13 German divisions attacked the Allied lines in the Ardennes in Belgium, commencing the Battle of the Bulge.
The German offensive was a desperate roll of the dice by Adolph Hitler, who was, in effect, aided by Allied intelligence, which missed several clues and hints. The American generals regarded the Ardennes as a quiet sector, suitable for unblooded or rebuilding units. The American line was lightly held. And the Allied armies had, at the time, pretty much outrun the limits of their supply chain.
The Germans made full use of bad weather, which grounded the Allied Jabos, as the German soldiers termed Allied fighter-bombers. The Germans achieved complete surprise. But far stiffer than expected resistance in places such as Lanzerath, Elsenborn Ridge, St. Vith and, of course, Bastogne, threw off the German timetable. Allied engineers blew up many bridges that the Germans planned to capture.
When the weather cleared almost a week after the attack began, Bastogne and other areas held by the Allies were resupplied by air. P-47s began shooting up German troops and slicing the crap out of the Germans' supply routes. The Luftwaffe made a major attack on Allied airfields, only to be shot to pieces by AA guns firing VT-fuzed shells. The Luftwaffe destroyed almost double of what they lost, but the Allies could well afford to lose those planes. The Germans couldn't. The Germans also used up tanks, vehicles and fuel that they couldn't replace. They had no reserve troops left. And the Red Army kicked off its January offensive a week early,
By the time the battle officially ended in late January, 1945, it turned out to be the bloodiest American fight of the entire war.
The best American movie of the battle, if not the war, probably is
Battleground.
Update:
Coverage of the commemoration in Bastogne, headed by King Phillipe and Queen Mathilde.