In December 1950, as U.S. Marines tried to escape from a massive trap near North Korea’s Chosin Reservoir, sprung by an enemy they hadn’t even known was there, Major Gen. Oliver P. Smith was asked how it felt to be retreating for the first time during the Korean War. “Retreat, hell!” snapped Smith. “We’re not retreating, we’re just advancing in a different direction.”

As you can see in Chosin: One Way Out, an episode of the Military Channel’s Ultimate Warfare series airing Tuesday, Smith wasn’t spin-doctoring anything. The general’s 17,000 Marines were surrounded by 120,000 Chinese troops. Their vicious, gut-wrenching breakout from their encirclement, though hardly remembered these days, is one of the most harrowing and heroic tales in U.S. military history.

The Chinese ambush was more than just a tactical surprise; it was a symptom of a much larger U.S. intelligence failure. American troops had been shocked and nearly annihilated when North Korea’s army rolled across the border into the south six months earlier. But a surprise U.S. amphibious assault behind enemy lines in September had routed the North Koreans, and American troops had been pursuing them north toward the China with little resistance.

What U.S. leaders didn’t know was that hundreds of thousands of Chinese soldiers had secretly crossed the border in North Korea. As the Marines moved into the area around the Chosin Reservoir, about 60 miles from the border, the Chinese struck with a terrible vengeance.

Not only were the Marines outnumbered by more than 6 to 1, the attack caught them in a dangerously thin line that spread out over about 30 miles in rugged, frozen terrain in which the Americans hadn’t even been able to dig foxholes when they bivouacked for the night. What followed were two weeks of what One Way Out calls “the most violent small-unit fighting in the history of American warfare.” As repeated human-wave attacks overran one U.S. position after another, the Americans found themselves in hand-to-hand combat that seemed scarcely human.

One Marine, cut off from his squad, recounts how his machine gun jammed and he lunged for a shovel lying on the ground where he’d dropped it after a fruitless attempt to dig shelter. As the Chinese swarmed toward him, he began swinging it. “A lot of them were young, they were confused, and they didn’t wear helmets,” he remembers. “It got a little nasty. … They call you a warrior, but actually you’re a savage.” Others remember the hideous smell of barbecued flesh as U.S. planes dropped napalm in an attempt to drive off the Chinese.

Eventually the Marines fought their way 78 miles south through the aptly named Hell Fire Valley to a port from which American ships could evacuate them. One soldier, his voice breaking, describes how they broke into a chorus of the Marines’ Hymn as they approached the U.S. line. Hundreds of others stayed behind, buried in the Hell Fire Valley, where North Koreans are still finding skeletons today.