On 16 December 1944 Hitler launched an offensive through the Ardennes aimed at driving a wedge between the allies, securing Antwerp and the important fuel supplies, cutting off the forces in the north and reversing the tide of the war in Germany's favour. The attack came as a complete surprise to the Allies. For this offensive, which became know as the Battle of the Bulge, Hitler assigned three armies. These were, in the north the Sixth SS Panzer Army which would attempt to break through to Liege, in the centre the Fifth Panzer Army which was to attack towards the St Vith and Bastogne, and in the south the Seventh Army directed on Luxembourg.
Determined American resistance deprived the Germans of early success and only the Seventh Army managed a breakthrough with the 1st SS Panzer Division which raced towards Malmedy, Stavelot and Liege. The offensive slowed but by 19th December the Germans were closing in on St Vith and Bastogne. Although the defenders of St Vith were forced to retreat they had inflicted a serious delay on the Germans. Meanwhile the refusal of Bastogne to surrender ( "Nuts", the US commander's reply to the request going down in history) prevented a breakout in the south.
Meanwhile on 19th December, Eisenhower cancelled the Allied offensive operations elsewhere and ordered Patton to attack the southern flank of the German salient and gave Montgomery command of additional US troops to assist his own in driving down on the northern flank of the German salient. It was as a part of this force, first guarding a number of the River Maas crossings between Liège and Namur and then moving forward on 7th January to take over from the 53rd Welsh Division as part of the allied counter offensive, that the 51st Highland Division found itself committed to the Ardennes.
The US counter attack began on the 22nd December. The Allied operations had been hindered in the first week of the offensive by thick fog making air operations impossible. When this lifted on 23rd December the air superiority of the allies was a crucial in both resupplying Bastogne and in attacking the Germans.
With the German offensive now stalled the Germans concentrated on Bastogne. The Allies now pushed forward slowing driving back the retreating Germans. Hitler's Ardennes offensive was over.