Called “the greatest American battle of the war” by Winston Churchill, the Battle of the Bulge in the Ardennes region of Belgium was Adolf Hitler’s last major offensive in World War II against the Western Front. Hitler’s aim was to split the Allies in their drive toward Germany. The German troops’ failure to divide Britain, France and America with the Ardennes offensive paved the way to victory for the allies.

Lasting six brutal weeks, from December 16, 1944, to January 25, 1945, the assault, also called the Battle of the Ardennes, took place during frigid weather conditions, with some 30 German divisions attacking battle-fatigued American troops across 85 miles of the densely wooded Ardennes Forest.

As the Germans drove into the Ardennes, the Allied line took on the appearance of a large bulge, giving rise to the battle’s name. The battle proved to be the costliest ever fought by the U.S. Army, which suffered over 100,000 casualties. The formerly serene, wooded region of Ardennes was hacked into chaos by fighting as the Americans dug in against the German advance at St.-Vith, Elsenborn Ridge, Houffalize and, later, Bastogne, which was defended by the 101st Airborne Division.

“Did you ever see land when a tornado’s come through? Did you ever see trees and stuff, twisted and broken off? The whole friggin’ forest was like that,” said U.S. Army Charlie Sanderson in My Father’s War: Memories from Our Honored WWII Soldiers.

READ MORE: 8 Things You May Not Know About the Battle of the Bulge

The surprise German attack broke through the front on day one as stories quickly spread of massacred soldiers and civilians, according to the U.S. Army Center of Military History.

“For those who had lived through 1940, the picture was all too familiar. Belgian townspeople put away their Allied flags and brought out their swastikas,” the center writes. “Police in Paris enforced an all-night curfew. British veterans waited nervously to see how the Americans would react to a full-scale German offensive, and British generals quietly acted to safeguard the Meuse River's crossings. Even American civilians, who had thought final victory was near were sobered by the Nazi onslaught.”

Hitler’s mid-December timing of the attack—one of the bloodiest of the war—was strategic, as freezing rain, thick fog, deep snow drifts and record-breaking low temperatures brutalized the American troops. More than 15,000 “cold injuries”—trench foot, pneumonia, frostbite—were reported that winter.

“I was from Buffalo, I thought I knew cold,” baseball Hall of Famer and WWII veteran Warren Spahn said in The Love of Baseball. “But I didn’t really know cold until the Battle of the Bulge.”

Another Nazi strategy was to attempt to infiltrate the Allied troops.

Veteran Vernon Brantley, a private first class in the 289th Regiment, told the Fort Jackson Leader in 2009 that his unit had just arrived in Germany from France when they were told to load up and return to Luxembourg.

“We got word that the Germans had dropped a lot of paratroopers behind our lines, and that they were dressed like American Soldiers and spoke English,” he said. “... They were there to create confusion.”

The Germans also changed road signs and spread misinformation.

“The Nazis were carefully groomed for their dangerous mission,” LIFE magazine reported in 1945. “They spoke excellent English and their slang had been tuned up by close association with American prisoners of war in German camps. ... Under the rules of the Hague Convention these Germans were classifiable as spies and subject to an immediate court martial by a military tribunal. After brief deliberation American officers found them guilty, and ordered the usual penalty for spies: death by firing squad.”

To stop infiltrators, the U.S. troops would ask suspected Germans to answer American trivia questions.

"Three times I was ordered to prove my identity," Gen. Omar Bradley recalled, according to the Washington Post. "The first time by identifying Springfield as the capital of Illinois; the second by locating the guard between the center and the tackle on a line of scrimmage; the third time by naming the then-current spouse of a blonde named Betty Grable."

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It wasn’t until Christmas Day that the weather conditions finally cleared, allowing Allied air forces to strike.

"It was on that bright, clear and cold Christmas morning in 1944 that the ground froze solid," Brantley told the Leader. "The tanks and air forces could finally maneuver, and get assistance to all of us who were previously blocked off. … It was a welcome sign to see the sun come up. It meant that we were alive for one more day."

Gen. Dwight D. Eisenhower, the supreme Allied commander, and Lt. Gen. George S. Patton Jr. led the American defense to restore the front. According to the National Archives’ Bloodiest Battle, Eisenhower gave Patton the Third Army, about 230,000 soldiers, and ordered him to head to the Ardennes.

In the small, pivotal Belgian town of Bastogne, the Germans surrounded thousands of Allied troops. Eisenhower, in response, sent in more units, including the famed 101st Airborne Division.

“When the Germans sent a message demanding the surrender of the 101st on December 22, they got a one-word response from its commander, Brig. Gen. Anthony McAuliffe: ‘Nuts!’” the Bloodiest Battle states. “This was interpreted by German officers as a more colorful—and negative—response to their demand. The day after Christmas, units of Patton’s rapidly approaching Third Army finally arrived, broke through the German lines, and rescued the troops.”

Claiming victory of the battle on January 25, 1945, and the Allies headed for Berlin. The war ended less than five months later with Germany’s May 7 surrender.

In all, according to the U.S. Department of Defense, 1 million-plus Allied troops, including some 500,000 Americans, fought in the Battle of the Bulge, with approximately 19,000 soldiers killed in action, 47,500 wounded and 23,000-plus missing. About 100,000 Germans were killed, wounded or captured.

“The Ardennes campaign of 1944-45 was only one in a series of difficult engagements in the battle for Europe,” wrote John S.D. Eisenhower, in his 1969 book, The Bitter Woods. “Nevertheless, it can be said that the Ardennes campaign epitomized them all. For it was here that American and German combat soldiers met in the decisive struggle that broke the back of the Nazi war machine.”