On December 14, 1944, American GIs stationed in the Belgian-German border town of Bastogne were in a jolly holiday mood. Hollywood star Marlene Dietrich was in town on a USO tour performing songs for a crowd of fresh-faced new arrivals and war-worn infantry on much needed R&R.

It was six months after the D-Day invasion at Normandy and the Allies had reason to celebrate. The Americans and British had chased the Nazis out of France and the Russian army was quickly closing in from the East. The surrender of the German menace was in sight.

But that’s not how Adolf Hitler saw it. The Nazi leader, paranoid and agitated after a failed assassination attempt by the Operation Valkyrie conspirators, believed Germany had one last chance to strike at the heart of the Allies in the West. Hitler ordered his commanders to prepare for an all-out offensive against a strategic soft spot in the Allied line located in the densely forested region known as the Ardennes.

Hitler’s last-ditch gamble would result in the largest land battle in American military history and cost tens of thousands of lives on both sides. But in the month-long slog known as the Battle of the Bulge, the Americans proved their mettle under frigid winter conditions to seal the fate of the Nazis for good.

The Battle of the Bulge is the Allied name for the massive German offensive that the Nazis codenamed Wacht am Rhein or the “Watch on the Rhine.” More than 1 million Allied and German soldiers fought in the Battle of the Bulge, nicknamed for the westward bulge created in the Allied line after Nazi tanks and infantry temporarily captured territory during Hitler’s surprise attack.

The Allies were indeed unprepared for the German onslaught, says Martin King, a World War II historian and author of eight books about the Battle of the Bulge. The Ardennes region was known to American GIs as the “honeymoon sector,” a remote stretch of the Allied front line where new arrivals could ease into the war effort and battle-weary vets could recuperate and refit for battle.

“In December of 1944, there were only four American divisions in the Ardennes covering an 89-mile front,” says King. “Two of the divisions had never fired a shot in anger and two were recovering from the grueling Battle of Hürtgen Forest.”

Allied commanders dismissed intelligence from the British ULTRA codebreakers that large numbers of German troops and equipment were being pulled from the fight with Russia and amassing along the Western front. The assumption, soon proven wrong, was that the Nazis were simply bracing their defenses for the coming Allied push into Germany. No one thought that Hitler would have the gall to attempt a counterstrike with a German army already decimated by months of heavy fighting on two fronts.

Hitler’s plan was to power through the weak link at the Ardennes and then move northeast to take the Belgian port city of Antwerp. Without Antwerp, the Allies would have difficulty resupplying for their final push toward Berlin. With that strategic position secured, Hitler believed he could negotiate terms with the Allies and avoid an unconditional surrender, allowing him to continue the war with Russia in the East.

Also, by attacking through the Ardennes in winter, Hitler bet on bad weather grounding the Allied air support. The Ardennes are famously fogged in during December, making it impossible for bombers and supply planes to hit their targets.

And so it was on the morning of December 16, with a thick mist blanketing the mountainous Ardennes Forest, that a German fighting force numbering 200,000 men and 1,000 tanks launched an all-out attack on the unsuspecting Allies. In King’s interviews with dozens of Battle of the Bulge veterans, they describe eerie red and purple lights streaking through the pea-soup fog followed by the bone-chilling sound of “screaming meemie” Nebelwerfer rockets and earth-shattering detonations in every direction.

Two regiments of the 106th Division were quickly surrounded by German infantry leading to the single largest field surrender of Allied troops in World War II. More than 6,800 American soldiers in the 422nd and 423rd regiments taken as prisoners. Elsewhere, near the Belgian town of Malmedy, 84 American prisoners were summarily killed by German Waffen SS in the largest mass execution of the war.

Just 24 hours after the initial bombardment, German tanks had broken through the thinly defended center of the Ardennes region and rolled West to the Meuse River, creating the infamous bulge in the Allied line that gave the battle its peculiar name. Using the weather and intelligence breakdowns to their advantage, the Nazi offensive appeared to be working.

But if Hitler thought that the outmanned Americans were going to lay down and let the German tanks roll all the way to Antwerp, he was mistaken. After the initial confusion and chaos of the Nazi surprise attack, American soldiers regrouped and relied on old-fashioned ingenuity to hold off the German advance until reinforcements could arrive.

“Talking to both American and German veterans, you realize how very different these two armies were in their methods and methodologies,” says King. “The Americans had this incredible capacity to improvise, to think on the hop and operate autonomously right down to the squad level. The Germans couldn’t operate below the regimental level without written orders.”

General James Gavin of the 82nd Airborne, already a hero at Normandy and during Operation Market Garden, shined yet again in defense of the strategic Belgian town of St. Vith’s. He hopped in his Jeep, spied on the enemy’s positions and divided his men into squads to hunt down the Nazis using the terrain to their advantage.

The Fighting 30th, nicknamed “Roosevelt’s SS,” used similar guerrilla tactics to halt the progress of the German army in the North.

“The Fighting 30th engineers were phenomenal—they blew up everything,” says King. “There’s a town called Trois-Ponts for the three bridges that crossed the Amblève River. Not after the engineers had been there.”

Albert Tarbell was a full-blooded Mohawk with the 82nd Airborne who was “the best scout you could possibly have,” says King, when you’re up to your knees in snow and hunting the enemy through the forests. Nazi commanders thought their soldiers were hallucinating when they swore they were taking fire from “Indians.”

But perhaps the most famous stand of the Battle of the Bulge happened at Bastogne, where the 101st Airborne was pinned down and surrounded by Nazi infantry for five long days. The Americans dug trenches in a wide perimeter around Bastogne, and relied on locals for warm clothing and rations.

Temperatures plunged to -20°C (-4°F) and soldiers who didn’t die of hypothermia were crippled by frostbite and trench foot. When medics ran out of supplies, life-saving amputations had to be performed with kitchen knives and cognac as the only anesthesia.

When things looked worst for the Americans, the Nazi field commander issued an official call for surrender. General Anthony McAuliffe of the 101st Airborne replied with one of the most famously blunt rebuffs in the history of warfare: “Nuts!”

(King says that there’s more to that much-told story. While McAuliffe’s gut reaction was indeed to yell out, “Nuts!” to the Nazi’s demands, he tried the more conventional approach. A West Point graduate, he dictated a two-page response to the German commander. When McAuliffe’s intelligence chief read back the formal letter, he said he liked McAuliffe’s first response better. So they scrapped it and went with “Nuts!”)

The 101st airborne held out long enough for the skies to clear and the first supply drops to arrive from Allied bombers. Within days, General George Patton had turned his 350,000-man army North and punched through the German flank to relieve the beleaguered 101st Airborne and turn the tide of the Battle of the Bulge.

By January 13, 1945, the Allies had fully repelled the German attack and ironed out the bulge on the Western front. But the Allied forces, particularly the Americans, paid a high price for victory in the Battle of the Bulge. All told, 19,000 U.S. soldiers died, 47,500 were wounded—many from the brutal conditions—and more than 23,000 went missing.

It’s estimated that more than 100,000 German soldiers were killed, wounded or missing in action as a result of Hitler’s ill-fated final gamble.