The commonest English name for the German army's attack on mainly American forces in the Ardennes in December 1944, also known as the Rundstedt Offensive or by its code name, Wacht am Rheinübung (Operation "Watch on the Rhine", after a traditional German song and chosen to be misleading.)
The plan for Wacht am Rhein was deeply flawed - indeed, completely futile. The strategic objectives - to cut off the northern end of the Western Front where it most closely threatened Germany, stable since the failure of the Arnhem operation, by driving to the sea at Antwerp - were completely unachievable, the fruit of Hitler's megalomaniac delusions; the outcome was foreseen by many of those involved on the German side. The ground may have been the same as that in which the somewhat fluky German offensive destroyed the French, Belgian and British armies in May 1940, but the weather, the opposition and the technology were not. The inevitable outcome was to get a lot of people killed in order to create a short-lived and indefensible salient.
The Panzer attack was launched under cover of bad weather - mist and snow - with low visibility effectively nullifying the Allies' air superiority. It struck a part of the front held by units which were either inexperienced (especially the 106th infantry division) or who had been moved to what was expected to be a quiet sector to recuperate after earlier fighting (e.g. the 101st Airborne Division which had been in heavy fighting during the abortive Arnhem attack).
The attack was basically two-pronged. The northern prong, made up of SS Panzer troops, brushed aside the earlier resistance advancing from the German frontier, taking many prisoners (a number of whom were shot after surrendering at Malmèdy, a massacre which spurred the Americans to more strenuous opposition) but ran aground in the defiles of the Amblève valley between Trois Ponts and Remouchamps and eventually ran out of fuel after Belgian partisans destroyed their major fuel dump at Spa; they were finally forced to abandon their vehicles and retire on foot. The museum at La Gleize, where most of the armour was abandoned, includes much material on this. Hulks of Panthers and the odd King Tiger serve as memorials in many towns and villages.
The southern prong via northern Luxembourg met heavier resistance on the Elsenborn ridge and in the Clervaux area, and was basically stalled by the resistance of the paratroopers in the Bastogne pocket and did not go much further. although some troops of the 5th army advanced via Houffalize and La Roche en Ardenne to reach the outskirts of Dinant, where Rommel had crossed the Meuse five years earlier; they were however already badly supplied and lacking flank cover and there was no prospect of crossing the river. Armoured forces under General Patton made a difficult forced march north from Alsace via Luxembourg to relieve the Bastogne pocket and then they and British forces advancing from the west fairly briskly recaptured the territory lost.
One casualty figure often neglected is that of the Belgian and Luxembourg civilians who suffered occupation and liberation for the second time in the war.
The end result of the battle was to enfeeble the German forces for no gain of ground - material losses were about even, but the Allies' capacity for resupply and replacement was far higher. In the end, the only effect was to ensure that in 1945 more of Central Europe fell to the Soviet army advancing from the east than to the delayed western allies.