Robert B. Rheault, a charismatic Army colonel who could scale mountains, dive to the ocean floor and speak flawless French, arrived for his second tour of duty in Vietnam in May 1969, when the war was at its raging peak. He had the job that had been his destiny, commander of the Green Berets, the elite Special Forces unit that often operated outside the standard Army chain of command.

Within a month, Col. Rheault (pronounced Roe) was embroiled in a case that spread to the highest levels of the Pentagon, White House, CIA and Congress and brought a premature end to his promising military career. The Green Beret murder case, which was splashed across magazine covers and in headlines for weeks, became one of the most puzzling, disturbing and tragic episodes of the war, but it has largely been forgotten in the decades since.

In the words of Time magazine, it was “a Vietnam War scandal second only to the My Lai killings” — in which U.S. troops killed hundreds of innocent civilians — “and one of infinitely more complex moral overtones.”

Col. Rheault died Oct. 16 at his home in Owls Head, Maine. He was 87.

His wife, Susan St. John, confirmed his death. She did not disclose a cause.

Col. Robert B. Rheault, a former commander of the Army’s Green Berets in Vietnam in 1969. (AP)

Col. Rheault, a 1946 graduate of the U.S. Military Academy at West Point, N.Y., joined the Army’s Special Forces in 1960. He traveled all over the world, engaging in actions along the East German border and peering into China from high in the Himalayas. He trained military units in Jordan, Pakistan, Tunisia and Iran.

In one training exercise, according to Jeff Stein’s 1992 book “A Murder in Wartime,” Col. Rheault’s commandos sneaked into the headquarters of a sleeping U.S. general, drew a red line across his throat and left a note on his pajamas that said, “You are dead.”

During Col. Rheault’s first tour in Vietnam in 1964, he was an intelligence and operations officer with the Green Berets. Few people knew what he did on his long solo forays into the jungle, but he always came back alive.

In the mid-1960s, when he received a master’s degree in international relations from George Washington University, Col. Rheault also worked as a counterinsurgency specialist for the Joint Chiefs of Staff. He had close contact with top officials at the White House, State Department, Pentagon and CIA. He was among the few people who knew about clandestine U.S. operations in Vietnam and neighboring Cambodia and Laos.

As the war in Vietnam began to be questioned on the home front, the Green Berets remained one of the few popular parts of the military during an unpopular war.

In 1966, “The Ballad of the Green Berets,” a song co-written and sung by Staff Sgt. Barry Sadler, was a ­No. 1 hit on the pop charts. In a flattering 1968 film portrayal of Special Forces units in Vietnam, John Wayne played a colonel in “The Green Berets.”

‘A scintillating leader’

When Col. Rheault returned to Vietnam in 1968 to take command of his old unit, the 5th Special Forces Group, the first thing he did was drag the queen-size mattress out of his bedroom and toss it in the dusty compound for all to see. He put in a requisition for a plain canvas cot.

“At forty-three, with gray-flecked hair, crinkled blue eyes, and an occasional cheroot stuck in the corner of his mouth,” wrote Stein, a journalist who was an Army intelligence officer in Vietnam at the time, “Bob Rheault looked like Clint Eastwood’s version of a Green Beret officer.”

Robert Bradley Rheault was born Oct. 31, 1925, in Boston. His mother came from a prominent Boston family, and his Canadian-born father once served in the Royal Canadian Mounted Police before becoming a financial adviser.

The family spoke French at the dinner table, and the young Col. Rheault spent summers as a teenager working as a cowboy in Wyoming. He graduated from the exclusive Phillips Exeter Academy in New Hampshire before entering West Point.

During the 1950s, he received a Bronze Star Medal in the Korean War, studied at the Sorbonne in Paris and was head of the French department at West Point.

He was considered “a scintillating leader with the aura of Lawrence of Arabia,” Stein wrote in “A Murder in Wartime.” “His men were fanatical about him. Rheault was one of those rare officers with the kismet, perhaps, for true greatness.”

Movie director Francis Ford Coppola once said that Col. Kurtz, the mysterious French-speaking graduate of Exeter and West Point portrayed by Marlon Brando in the Academy Award-winning film “Apocalypse Now,” was modeled in part on Col. Rheault.

In Vietnam, the Green Berets were often said to embark on secret missions as “hired assassins, master kidnapers, CIA enforcers,” as a 1969 Washington Post story put it. They often worked outside the standard military hierarchy, with weapons that came from sources that could not be traced. The top Army general in Vietnam at the time, Creighton Abrams, was said to be annoyed by these arrangements, concerned that Special Forces soldiers lacked accountability and discipline.

Col. Rheault had 4,500 troops under his command, plus a wide network of Vietnamese informants and irregular combatants throughout the country. In 1969, his unit received word that a Vietnamese informant named Thai Khac Chuyen was suspected of being a double agent for the enemy North Vietnamese.

Members of Col. Rheault’s unit picked up Chuyen and interrogated him for a week. Chuyen’s role and significance were never firmly established, but this much is known: Col. Rheault’s troops believed they were authorized to deal with him as they saw fit.

On June 20, 1969, three junior Green Beret officers drugged Chuyen with morphine, wrapped him in chains and tire rims and took him out in a small boat on the South China Sea. He was shot in the head and dumped overboard.

A day later, a message arrived from the CIA station chief in Saigon, telling Col. Rheault’s unit that the CIA had no interest in Chuyen and that killing him was “immoral and has the highest flap potential.”

Col. Rheault was summoned to Abrams’s headquarters in Saigon and asked what had happened to Chuyen. According to information obtained by Stein, who interviewed dozens of people and reviewed hundreds of documents for his book, Col. Rheault said Chuyen was on a secret mission in Cambodia, despite knowing that he was already dead.

When Chuyen failed to return, Army investigators began to look into the matter. A sergeant working for the Green Berets typed out a statement, attesting to Chuyen’s murder, and seven of Col. Rheault’s subordinates were taken into custody.

On July 21, 1969, Col. Rheault was arrested. Even though he had not pulled the trigger, he was held in a military stockade, charged with murder and conspiracy. He was one of the ­highest-ranking officers ever accused of such serious crimes during a war.

Two lower-ranking associates were soon released, but the Army conducted hearings and proceeded with plans to prosecute Col. Rheault and five other defendants. Time magazine noted that the trials “could turn into the most sensational courts-martial in U.S. history.”

Celebrity lawyers take case

The news media swarmed over the story, and Chuyen’s widow and children appeared at the U.S. Embassy in Saigon, pleading for justice. Celebrated civilian lawyers took an interest, as Edward Bennett Williams flew to Saigon to defend Col. Rheault. F. Lee Bailey signed on to handle another soldier’s case.

In an informal meeting with congressmen who supported Col. Rheault, Army Secretary Stanley R. Resor made an unintentionally ironic statement while trying to explain why the case would go forward: “But gentlemen, the Army simply cannot condone murder.”

As lawyers picked apart the case, it became clear that sensitive secrets about how the Vietnam War was being conducted would be revealed. The perceived feud between Abrams and the Green Berets lay just beneath the surface.

The lawyers said Col. Rheault and the Green Berets were merely carrying out a CIA order, but the CIA maintained a wall of stony silence. The CIA reportedly appealed to Abrams and Resor, asking to be absolved of any complicity in Chuyen’s death. When that effort failed, the CIA said it would not allow anyone to testify at a trial.

An influential congressman, L. Mendel Rivers (D-S.C.), chairman of the House Armed Services Committee, got the attention of President Richard M. Nixon when he threatened to withhold military funding unless the case was dropped.

Finally, under pressure from the White House and CIA, Resor announced on Sept. 29, 1969, that all charges against Col. Rheault and his officers were dismissed. When the news was announced on the floor of the House, the chamber broke into applause and cheers that went on for minutes.

“If there had been a trial,” Bailey told Time magazine in 1969, “the defendants would have become Abrams, [CIA director Richard] Helms and Nixon. The only winner would have been North Vietnam.”

Col. Rheault was offered other jobs in the Army, but the only one he wanted was the one he couldn’t have: command of the Green Berets. He retired from the Army on his 44th birthday, Oct. 31, 1969. Two weeks later, he was featured on the cover of Life magazine, in a sympathetic story.

“The biggest loss was that he left the service,” Martin Linsky, one of his Army lawyers, said last week. “He was a crackerjack officer.”

To this day, the case against Col. Rheault remains murky and confused, compounded by the adulation in which he was held and by the bewildering fog of war. After years of reviewing the case, Stein concluded in his book that Col. Rheault was held accountable for a simple and inviolable reason: He had lied to Abrams about the death of Chuyen, breaking an officer’s code of trust.

In 1971, Col. Rheault began working in Maine with Outward Bound, the rigorous outdoor leadership program. He began organizing Outward Bound expeditions for troubled Vietnam veterans in 1982, which have been extended to other veterans. In 1989, he led a joint trek in Uzbekistan for U.S. veterans and veterans of the Soviet military.

He became the leader of Maine’s Hurricane Island Outward Bound School and was active in conservation efforts in New England until his retirement in 2001.

“The Green Beret case,” he often said, according to his wife, Susan St. John, “is not what I am about.”

His first marriage, to Caroline Anna “Nan” Young, ended in divorce. Survivors include his wife of 36 years, Susan St. John of Owls Head; three children from his first marriage; two children from his second marriage; a brother; and four grandchildren.

Describing why he left the Army, Col. Rheault told Life magazine in 1969, “I didn’t want to become a bitter old fud of a colonel mumbling into my martinis about the star I should have had but never got.

“And even if I had been permitted to finish my tour in Vietnam, I would never be able to serve in the Special Forces again — and that experience was so great that it spoiled me for any other Army service.”