'Kaput' was the US defeat of the German counterattack by Generalleutnant Martin Unrein’s Panzerdivision 'Clausewitz' of General Walter Wenck’s 12th Army in the sector of Major General Alvan C. Gillem’s XIII Corps of Lieutenant General William H. Simpson’s 9th Army in north central Germany with the object of regaining a portion of the Harz mountains, and the reoccupation of a length of the Elbe river (15/24 April 1945).
Together with the reduction of Magdeburg, this was one of the last two combat tasks of the 9th Army, and was the defeat of an incursion into the zone of the XIII Corps from the north and the clearance of a newly assigned sector along the Elbe river which had previously been a British responsibility. According to the Oberkommando der Wehrmacht’s orders, two of the 12th Army's newly formed divisions were to have attacked on a southerly axis into the 9th Army’s flank to reach the Harz mountains, but one of these formations became embroiled in combat with the British and the undertaking thus fell on Unrein’s incomplete Division 'Clausewitz', formed from staff and students of a Panzer training school, off the 9th Army’s flank near Uelzen.
At full strength in men, the division had at least 50 tanks and additional armour that included experimental and obsolete vehicles from the school’s inventory. The operational conditions favouring an attack on the 9th Army’s flank had begun to develop on about 15 April as Lieutenant General Sir Richard O’Connor’s flanking British VIII Corps of Lieutenant General Sir Miles Dempsey’s British 2nd Army moved to the north at Uelzen in accordance with the progress of Field Marshal Sir Bernard Montgomery’s Allied 21st Army Group in the direction of Hamburg. This exposed a 60-mile (100-km) sector of the 9th Army’s northern flank from Uelzen to the Elbe river except for British light armour patrols and a screen maintained by the 11th Cavalry Group. Gillem’s XIII Corps had already experienced problems in its rear areas as a result of German remnants who had been bypassed and taken refuge in some of the vast expanses of forest in the XIII Corps' zone and refused to surrender.
Astride the Wesergebirge on the eastern bank of the Weser river, Major General Frank A. Keating’s 102nd Division fought for two days against more than 2,000 Germans who had come together in the forested highlands. Farther to the east, on 13/14 April elements of the 102nd Division and Major General Alexander R. Bolling’s 84th Division fought to eliminate two other pockets of resistance, one of which included eight tanks.
The first indication of the presence of the Panzerdivision 'Clausewitz' came on 15 April, when the British identified part of the division in a counterattack at Uelzen. Before daylight the following day, a force estimated at 1,000 men and 30 armoured fighting vehicles severed the XIII Corps' primary line of communication near Gillem’s headquarters at Kloetze, some 36 miles (58 km) to the south-east of Uelzen. For another four days, contingents of the new Panzer division appeared at various points in the corps' rear, and the fight to destroy these contingents involved, at one time or another, all three of the corps' divisions, including Major General Lunsford E. Oliver’s 5th Armored Division and Major General Charles H. Gerhardt’s newly assigned 29th Division. Telephone communications and motor messenger services between the XIII Corps and the 9th Army were disrupted for two days. In most cases the German thrusts were stopped well short of the corps' southern boundary, although part of one column did penetrate into the zone of Major General Raymond S. McLean’s XIX Corps, where it was destroyed by Major General Isaac D. White’s 2nd Armored Division.
The most persistent of the German groups hid in a forest near Klötze. A chemical mortar battalion tried in vain to burn out the Germans with white phosphorus shells, and it required one of the 29th Division’s regiments, supported by 155-mm (6.1-in) and 8-in (203-mm) artillery, to destroy it.
Although more annoying than militarily significant, the activities of the Panzerdivision 'Clausewitz' imposed a one-day delay on the start of the XIII Corps' new task, namely the assumption of responsibility for the large triangle of uncleared territory north of the 9th Army’s boundary between Uelzen and the Elbe river. When the attack finally began on 21 April, the 5th Armored Division encountered and destroyed the remnant of the Panzerdivision 'Clausewitz'.
In total, the US forces had destroyed 47 tanks and more than 12 assault guns. The corps' sweep to the north of the former army boundary made rapid progress on 22 April, and by 24 April the corps had completed its task, the 9th Army Army now holding the line of the Elbe river to a point near Dannenberg, some 30 miles (50 km) downstream of Wittenberge.