'Wallop' was the Allied advance, otherwise known as 'Grouse' (iii), toward Tinchebray in northern France (12/15 August 1944).

The undertaking involved Major General Leonard T. Gerow’s US V Corps of Lieutenant General Courtney H. Hodges’s US 1^st Army within Lieutenant General Omar N. Bradley’s US 12^th Army Group in the aftermath of the successful implementation of 'Cobra' and defeat of 'Lüttich' as the Allies swept out of their Normandy lodgement to the south and south-west, and started their effort to trap major portions of Generalfeldmarschall Günther von Kluge’s Heeresgruppe 'B' by closing the Falaise/Argentan gap. The task of plugging the gap on the left flank of Major General Wade H. Haislip’s XV Corps, which had been caused by 'Lüttich' in the Mortain area, belonged to the 1^st Army, and more specifically Major General J. Lawton Collins’s VII Corps in the area to the north-west of Avranches. While the V Corps and Major General Charles H. Corlett’s XIX Corps on the north exerted pressure on the Germans by attacking toward Tinchebray and Flers respectively, the VII Corps to their south-west was to drive north-east from Mayenne in the direction of Fromental to cover the XV Corps' left flank.

In the case of each corps, the objective was the army group boundary, which corresponded with the right-flank boundary of Lieutenant General Sir Miles Dempsey’s British 2^nd Army (Lieutenant General Sir Richard O’Connor’s VIII Corps and Lieutenant General B. G. Horrocks’s XXX Corps) of General Sir Bernard Montgomery’s Anglo-Canadian 21^st Army Group. In advancing to the south-east, the British troops would pass across the fronts of all three of the 1^st Army’s corps.

Hodges had ordered the 1^st Army to attack as early as 9 August, but it was not until SS-Oberstgruppenführer und Generaloberst der Waffen-SS Paul Hausser’s 7^th Army of von Kluge’s Heeresgruppe 'B' had pulled back from Mortain that the operation get under way.

On 12 August Gerow’s V Corps and Corlett’s XIX Corps began the operation. Collins’s VII Corps needed an additional day to redeploy from its current position to the south of Mortain to Mayenne farther to the south-east. In the sector of the V Corps, Major General Walter M. Robertson’s 2^nd Division and Major General Charles H. Gerhardt’s 29^th Division attacked abreast through a narrow sector of rough terrain lacking good roads, and three days later captured Tinchebray and the high ground lying to the south of this town. With the corps' front facing to the east and the troops out of contact with the Germans, the advance then came to a halt. Hodges had hoped to trap a considerable number of Germans, but the prisoners taken during the four-day attack came to the disappointing total of 1,200, less than the number of casualties sustained by the V Corps.

From positions near Sourdeval, Corlett’s XIX Corps had attacked with the 28^th Division: hoping to improve the division’s performance, which he deemed unsatisfactory, Corlett on 12 August replaced Major General Lloyd D. Brown as commander of the formation with Brigadier General James E. Wharton, formerly the assistant commander of the 9^th Division. Several hours later Wharton was mortally wounded, and the following day Major General Norman D. Cota arrived from the 29^th Division to assume command.

On 13 and 14 August, respectively, the Major General Edward H. Brooks’s 2^nd Armored Division and Major General Leland S. Hobbs’s 30^th Division, hitherto part of the VII Corps, were shifted to reinforce the XIX Corps. Pivoting on Ger, the corps moved to the east against only light resistance and seized Domfront, which was held by a battalion of stragglers, depot personnel, and soldiers recovering from minor wounds. On 15 August the corps made contact with the British several miles to the west of Flers, and on the following day British forces swept south across the XIX Corps' front, as they had already done across the V Corps' front.

Although their advance had been quite fast and casualties light, the US formations had succeeded in trapping only a few German troops.

The VII Corps started in attack on 13 August after Collins had released Major General Paul W. Baade’s 35^th Division to Lieutenant General George S. Patton’s new US 3^rd Army, reunited the combat commands of the 3^rd Armored Division under a new commander, Major General Maurice Rose, brought Major General Manton S. Eddy’s (from 19 August Major General Louis A Craig’s) 9^th Division to join Major General Clarence R. Huebner’s 1^st Division at Mayenne, and placed Major General Raymond O. Barton’s 4^th Division in reserve to the south of Barenton. Against an estimated 7,600 combat effectives, the 1^st Division on the left and the 3^rd Armored Division on the right drove more than 20 miles (32 km) to the north-east from Mayenne on the first day. Fairly heavy fighting occurred on the following day around Ranes as German resistance stiffened in defence of the road linking Flers and Argentan. Though the 9^th Division moved into the centre to strengthen the corps' attack, strong opposition slowed the advance.

Montgomery approved a request for a crossing of the army group boundary, and at the end of 17 August the corps made contact with British troops at several points along its front. In the five-day action, the VII Corps had closed the gap on the left flank of the XV Corps, taken more than 3,000 prisoners, and destroyed large quantities of German matériel.

Though on 13 August the VII Corps had been well on its way to closing the gap on the left flank of the XV Corps, the XX Corps, recently committed command of the 3^rd Army, had also been involved. The fluidity of the tactical situation had led to some confusion: events outran decisions, and communications reflected outdated missions. The difficulties started on 8 August, when Patton ordered Haislip’s XV Corps to advance to the north from Le Mans to secure the line between Sées and Carrouges. Patton also alerted Walker’s XX Corps for a possible commitment beside the XV Corps, though the 3^rd Army could not currently say on which side of the XV Corps the XX would eventually operate. Three days later the 3^rd Army instructed the XX Corps to assemble on the line between Mayenne and Le Mans for an attack to the north-east to secure the line between Sées and Carrouges, the objective previously allocated to the XV Corps. The 3^rd Army had apparently decided to commit the XX Corps on the XV Corps' left. The only unit immediately available to XX Corps was Major General Horace L. McBride’s 80^th Division, which had been clearing the area of Evron.

One day later, following telephone conversations between the staffs of both headquarters, the 3^rd Army confirmed the previous attack order but changed the corps' objective. The XX Corps was now to advance only until it came in contact with the XV Corps around Alençon, which the latter had taken that morning, or farther to the north, and there await further orders. Completion of this mission would clear the XV Corps' left flank. The XX Corps issued its own orders shortly before 24.00. In an area between the VII Corps on its left and the XV Corps on its right, the XX Corps designated zones of advance for two divisions to attack abreast, the 80^th Division on the right and Major General Lindsay McD. Silvester’s 7^th Armored Division, only recently arrived in France, on the left. Because the armoured division would not arrive in the area until the afternoon of 13 August, the XX Corps ordered the 80^th Division to initiate the attack at 08.00 on 13 August: the armoured division was then to follow and pass through the 80^th Division to take the lead. With two regiments abreast, the 80^th Division was to attack from the area of Evron and Sillé le Guillaume to take the line between Argentan and Sées. The advance’s north-easterly axis cut directly across roads being used by the XV Corps going to the north from Alençon toward Argentan. As a result of an oversight, the XX Corps' order made no mention of the 3^rd Army’s order to halt the advance after establishing contact with the XV Corps in the area of Alençon or farther to the north.

The 80^th Division’s order indicated the railway linking Argentan and Sées as its objective. The troops were to destroy the German forces in their zone and establish contact with the XV Corps' armoured elements as and when these latter crossed the division’s front. The attached overlay designated routes of advance to the objective and also showed a route presumably to be taken by the XV Corps' armoured elements, which were to enter the 80^th Division’s sector from the direction of Alençon and move through the Forêt d’Ecouves to Argentan, thereby cutting diagonally across the XX Corps' sector, which was oriented to the north-east. Like that of the corps, the division’s order made no mention of halting upon contact with elements of the XV Corps but in fact emphasised that 'rapid progress…is essential to the success of the mission. Forces…will advance without regard to progress of forces to right and left.'

The attack was committed on 13 August, and during the afternoon of that day the right-hand regiment, the 318^th Infantry, became completely entangled with part of the 90^th Division which, under XV Corps command, was moving to the west of Alençon to protect the corps' deep left flank. Intent on its own mission, the 318^th Infantry cut across the 90^th Division’s allocated routes and thereby caused major traffic-flow problems. The 90^th Division ordered the 318^th Infantry off the road, but the regiment refused on the grounds that it was on the correct road to its objective. The 90^th Division informed the 318^th Infantry that another unit, also under XV Corps' command, had already captured and was occupying the 80^th Division’s objective. The 318^th Infantry was adamant, however, on the grounds that its orders were clear and that it planned to carry them out.

The 90^th Division then radioed the headquarters of the XV Corps, and the 318^th Infantry radioed the headquarters of the 80^th Division. Haislip despatched an officer to tell the 318^th Infantry to 'get the hell off the road', but the regimental commander retorted that his unit was under the command of the XX Corps and then sent a liaison aeroplane to divisional headquarters for help. Elements of Général de Division Philippe François Marie de Hauteclocque’s French 2ème Division Blindée then arrived and added further to the confusion. The commander of the 318^th Infantry finally got a message through to the headquarters of the 80^th Division, informing McBride that the XV Corps had ordered him off the road, then added 'My mission requires speed. What is decision?' What he did not know was that his mission had been rendered superfluous by the rapid passage of events.

The VII Corps had started to close the gap on the left flank of the XV Corps during that morning, and at the headquarters of the 12^th Army Group, Bradley had decided to halt the XV Corps before it reached Argentan. The commitment of the XX Corps on the left of the XV Corps proved unnecessary. At 13.000 the XX Corps, ordered to regroup, had instructed the 80^th Division to concentrate in the area of Laval and Evron. McBride therefore radioed the both the 317^th and 318^th Infantry to 'halt in place, clear road, bivouac present position for night…and await further orders'. This did not quite end the confusion. The regiment halted and bivouacked, but it now emerged that the area lay in the path of the 90^th Division’s advance. After more dispute the commander of the 318^th Infantry chose another bivouac. During the following morning McBride arrived and ordered both regiments back to the area of Laval and Evron.

By then the 1^st Army’s V, XIX and VII Corps were approaching the army group boundary. When they completed their moves, the Allied front resembled an irregular horseshoe virtually encircling SS-Oberstgruppenführer und Generaloberst der Waffen-SS Joseph Dietrich’s 5^th Panzerarmee, Hausser’s 7^th Army and General Heinrich Eberbach’s Panzergruppe 'Eberbach', the major part of the German forces in Normandy.

Surrounding these formations, Allied troops held a line from the Canadian positions at Falaise westward to the British near Flers, then eastward to Argentan, thereby forming the Argentan and Falaise pocket. Yet this pocket was still open at its eastern end, and through this 15-mile (24-km) bottleneck the German formations were now trying to escape complete encirclement.