From mid-December 1944 through the end of January 1945, in the heavily forested Ardennes Mountains of Belgium, thousands of American, British, Canadian, Belgian and French forces struggled to turn back the final major German offensive of World War II. While Allied forces ultimately triumphed, it was a vicious six weeks of fighting, with tens of thousands dead on both sides. Today, the conflict is known as the Battle of the Bulge.

Here LIFE.com presents a series of photographs made by LIFE photographers throughout the fighting. Many of these pictures never ran in LIFE magazine, or anywhere else.

For its final offensive to succeed, Germany needed four factors to work in its favor: catching the Allies off-guard; poor weather that would neutralize air support for Allied troops; the dealing of early, devastating, demoralizing blows against the Allies; and capturing Allied fuel supplies intact. (Germany originally intended to attack on November 27, but had to delay its initial assault due to fuel shortages). On December 16, 1944, the German attack began: the Wehrmacht (the Third Reich’s unified armed forces) struck with 250,000 soldiers along an 85-mile stretch of Allied front, stretching from southern Belgium to Luxembourg.

The attack proved stunningly effective, at first, as troops advanced some 50 miles into Allied territory, creating the “bulge” in the American lines that gave the battle its memorable name.

American forces had been feeling triumphant—Paris had been liberated in August and there was a sense among some American and other Allied leaders that Germany was all but defeated. The attack in December 1944, officially labeled the “Ardennes-Alsace Campaign” by the U.S. Army, showed that any complacency  was dangerously misplaced.

Nevertheless, as effective as the initial German efforts were, they failed to achieve the complete and early knockout of Allied forces that German military brass had counted on. (Wehrmacht Field Marshal Walter Model had given the attack only a 10 percent chance of success to begin with. The German name for the operation: Wacht am Rhein, or “Watch on the Rhine.”)

One of the most difficult aspects of the Bulge was the weather, as extreme—indeed, historic—cold wreaked havoc and turned relatively simple logistics of travel, shelter, and meals into a daily struggle. January 1945 was the coldest January on record for that part of Europe, and over the course of the battle more than 15,000 Allied troops were treated for frostbite and other cold-related injuries.

Before the attack, some German troops who spoke English disguised themselves as Allied soldiers. They made a point of changing road signs and generally spreading misinformation. Germans who did that and were captured were executed by firing squad. A few images in this gallery chronicle one such execution. The three Germans, LIFE magazine reported in June 1945 when the U.S. War Department released the images were German intelligence officers who were captured, tried and shot.

“The Nazis were carefully groomed for their dangerous mission [LIFE wrote]. 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.”

Other German efforts at sabotage, meanwhile, proved largely ineffective, including attempts to bribe port and railroad workers to impede Allied supply operations.

Perhaps the defining moment in the Battle of the Bulge came when the Germans demanded the surrender of American troops who were outnumbered and surrounded in the town of Bastogne. United States General Anthony McAuliffe replied to the ultimatum with a now-legendary one-word response “Nuts!” His men withstood several German attacks until they could be relieved by the 4th Armored Division.

“This is undoubtedly the greatest American battle of the war,” Winston Churchill said in the House of Commons following the Battle of the Bulge, “and will, I believe, be regarded as an ever-famous American victory.”

While the Allied forces triumphed, victory came at a heavy price, with nearly 20,000 Americans killed and tens of thousands more wounded, missing or captured. British troops suffered more than 1,000 casualties. For American forces, the Bulge was the bloodiest battle on the Western Front during the Second World War.

German losses were severe, with estimates ranging from 70,000 to 100,000 casualties (depending on the source).

With victory on January 25, 1945, the final triumph over Nazi Germany was in reach; Allied forces pressed their advantage and began the last push toward Berlin. On May 7, Germany agreed to an unconditional surrender. Less than five months after the Battle of the Bulge ended, the war in Europe was over.

—gallery by Liz Ronk

American troops in a snow-filled trench during the Battle of the Bulge.

John Florea The LIFE Picture Collection/Shutterstock

American GI’s chop a foxhole in the frozen ground by a haystack during the Battle of the Bulge. The machine gun was set up in preparation for a German counterattack, expected at any moment.

John Florea The LIFE Picture Collection/Shutterstock

An American artilleryman shaves in frigid cold, using a helmet for a shaving bowl, during the Battle of the Bulge, 1944.

John Florea The LIFE Picture Collection/Shutterstock

American troops man trenches along a snowy hedgerow in the northern Ardennes Forest during the Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

Allied troops around a fire in the Ardennes Forest during the Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

Shell craters left by an Allied barrage laid down to clean German infantry out of the woods and fields during the Battle of the Bulge, Belgium, 1944.

William Vandivert The LIFE Picture Collection/Shutterstock

American trucks and half-tracks in a snow-covered Ardennes field, Battle of the Bulge.

William Vandivert The LIFE Picture Collection/Shutterstock

Battle of the Bulge

John Florea The LIFE Picture Collection/Shutterstock

German POWs on grave-digging duty during the Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

A corpse beside a road during the Battle of the Bulge.

John Florea The LIFE Picture Collection/Shutterstock

German military wreckage, Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

The frozen corpse of a German soldier killed during the Battle of the Bulge.

John Florea The LIFE Picture Collection/Shutterstock

Allied troops and the German dead, Battle of the Bulge.

John Florea The LIFE Picture Collection/Shutterstock

Some of the 115 Americans who, LIFE reported, were “massacred at point-blank range” in a field after being captured by Germans in the early days of the Battle of the Bulge, 1944. The soldiers were herded into a field and machine-gunned; when found, many of the frozen bodies still had their hands above their heads.

John Florea The LIFE Picture Collection/Shutterstock

Belgian civilians are evacuated by American troops, 1944.

George Silk The LIFE Picture Collection/Shutterstock

An American tank moves past another gun carriage which slid off an icy road in the Ardennes Forest during the Battle of the Bulge, Dec. 20, 1944.

George Silk The LIFE Picture Collection/Shutterstock

Belgian residents of a northern Ardennes hamlet flee the fighting during the Battle of the Bulge, 1944.

George Silk The LIFE Picture Collection/Shutterstock

American GI, Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

Portraits of American soldiers during the Battle of the Bulge, December 1944.

George Silk The LIFE Picture Collection/Shutterstock

American troops with Belgian children, Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

A wounded German soldier rests on makeshift bedding after being taken prisoner during an attack on an American fuel depot on Dec. 16, 1944, the first day of the Battle of the Bulge.

John Florea The LIFE Picture Collection/Shutterstock

Germans surrender during the Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock

German POWs, Battle of the Bulge, January 1945.

George Silk The LIFE Picture Collection/Shutterstock

German prisoners, some of them wearing coveralls for camouflage in the snow, are herded by guards. (In close fighting, U.S. troops also used snow-camouflage suits.)

John Florea The LIFE Picture Collection/Shutterstock

Photographed on Dec. 23, 1944, and published in LIFE in June 1945. Behind a cell block, German prisoners are bound to stakes by MPs. Tried and convicted as spies, they are about to be executed.

John Florea The LIFE Picture Collection/Shutterstock

A blindfolded prisoner is securely bound, hand and foot, to a stake in front of a concrete wall. A large white paper target is pinned over his heart. American MPs stand at attention until the firing squad’s commanding officer inspects the final arrangements. Belgium, 1944.

John Florea The LIFE Picture Collection/Shutterstock

The volley is fired and three white puffs of smoke appear against the wall of the concrete block. The initial burst killed all three almost instantaneously. The firing squad, all military police, consisted of three groups of eight men, each with one additional marksman along as a spare.

John Florea The LIFE Picture Collection/Shutterstock

A German shot as a spy in the early days of the Battle of the Bulge, 1944.

John Florea The LIFE Picture Collection/Shutterstock

American troops in Belgium during the Battle of the Bulge.

George Silk The LIFE Picture Collection/Shutterstock