Operation Wacht am Rhein was the final major offensive and last gamble Hitler made. Mohnke, now in command of his home division, led his formation as the spearhead of the entire operation in the Ardennes. Attached to the I SS Panzer Corps, the LSSAH was one of the most elite and highly trained units in the entire German military. The crisis in the Reich meant that the LSSAH had dangerously low amounts of fuel for the vehicles that they depended on to make the division a viable fighting force. On 16 December 1944 the operation began, with Mohnke designating his best colonel, SS-Obersturmbannführer Joachim Peiper, and his Kampfgruppe to lead the push to Antwerp.
By 0700 on 17 December 1944, Peiper's Kampfgruppe had seized the American fuel dump at Büllingen. At 1330 that same day at a crossroads near Malmedy, men from Peiper's combat formation shot and killed at least 68 United States POWs. The Malmedy massacre, as it was to become known, is one of the most infamous killings of the war. Since Kampfgruppe Peiper, the perpetrators of the massacre, were under Mohnke's overall command, there were several accusations that he should be held personally responsible, yet he was never found guilty of the crime. By the evening of 17 December, the leading element of the LSSAH was engaged with the 99th US Division at Stavelot. Mohnke's division was behind their deadline by at least 36 hours by the end of the second day. Progress was further delayed by the retreating troops blowing up important bridges and fuel dumps that Mohnke and Peiper had counted on taking intact.
With each passing day, enemy resistance stiffened and the advance was eventually halted on all fronts. Desperate to keep the assault going, the German High Command ordered that a renewed attack begin on 1 January 1945. Yet this time, the Allies had regrouped their forces and were ready to repulse any attacks launched by the Germans. The operation formally ended on 27 January 1945, and three days later Mohnke was promoted to SS-Brigadeführer. A short while later the LSSAH and 'I SS Panzer Korps' were transferred to Hungary to bolster the crumbling situation there. Mohnke was injured in an air raid where he suffered, among other things, ear damage. He was removed from front-line service and put on the Führer reserve.
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