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SEOUL, South Korea — Dawn was breaking over snow-covered Sambong Mountain a half century ago as the four Woo brothers set out to cut wood.

In a clearing they found 31 men dressed in South Korean army uniforms. Assuming it was a patrol, they shouted a greeting.

The soldiers were hollow-cheeked and drenched in sweat despite the sub-zero temperatures and the bitter wind in Paju, just 10 miles from South Korea's border with the North.

Most had removed their boots and wrapped their hands and feet in blankets to stave off frostbite. The leader introduced himself as "Captain Kim," with his sophisticated Seoul accent putting the siblings at ease.

That was when one of the brothers noticed something strange: One soldier's rank insignia was upside down. It made him suspicious: For months there had been broadcasts in the South warning citizens to be on the lookout for infiltrators.

“Gentlemen, are you from the North?” the eldest brother asked Kim.

“Yes, comrades. We are here to liberate you and bring communism to South Korea,” Kim told the woodcutters.

"You can join us, or you can die"

The "soldiers" were actually North Korean commandos who had trained for two years for this mission: assassinate South Korean President Park Chung-hee, a former general who had risen to power through a military coup seven years earlier. They had been sent by Kim Il Sung, North Korea's founder and the grandfather of its current leader, Kim Jong Un.

The commandos had spent the final months of their training practicing the assault on a mock-up of Park's Blue House presidential residence that had been built inside North Korea.

“I was in charge of the assault element, which would secure the first floor, allowing the rest of the team to proceed upstairs and kill Park,” one commando, Lt. Kim Shin-jo, said in an interview with NBC News on the 50th anniversary of the day in 1968 that he crossed the Demilitarized Zone into South Korea.

Kim Shin-jo Mac William Bishop / NBC News

A debate broke out among the North Korean commandos about what to do with the Woo brothers. If the woodcutters revealed the existence of the team to South Korean authorities, it would jeopardize the raid.

The decision seemed clear cut to Kim Shin-Jo. A fanatical communist recruited at 23 from the regular ranks of North Korea’s military to join an elite special forces team dubbed Unit 124, he supported killing the Woo brothers.

He argued that they needed to be sacrificed on the altar of the glorious revolution that would ensue when Park was dead.

Guided by North Korean infiltrators like himself, he believed the South Korean people would rise up to overthrow their capitalist puppet state and oust the American imperialists who had divided Korea.

But the ground was frozen, making it impossible to bury the bodies.

So Captain Kim, whose real name was Kim Jong-ung, drew up a contract, lecturing the brothers on the virtues of communism and the unity of the Korean people, and promising them a place in the revolutionary government that would be formed once Unit 124’s mission was complete.

“You can join us, or you can die,” Captain Kim told them.

The woodcutters signed the pact and were released.

Once the Woo brothers were clear of the commandos, they went straight to a South Korean military post and reported the encounter. Unit 124 didn’t know it yet, but their mission was doomed.

'The year that mattered most'

With North and South Korea engaged in high-stakes Olympic diplomacy, the Trump administration has said it is considering limited strike options against Pyongyang, the North's capital, after a period of nuclear tensions reminiscent of the height of the Cold War.

Moon Jae-in, South Korea's president, wants next month's PyeongChang Olympics to be an "important turning point in solving North Korea’s missile issues." The neighbors — which technically remain at war— have agreed to both contribute players to a unified Korean women's hockey team.

But the history of the two Koreas is an endless series of turning points.

“It was 1968 that was the year that mattered most in terms of North-South relations,” Kim Shin-jo said. “It was really the turning point in terms of the power dynamic between the two countries, when North Korea was at its peak economically and militarily compared to the South.”

On the day that Kim Shin-jo was trying to persuade his comrades to kill the Woo brothers, the USS Pueblo — an American spy ship — was operating off the coast of North Korea.

The USS Pueblo on Jan. 23, 1968. Bettmann file

Having set sail a week earlier from Sasebo, Japan, the Pueblo and its 83 crew members were tracking Soviet naval activity in the area, as well as trying to gather intelligence on North Korean military activities.

"Estimate of risk: minimal, since Pueblo will be operating in international waters for entire deployment," read the Pueblo’s operational orders.

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By Jan. 20, 1968, the Pueblo was being shadowed by a North Korean patrol vessel. This was in and of itself nothing out of the ordinary.

For nearly two years, a low-level conflict had been fought between the two Koreas, relying on unconventional tactics. The authoritarianism of the Park junta and a relatively weak economy in South Korea meant that the North was in some respects the stronger and more stable of the two countries. American preoccupation with the war in Vietnam gave Pyongyang the opening Kim Il Sung needed to attempt to reunify the Korean Peninsula.

Given the gravity of events on the ground in South Vietnam, the United States did not want to involve itself more deeply in Korea. But it was aware of Pyongyang’s intentions.

On Jan. 22, two more North Korean vessels joined the flotilla shadowing the Pueblo.

According to a declassified investigation, the Pueblo experienced difficulties communicating with its base in Japan before managing to send a report detailing its situation. In return it received a message containing the latest NBA scores.

What the crew of the Pueblo didn’t know was that an attack on the Blue House was unfolding in Seoul.

The North Korean commandos of Unit 124 were trying to complete their mission; the Korean cold war was heating up.

Trenchcoats and submachine guns

Unit 124’s plan had fallen apart almost immediately after the encounter with the Woo brothers. The South Korean military had mobilized in search of the North Korean commandos, who were racing to escape their pursuers and complete their mission.

Dressed in civilian clothes, including trenchcoats hiding their Russian-made submachine guns, pistols and hand grenades, the 31 commandos had managed to work their way through heavy security to an intersection near their target.

The South Korean military had hunting them for nearly two days, but somehow the commandos had managed to dodge patrols or talk their way past checkpoints.

Now, a mere 350 yards from their objective, with the Blue House in sight, they were stopped by a police chief and two officers at a checkpoint near Cheong-un Middle School.

The Blue House in Seoul, South Korea. Chung Sung-Jun / Getty Images file

Captain Kim told the chief, Choi Gyu-sik, that they were a South Korean counter-infiltration unit returning from an exercise.

But Choi didn’t buy it. Noticing their bulging trench coats, he unholstered his pistol and demanded to know what they were carrying. One of the commandos shot him.

The heavily armed Capital Garrison Command protecting the presidential residence responded immediately, and an intense firefight ensued with the well-trained North Koreans.

A bus carrying civilians was caught in the crossfire, killing several. By the time it was over, nearly 100 people were dead or wounded.

But try as they might, Unit 124 could get no closer. Their opponents had heavy weapons, even a tank, and reinforcements were materializing from every corner. There was no way to battle through.

Captain Kim gave the order for the commandos to disengage and disperse. They fled, most heading north.

Kim Shin-jo followed his own instincts, not his orders. He was bitter at the failure of the mission, and felt poor decisions by Captain Kim were to blame. Kim Shin-jo was willing to sacrifice his life to kill Park, but he was unwilling to die for nothing.

“I thought, first of all, I want to live,” Kim Shin-jo said. He added that he felt like he had accomplished nothing with his life: He’d never had a serious relationship with a woman, as commandos were required to be bachelors. He hadn’t eaten in days of bitter cold; and he hadn't even fired a single round during the firefight. He was questioning not just his leader’s commands, but his own existence: “Who am I? Who am I? I want to live.”

So, alone, he ran away from his comrades, and away from North Korea.

Surrender

At about 1:20 p.m. on Jan. 23, North Korean vessels circling the USS Pueblo opened fire as the American ship tried to maneuver away.

There had been a back-and-forth all morning, with the American and North Korean ships exchanging signals: The North Koreans were demanding the Pueblo heave-to for inspection, and indicated they were prepared to tow it into harbor. The American ship signaled that it was operating legally in international waters and that the North Koreans had no right to interfere.

According to official accounts from the U.S. Navy, the Pueblo stayed in international waters at all times, but the North Korean government disputes this, saying the ship repeatedly entered its territorial waters.

What isn’t disputed is that the ensuing battle saw the lightly armed Pueblo engage enemy torpedo boats and fighter aircraft. The battle ended with the surrender of the Pueblo to North Korean forces; one American was killed, while the remaining 82 crew members were taken prisoner.

The capture of the Pueblo came as South Korean and American officials were already meeting to discuss a possible military response to the attempted raid on the Blue House. For the Americans, the ship’s capture overshadowed all other considerations.

With dozens of U.S. servicemen now in North Korean hands, President Lyndon B. Johnson was disinclined to do anything that might make the situation worse.

Kim Shin-jo (second from left) is questioned at a police station in Seoul, South Korea, on Jan. 22, 1968. AP file

Meanwhile, the South Korean military was chasing down the commandos who had targeted the Blue House. By the time the Pueblo was captured, South Korean soldiers had killed at least five, while one had surrendered: Kim Shin-jo.

He was captured after being surrounded by South Korean soldiers in a house on Inwang Mountain near central Seoul.

“If I die after killing Park Chung-hee, then I've fulfilled my duty,” Kim Shin-jo said he remembered thinking. “But if I don't kill Park Chung-hee, why should I die?”

All but one of his comrades in Unit 124 were killed as they fought to the death against South Korean forces.

The surviving commando, Pak Jae Gyong, was hailed as a hero upon his return to North Korea. He is now a senior military and political official, having surviving three generations of North Korean leaders.

'They executed my family'

For Kim Shin-jo, there would be no victory. In South Korean hands, he was interrogated and his weapons were inspected as the intelligence services sought to ascertain his role in the failed Blue House raid. He cooperated, and the authorities noted that his weapons had not been fired.

Kim Shin-jo speaks to NBC News on the 50th anniversary of the day his mission began. Mac William Bishop / NBC News

But this decision not to take part in the fighting, and to surrender willingly, came at a terrible price.

“If I had fired my weapon alongside my comrades, and if I had gotten a life sentence or the death penalty, I would have been deemed a revolutionary. But because I raised my hands, because I wanted to live, I pledged my allegiance to South Korea,” Kim Shin-jo said.

“So I was deemed a defector, and they executed my family.”

It took years for Kim Shin-jo to learn the fate of his parents and six siblings. He finally learned from another North Korean defector that they had been publicly tried and shot before a People’s Court.

The Johnson administration made securing the release of the hostages in North Korea their top priority on the peninsula. The 82 men of the Pueblo were released in Panmunjom, walking across the "Bridge of No Return," on Dec. 23, 1968, after nearly a year in captivity.

They had faced torture, indoctrination campaigns and deprivation, and were forced to take part in propaganda efforts. Their capture and captivity was viewed with embarrassment in Washington, and there was no hero’s welcome upon their return.

Released crewmen of the USS Pueblo are escorted by military police upon their arrival at the U.S. Army 121st Evacuation Hospital at Ascom City, South Korea, on Dec. 23, 1968. AP file

As for their ship — a museum and tourist attraction in Pyongyang — it remains on the U.S. Navy's active service roster as the only American vessel classified as “in enemy hands.”Its crew has battled for years, with measured success, to change the way the military and the public view their mission and the ensuing hostage crisis.

Over the years, Kim Shin-jo has been rehabilitated: The South Koreans deemed that as a soldier, he was merely executing orders. That he hadn’t fired his weapon, and that he had cooperated with security officials in explaining North Korea’s special forces units and training, weighed heavily in his favor.

He was released without charge on April 10, 1970. Six months later, he married a South Korean woman who had become his pen pal while he was being held by the authorities — and who he had the intelligence services investigate as a potential North Korean assassin before he agreed to meet her.

She wasn't a sleeper agent. But she did succeed in converting Kim Shin-jo to her faith, and he is now a Christian pastor at a megachurch in Seoul.

Now aged 76, the decisions he made five decades ago weigh heavily on him, even as the events of the past have faded into obscurity.

“I lived, but my heart aches when I think about my parents and my siblings who I left behind," he said. "This is something I have to carry with me to my grave.”