On February 21, 1959, Dap Chhuon, the anti-communist warlord who ruled over Siem Reap province, was doing calisthenics in his home when soldiers from the Royal Cambodian Army showed up. Sensing danger, Chhuon, a thin, superstitious strongman who had convinced some that he was impervious to bullets and fire, fled into the jungle, wearing only a sarong.

Then-prime minister Norodom Sihanouk, who ordered the operation, would later deem Chhuon, once a decorated colonel, a traitor with mutinous intent. In the preceding weeks, Chhuon, a former anti-French Issarak rebel, had amassed weapons and an estimated army of 3,000 men. The strongman also had the support of 1,200 Khmer Serei fighters on the Thai side of the border, archived US State Department records note.

A CIA National Security Council briefing drafted in the month following the coup summed up his doomed plan.

“Chhuon hoped that Sihanouk, realising that the Cambodian people were disaffected and lacking support from his own armed forces, would come to terms and agree to the installation of a pro-Western regime. If Sihanouk was not reasonable, Chhuon planned a guerrilla war,” it read. 

The affair reeked of foreign subterfuge. At Chhuon’s villa following the assault, soldiers discovered gold, radio equipment and two Vietnamese technicians.

The Royal Army also arrested Chhuon’s brother, Slat Peau, who later testified that the gold came from a South Vietnamese agent. The radio equipment, he said, was provided by a Japanese-American with the US State Department named Victor Matsui. The attache was quickly transferred out of the country. (In 1966, Matsui would be expelled from Pakistan, also for activity unfitting of a diplomat. He died in 2012.)

Washington has never admitted involvement in the botched coup, nor satisfactorily explained Matsui’s connection to the warlord. As with similar Cold War allegations, their official stance has always been one of denial.

As Washington’s line goes, the radio was provided by Matsui only to keep tabs on Chhuon’s scheming. But if that is so, probe sceptics, why then did the US not warn Sihanouk, as the French, Chinese and Soviet intelligence agencies did, of the plot to topple him?

Now, a US historian using, in part, previously classified documents, claims to have produced the most compelling case to date that the US colluded in the 1959 conspiracy. 

“I would say that no other author has put together as strong a case for US government complicity in the Dap Chhuon plot,” said William J Rust, author of Eisenhower and Cambodia: Diplomacy, Covert Action, and the Origins of the Second Indochina War, out this month through the University Press of Kentucky.

Rust, who has written three other books on US policy in Southeast Asia, committed 100 pages of his new work to Cold War “plots against Sihanouk”. He compiled archival documents: diplomatic cables, internal CIA briefs, policy papers, personal letters, memorandums, interviews – evidence that he says “has been hiding in plain sight”.

In a 1990 oral history interview with William Trimble, the US ambassador to Cambodia following the botched coup (the former ambassador Carl Strom left less than a month after the fiasco), Trimble tells the interviewer: “The CIA station chief in Phnom Penh had been instructed to establish contact with Dap Chhuon . . . and to provide him through a South Vietnam intermediary with a sum in gold . . . The Dap Chhuon operation was stupid, very stupid,” he said. 

Trimble’s recollection, Rust noted, is consistent with that of assistant secretary of state for far eastern affairs Roger Hilsman in a 1963 recorded telephone conversation with president John F Kennedy.

“Was that a true story about the ’59 or something?” asked Kennedy. “Yes, it really is true,” replied Hilsman. “CIA did do it?” the president reiterated. “Sure, they supplied money and they were involved in a plot against Sihanouk back before this administration.” 

James Lilley, a CIA officer in Cambodia who arrived soon after the coup, spoke directly about US complicity in a 1998 oral history interview, noted Rust. 

“This was set up by a Japanese-American guy attached to our station there. This was the so-called ‘Dap Chhuon’ plot centred in Siem Reap. The Cambodian authorities exposed the operation. In this operation, we were working with the South Vietnamese,” Lilley said. 

As far back as 1956, said Rust, “the US government was contemplating getting rid of Sihanouk”. In an August letter from Robert McClintock, then-US ambassador to Cambodia to Daniel Anderson, counselor of the US embassy in Saigon, McClintock wrote that he had given Colonel Edward Lansdale, an intelligence operative, “our tentative appraisal of Dap Chhuon as a possible leader in the event Sihanouk has to be got rid of”. 

Collectively, these notes and others point to US complicity in the half-century-old coup attempt, said Rust. “In my line of work, there’s a very big difference between what can be assumed and what can be proven,” he said. “Strong claims require strong evidence, and here it is in black and white.”

Elizabeth Becker, author of When the War Was Over: Cambodia and the Khmer Rouge Revolution, said that while “Rust may not have altered the general historical narrative, he certainly removes any lingering doubts that too often the United States considered Sihanouk fair game in its pursuit of victory in Vietnam”.

According to Kenton Clymer, a Cambodia historian at the University of Northern Illinois, “[Rust] has done the most thorough research on the topic to date”.

Julio Jeldres, a scholar at Monash University and Sihanouk’s good friend and official biographer, said the evidence was conclusive. “For many years, various scholars have written that there was never CIA plotting against Sihanouk and that they doubted that Dap Chhuon had been infiltrated by the CIA, but this book tells the story as it was.”

In The Tragedy of Cambodian History: Politics, War and Revolution Since 1945, historian David Chandler wrote that after the Dap Chhuon affair “Sihanouk became permanently distrustful of American intentions”. However, he said this week that relations were not seriously damaged.

“The relations calmed down in 1960 until they nose-dived in November 1963,” he wrote in an email. “Ambassador Trimble got along with Sihanouk, but Ambassador Sprouse did not.”

Current US Embassy spokesman Jay Raman had this to say about Rust’s findings: “We’re pleased to hear that more information about that era has been made available for use by scholars and researchers, but we don’t have any comment on specific allegations in the book.”

While the US may have gotten away from the affair relatively cleanly, the same cannot be said for Chhuon. A few days after the army assault on his villa, he was found in the jungle and shot in the foot then carried to a main road and executed.