This is another name for the Battle of the Bulge (16 December, 1944 - 25 January 1945), a German offensive through the Ardennes Mountains region of Belgium. Its official name was the Ardennes-Alsace campaign, and the German army referred to it as Unternehmen Wacht am Rhein (Operation Watch on the Rhine, a reference to the German hymn).
The German goal was to split the British-American line and capture the city of Antwerp, before encircling and isolating the Allied armies, forcing them to negotiate an armistice in Germany's favour, and enabling Germany to devote more resources to the battle on the eastern front.
The operation was planned and coordinated in secret, and it took the Allies by surprise. Allied air forces were grounded by the heavy winter conditions. However, fierce resistance, especially around the town of Bastogne, disadvantageous terrain, the arrival of reinforcements in the form of General Patton's legendary Third Army, and a change in weather conditions that allowed the Allies to mobilise their superior air force rendered the German offensive a failure.
Both sides suffered heavy casualties, and the Battle of the Bulge represented the most damaging battle of the war for American forces.