National Review has a much better article than I could ever write so I will just do a short survey.
Winter 1944 was one of a relative calm in Europe compared to the last few years. The Germans were unable to capitalize on the Allies' disaster of Market-Garden and were thought of as contained on the Western Front. The Allies were ready for a peaceful Christmas. The Germans had never launched a major winter offensive since the time of Fredrick the Great.
The Nazis attacked without warning. The Ardennes Forrest was the scene of the massive German drive to the sea. If the Germans could reach the port cities in Belgium they could destroy several Allied divisions, cut off supply-lines, and score a huge dramatic victory.
The blitzkrieg continued deep into Belgium. Only one thing stood between Hitler and victory. The 101st Airborne was rushed to the front. Many immediate suffered from the cold as there was no time to gather supplies and morale suffered as battered soldiers warned of the coming German war machine.
The 101st held the city the Bastogne while surrounded and held the German at bay. When the Germans demanded their surrender the 101st replied "Nuts." Patton led his tanks in a fighting drive and relieved the 101st the day after Christmas.
Today the Mardasson Memorial is dedicated to 76,890 Americans who died or were wounded in the battle. Their bravery help end the war. Let us remember them.
For those who want to know everything about the Battle of the Bulge the official army history can be read here.