-tank mines, of which the US forces had to be especially cautious, but the German artillery fire was only occasionally heavy and in general only light.

This latter resulted largely from the fact that in the days immediately before the start of 'Undertone', the German artillery had been specifically targeted by the fighter-bombers of Brigadier General Glenn O. Barcus’s XII Tactical Air Command. At the same time the bombers of Lieutenant General James H. Doolittle’s 8th AAF had struck at the 'Siegfried-Linie' defences and industrial targets in cities such as Zweibrücken and Kaiserslautern. The weather was clear, and this made it possible for the warplanes to strike large numbers of targets.

The XV Corps was unusually large, with six rather than the otherwise standard three divisions, and only one regiment of Frederick’s 45th Division faced a water barrier at the start of its advance. That regiment had to cross the Blies river at a point upstream of that at which the river turns to the north-east to meander up the Kaiserslautern corridor. Yet even before dawn men of the regiment had driven through the main German defence beyond the river. Aided by searchlights, the US infantrymen bypassed strongpoints, leaving them for the following reserves to take or destroy. As night fell the 45th Division had driven almost 3 miles (4.8 km) beyond the Blies river to match a rate of advance that was general everywhere except in the fortified belt near Saarbrücken and on the flat land near the Rhine river.

On the XV Corps' right wing, Major General Withers A. Burress’s 100th Division drove quickly to the outskirts of the fortress town of Bitche. Perhaps aided by the fact that it had undertaken the same task in December, it gained dominating positions on the fortified hills around the town, leaving no doubt that it would clear the entire objective on the next day, 16 March. The only counterattack to cause appreciable was that which fell on one battalion of the 7th Infantry in O’Daniel’s 3rd Division. This division was responsible for the main effort in the centre of the XV Corps in the direction of Zweibrücken and the Kaiserslautern corridor. Although one company of supporting tanks ran into a dense minefield, disabling four tanks and stopping the others, one of the 7th Infantry’s battalions fought its way into the village of Uttweiler, just across the German frontier. Then an infantry battalion of the 17th SS Panzergrenadierdivision, supported by nine assault guns, struck back. The Germans quickly isolated the US battalion but could not expel it from the village. Supported by one platoon of tank destroyers and the regimental anti-tank company organised as a bazooka detachment, another of the 7th Infantry’s battalions counterattacked, destroying four 20-mm anti-aircraft vehicles and seven assault guns, and relieving the besieged battalion.

On the 7th Army’s wing, aimed at Wissembourg gap, the divisions of Brooks’s VI Corps generally had a similar experience: the sole exception was the 3ème Division d’Infanterie Algérienne. Although all four attacking divisions had to overcome the initial obstacle of a river, either the Moder or a tributary, they accomplished the task quickly in pre-dawn assaults as the Germans were too thinly stretched to do more than man a series of strongpoints.

On the corps' left wing, Major General Harry J. Collins’s 42nd Division overcame the geographical obstacle of attacking along the spine of the Lower Vosges by avoiding the roads and villages in the valleys and following the crests of the high ground. Pack mules, already proved in earlier fighting in the High Vosges, provided the division’s means of supply. As with the 3rd Division, a battalion of Major General Anthony C. McAuliffe’s 103rd Division was struck by a counterattack. Having entered Uttenhoffen, to the north-west of Hagenau, the battalion encountered such intense small arms fire and shelling from self-propelled guns that the regimental commander authorised a tactical withdrawal. Soon after the fall of night, German infantry counterattacked with support from four self-propelled guns, and the battalion pulled back another few hundred yards to better positions on the edge of a copse.

Meanwhile the 3ème Division d’Infanterie Algérienne crossed the Moder river without difficulty, but then became involved in bitter house-to-house fighting. Despite accurate artillery support, made possible by the clearness of the day, automatic weapons fire prevented the men of the French division from crossing a stretch of open ground facing the buildings of a former French frontier post. A minefield and two counterattacks, of which both were driven back by artillery fire, added to the division’s problems. As night fell, none of the division’s units had advanced more than 1 mile (1.6 km).

On 16 March, the offensive’s second day, all the indications were that the Germans were still fighting no more than a delaying action except on the two flanks. Nowhere did this seem clearer than in the XV Corps' sector, where all three attacking divisions improved on their gains of the first day’s fighting. Mines, demolitions and strongpoints, usually protected by a tank or assault gun, were the primary obstacles. By the fall of night the 3rd and 45th Divisions were well across the German frontier and only a short distance from the outposts of the 'Siegfried-Linie', and the 100th Division, relieved at Bitche by a follow-up infantry division, had begun to come abreast of the two other division. Fighter-bomber support by the XII Tactical Air Command was also very evident. Even though it seemed that the Germans were pulling back in accord with a predetermined plan, they were in fact intending a deliberate defence. Although corps commanders had requested authorisation to withdraw into the 'Siegfried-Linie' defences even before the start of 'Undertone, Foertsch and Hausser had refused. Generalfeldmarschall Albert Kesselring, the new Oberbefehlshaber 'West', remained as faithful as his predecessor to Adolf Hitler’s belief in holding ground until actually driven off it. In fact no order to pull back into the fortifications ever emerged above corps level.

Beginning the night of 16 March, German commanders facing the XV Corps simply did the obvious, ordering their formations and units to seek refuge in the 'Siegfried-Linie' defences when US pressure dictated retreat or destruction, and on the following day the commanders of formations facing the VI Corps did the same. At this time Patch came to the conclusion that it was a matter more of logistics than combat before all of the 7th Army’s divisions were fighting their way through the 'Siegfried-Linie' to enter the Saar-Palatinate triangle. At the same time German commanders came to appreciate that whether or not there developed any real battle for the 'Siegfried-Linie' depended not on the issue of orders from higher command levels but on the German formations and units, understrength and increasingly demoralised, trying to check the 3rd Army’s deeper enveloping thrust from the west and north-west into the German rear.

As the XX Corps' breakthrough headed toward Kaiserslautern, the 1st Army became increasingly concerned that its formations and units in the 'Siegfried-Linie' defences round Saarbrücken and Zweibrücken might be trapped. Once Kaiserslautern fell, the only routes of withdrawal left to those troops led through the Haardt mountains to the south of Kaiserslautern. Covered by an area of dense woodland, the Pfälzerwald, the region was crossed laterally by only one main highway, a secondary highway close behind the 'Siegfried-Linie', and a few minor roads and trails. The natural difficulties posed by these twisting, poorly surfaced routes had already been rendered still more tricky by the litter of German vehicles destroyed by US fighter-bombers.

Using the authority granted by Kesselring on 17 March to pull back units threatened with encirclement, Foertsch permitted the staged withdrawal of his westernmost troops, those of the LXXXV Corps. Over a period of three days, the corps' formations and units were to drop back to the east and move into position to block the main highway leading to the north-east through the Kaiserslautern gap. Foertsch’s plan was posited on his belief that the main threat to the Kaiserslautern gap would emerge from the west or south-west, whereas in fact it developed from the north-west, where the XX Corps was streaming through Hahm’s LXXXII Corps. The arrival of Major General William H. H. Morris’s 10th Armored Division at Kaiserslautern on 20 March meant not only that the gap was compromised by a force well in the rear of von Lüttwitz’s formations, but also that the only escape route available to von Lüttwitz’s forces and those of the adjacent XIII SS Corps was through the Pfälzerwald. The retreat of von Lüttwitz’s formations effectively opened a path through the 'Siegfried-Linie' for the left wing of the 7th Army.

Despite meeting a stubborn rearguard, the 63rd Division of the XXI Corps broke through the main belt of fortifications near St Ingbert late on 19 March. Had events moved according to plan, Milburn then would have despatched an armoured column northward to link with Walker’s XX Corps near St Wendel, but in the event the XX Corps had advanced so rapidly that all the significant objectives in the XXI Corps’s sector to the east of the 'Siegfried-Linie' had already been taken. Thus Milburn and his XXI Corps had achieved a breakthrough of the 'Siegfried-Linie' only to find they had nowhere to go.

Patch seized on the situation to provide a boost for his 7th Army’s main effort, the XV Corps' attack through Zweibrücken toward the Kaiserslautern gap. In two days of hammering at Simon’s XIII SS Corps, the XV Corps had still to break a gap in the 'Siegfried-Linie' for an exploitation by its armoured forces. Reinforced by an armoured combat command, Patch ordered Haislip to move through the 63rd Division’s gap and wheel round into the rear of the defenders of the 'Siegfried-Linie' still checking the XV Corps.

That the Americans would exploit the withdrawal of his formations was obvious to Foertsch who, during the night of 19/20 March, extended his authority to withdraw to the western wing of the XIII SS Corps. Thus, hardly had the new US combat command begun to move, at a time early on 20 March, to exploit the 63rd Division’s penetration, than the 45th Division of the XV Corps also advanced past the last pillboxes of the 'Siegfried-Linie' near Zweibrücken. During the night of 20/21 March the rest of the XIII SS Corps also began to pull back, and the progress of the 3rd Division accelerated. The German problem was to get the survivors of the LXXXV Corps and XIII SS Corps through the Pfälzerwald in the face of three threats: one from the closely following troops of the 7th Army; another from the 10th Armored Division of Walker’s XX Corps, which at Kaiserslautern was in a position to swing to the south and south-east through the Pfälzerwald and cut the escape routes; and a third from the ever-present fighter-bombers of the XII Tactical Air Command. It was the last threat which was most apparent to the soldiers of the retreating German divisions. As speed was imperative, the men had to move by day as well as by night, and were thus horribly vulnerable to detection and therefore attack from the air. Since almost everybody, including the troops of the motorised 17th SS Panzergrenadierdivision, had to use either the main east/west highway through the forest or the secondary road close behind the 'Siegfried-Linie', US fighter-bomber pilots had only to aim their weapons in the general direction of those roads to be assured of hitting some target. The German tactical problems were compounded by an acute shortage of vehicle fuel, and as a result the two roads were soon clogged by abandoned, damaged or wrecked vehicles, guns, and equipment.

The destruction in the Pfälzerwald was typical of the fighting over the whole of the 7th Army’s sector. Even as they ran the gauntlet of of the US fighter-bombers as they struggled through the Pfälzerwald, the disintegrating German forces now faced a fourth threat to their survival in the form of Brooks’s VI Corps, which had pursued the German withdrawal from the north-eastern part of Alsace and on 19 March began its attack on the 'Siegfried-Linie' to each side of Wissembourg. Here Petersen’s XC Corps was tasked to hold the fortifications and deny the US forces access to the flatter terrain along the Rhine river. In the 7th Army’s original plan, the 3ème Division d’Infanterie Algérienne attached to the right wing of the VI Corps along the Rhine river was to have been pinched out after it reached the Lauter river at the German frontier. The planners had not reckoned with the aspirations of the French in general and de Lattre de Tassigny, commander of their 1ère Armée, in particular. Assured of the support of the provisional French head of state, Général de Brigade Charles de Gaulle, de Lattre de Tassigny was determined to acquire a zone along the Rhine river to the north of the Lauter river in order to assure his forces of a Rhine river crossing site for the final drive into Germany.

As the 3ème Division d’Infanterie Algérienne matched and sometimes bettered the progress of the VI Corps' US formations and reached the Lauter river on a 10-mile (16-km) front, de Lattre de Tassigny was able to lay a convincing case before Devers. Using the 3ème Division d’Infanterie Algérienne and one combat group of Général de Division Henri Jacques Jean François de Vernejoul’s 5ème Division Blindée as Général de Division Joseph Jean de Goislard de Monsabert’s Groupement 'de Monsabert', again to be attached to the VI Corps, the French were to press to the north some 12 miles (20 km) beyond the Lauter river, thereby gaining a limited front along the Rhine river inside Germany. The French advance then pushed through the Bienwald, a large forest just to the north of the Lauter river and through which the 'Siegfried-Linie' defences passed. In the combat which followed, elements of Generalmajor Erich Seidel’s 257th Volksgrenadierdivision and Generalmajor Hermann von Witzleben’s 905th Division, the latter of less than brigade strength, were forced to retreat to the north in fighting dominated by the forested terrain. The adjustment meant that the assault on the 'Siegfried-Linie' by the VI Corps' four US divisions had to be concentrated into a sector less than 20 miles (32 km) wide.

Since the XC Corps had only the remnants of two Volksgrenadier divisions and one training division with which to defend its sector against French and US attacks, a breakthrough of the fortifications was only a matter of time. Yet just as had been the case in the zones of the XXI Corps and the XV Corps, it was not the attacks of the VI Corps but rather the powerful thrusts of the 3rd Army’s XX Corps in the German rear which in fact determined the time at which the 'Siegfried-Linie' would be penetrated. The VI Corps' divisions had been probing the pillbox belt for less than one day before Walker, leaving the task of reaching the Rhine river to Major General Roderick R. Allen’s 12th Armored Division and of capturing Kaiserslautern to an infantry unit, wheeled the 10th Armored Division to the south and south-east into the Pfälzerwald. By the fall of night on 20 March, two of the 10th Armored Division’s columns were just a few hundreds of yards from the main highway through the forest, one almost at Pirmasens on the western edge and the other not far from the eastern edge. A third column was approaching Neustadt, farther to the north beyond the forest’s edge. Meanwhile the 12th Armored Division was nearing the Rhine river near Ludwigshafen. Not only were the German lines of retreat through the Pfälzerwald about to be severed, but a rapid blow down the Rhine river plain from Neustadt and Ludwigshafen against the last escape sites for crossing the Rhine river appeared imminent.

On 20 March a desperate Luftwaffe launched about 300 warplanes of various types, including Messerschmitt Me 262 jet-powered fighters, in a fruitless attack on the 3rd Army’s columns. The US losses were small, but anti-aircraft units, in an increasingly rare opportunity to do the job for which they were trained, shot down 25 of the attackers, and pilots of the XII Tactical Air Command claimed another eight.

Reflecting the advance of the 10th Armored Division, the 1st Army late on 20 March signalled the XC Corps' most westerly elements to begin their withdrawal, and when the 42nd Division, in the mountains on the VI Corps' left wing, began a major assault against the 'Siegfried-Linie' late on the next day, its attack struck a vacuum. Soon after dawn on the morning of the next morning, 22 March, a regiment of the 42nd Division cut the secondary highway through the Pfälzerwald, and a column of the 10th Armored Division had moved astride the main highway through the woods and emerged on the Rhine river plain at Landau. Thus any Germans attempting to escape the forest had to do so off the roads and in small parties. By the fall of night on 22 March the Germans on the western side of the Rhine river could measure the time left to them in hours.

In the 'Siegfried-Linie' defences on each side of Wissembourg, the men of Petersen’s XC Corps continued to fight with determination from their pillboxes and bunkers. Major General Albert C. Smith’s 14th Armored Division pressed forward into the Wissembourg gap on 20 March and then fought units of the XC Corps for Steinfeld over the next two days. At Neustadt and at Landau the remnants of two of the XIII SS Corps' divisions, including the much depleted 17th SS Panzergrenadierdivision, checked the Americans through the day, but early in the evening their defence collapsed. Beyer’s LXXX Corps, transferred from the 7th Army in an effort to close the gap from the north alongside the Rhine river, had hardly anything left with which to prevent the 12th Armored Division from driving to the south from Ludwigshafen toward Speyer. By the fall of night on 22 March a column of the 12th Armored Division was just 6 miles (10 km) from Speyer, and on the following day the 14th Armored Division broke through the 'Siegfried-Linie' at Steinfeld and began its advance on Germersheim.

To forestall a repeat of the events which had occurred farther to the north at Remagen, where on 7 March elements of Lieutenant General Courtney H. Hodges’s US 1st Army had taken an incompletely destroyed railway bridge over the Rhine, by 19 March the Germans had blown all the Rhine river bridges to the north of Ludwigshafen. Of three that remained farther upstream to the south, the most southerly, at Maximiliansau, was destroyed on 21 March when a US artillery shell struck a detonator, setting off prepared demolitions. A second, at Speyer, was too immediately threatened and too far removed from the main body of German troops to be of much use to any but the defenders of Speyer itself and was blown late on the 23 March. Over the remaining bridge, at Germersheim, to the east of Landau, as many vehicles and field pieces as could be salvaged began to pass during the night of 22/23 March.

In these dire circumstances there was still no order from Kesselring for a German retirement across the Rhine river, and the headquarters of the 1st Army and Heeresgruppe 'G' were still to the west of the river. Some German officers feared that the last elements of the 1st Army were to be destroyed when at last, on 23 March, there arrived from Kesselring authority for them to fall back across the Rhine river. While the bridge at Germersheim continued to serve artillery and vehicles, troops on foot began to evacuate the western bank at three ferry sites to the south of the town. A small miscellany of infantrymen, an occasional tank or assault gun, and a regiment of anti-aircraft guns operating against ground targets formed a rearguard perimeter to the west of the ferry sites.

Although all of the VI Corps' divisions achieved clear breakthroughs on 23 March, they came into contact only with rearguards and failed to exercise any material effect of the German retreat across the Rhine river. Because a German force in Speyer fought doggedly, contact between the 12th and 14th Armored Divisions was delayed. Early on 24 March both divisions despatched task forces to locate and if possible to securing the single remaining bridge across the Rhine river at Germersheim, but neither had reached the fringes of the town when at 10.20 the Germans blew the bridge. Formal German evacuation of the western bank ended during the night of 24/25 March, while US units continued to mop up rearguards and stragglers up to 25 March.

It is impossible to ascertain how many Germans escaped from the Saar-Palatinate triangle, or how much in the way of weapons, equipment and other matériel they managed to take with them. Yet German losses clearly were very severe. The 7th Army’s staff estimated that the 1st Army and 7th Army had lost 75 to 80% of their infantry in 'Undertone'. The 7th Army and its attached French units captured 22,000 Germans during the campaign, and the 3rd Army took more than 68,000. The 3rd Army estimated that the German units opposing its advance lost approximately 113,000 men, including prisoners, while its own casualties totalled 5,220, including 681 killed. The 7th Army, much of its fighting along the 'Siegfried-Linie', probably lost about 12,000 men including slightly fewer than 1,000 killed.

'Undertone' was capped on the night of 22/23 March when units of the 5th Division secured a bridgehead across the Rhine river opposite Oppenheim. Two days later the XII and XX Corps had passed five divisions over the Rhine river and were developing their advance toward Aschaffenburg and Hanau. In view of the operation’s overall success, criticism might seem unnecessary. Nonetheless, it must be noted that the 1st Army, and to a lesser extent the 7th Army, fought a skilful delaying action to the end in the face of overwhelming strength on the ground and in the air. Neither of these formations suffered any wholesale encirclement despite the high command’s reluctance to sanction any withdrawal. In the process the Germans had withstood the clear threat of a rapid drive by some formations and units of the 3rd Army and 7th Army along the western bank of the Rhine rover to trap the 1st Army. In preserving their forces, however, the Germans had to surrender the important industrial area around Saarbrücken as well as the readily defensible terrain of the Pfälzerwald.