Pen Sovann, who served as Cambodia’s first prime minister after the fall of the Khmer Rouge but ended up jailed in Hanoi for a decade after displeasing Vietnam, died on Saturday. He was 80.

Unique as perhaps the only major figure to have opposed every regime in the country’s modern period – from the French colonial regime to Norodom Sihanouk, Lon Nol, Pol Pot and Hun Sen – Sovann died at his home in Takeo at about 7pm after a long battle with poor health, his family said.

The former prime minister had most recently been elected to the National Assembly for the opposition at the 2013 election – his third attempt – and made regular appearances at the protests that followed, although a serious stroke in January 2015 put an end to his public activities.

Like many in his generation, Sovann’s political ideas in his later years bore little resemblance to those of his earlier life, and he ended a career marked by his appointment as prime minister by the invading Vietnamese by campaigning against Vietnam’s influence in Cambodia.

Born in Takeo’s Tram Kak district in 1936, Sovann joined the Khmer Issarak armed resistance against the French at age 15 and soon became secretary and bodyguard to Ek Choeun, who as a Khmer Rouge leader would later become known as the notoriously sadistic Ta Mok.

After Sihanouk gained independence from France in 1953 and the Geneva Conference in 1954 divided control of the former Indochina colonies amid surging global communism, he was one of 189 Cambodians illegally “regrouped” to Hanoi on boats meant only to ship out Viet Minh forces.

Receiving military training at a school in Hanoi for senior officials, the future premier remained in Vietnam until March 1970, taking a local wife and developing a taste for Vietnamese food that would last the rest of his life – even as he campaigned against the Vietnamese in his final years.

The academic Ben Kiernan has estimated that up to 1,000 Cambodian communists had been sent to Hanoi for similar training by March 1970, the month that Lon Nol ousted Sihanouk and hastened North Vietnam’s efforts to finance and promote a revolution inside Cambodia.

Popular Cambodian politician Pen Sovann, a fierce critic of second Prime Minister Hun Sen, speaks to reporters at Phnom Penh’s international airport in 1997 before fleeing the country, in fear of his life. Matthew Lee/AFP

Vietnamese-trained cadres were far more skilled than those who had stayed in Cambodia’s forests since 1954 fighting Sihanouk, but were less trusted by the Khmer Rouge leadership, wrote Lien-Hang Nguyen in her book Hanoi’s War: An International History of the War for Peace in Vietnam.

“Their expertise in Marxism-Leninism and military combat might have made them attractive to Pol Pot, who had poorly trained and undisciplined troops,” Nguyen wrote of the returnees. “However, their exposure to Vietnamese culture and society rendered them suspect.”

While alive, Sovann said figures like Pol Pot began to change to a more violent disposition in 1968, after the Chinese became more involved in the Cambodian struggle, and it was indeed suspicions about his links to Vietnam that led Sovann to flee before the 1975 fall of Phnom Penh. Having worked in Cambodia for four years, including alongside future foreign minister Ieng Sary on the Khmer Rouge’s radio station, he escaped back to Hanoi in January 1974 and tried to persuade the Vietnamese to replace Pol Pot and Ieng Sary as the leaders of the Cambodian communists.

“Early in 1974, Ieng Sary and Pol Pot wanted to kill me, so I fled to Vietnam,” Sovann explained in an interview in July 2002, adding that he had seen the seeds of the later terror early on. “They wanted to kill me because I didn’t carry out their dictatorship policies.

“When I was back in Vietnam, I found out the Khmer Rouge was killing people, so I started to build a group to fight against the regime,” he said.

Yet Sovann’s efforts were rebuffed by Vietnam until March 1978 – nine months before its invasion of Cambodia was executed – when increasingly frequent border raids by the Khmer Rouge led the Vietnamese to decide to invade Cambodia and install a new government.

Read more: Pen Sovann remembered as ‘principled’

With a new revolutionary party in Cambodia from 1979, Sovann was named to the powerful role of general secretary, and when a new government was formally created in June 1981, he was named prime minister and defence minister – only to be stripped of all positions six months later.

The decision was made by Lê Đức Thọ, the top Vietnamese general who famously turned down a Nobel Prize in 1973, and then led the December 1978 invasion of Cambodia, according to Bùi Tín in his 1995 Following Ho Chi Minh: The Memoirs of a North Vietnamese Colonel.

“The removal of Pen Sovann from his positions . . . was also the work of Le Duc Tho acting together with general Le Duc Anh,” Bùi wrote. “On their recommendation, the Politburo in Hanoi accepted an ‘appeal’ from several members of the Cambodian Communist Party.”

Bùi, who was one of the first Vietnamese in Phnom Penh after the 1979 fall of the Khmer Rouge, wrote that Vietnamese officials were in those early days very much still in control of the regime they installed and were wary of any of the historical anti-Vietnamese sentiment re-emerging.

“The Cambodian people had nothing to do with the rise and fall of Pen Sovann,” Bùi wrote. “So what was his mistake? According to a Vietnamese adviser in charge of training Cambodian cadres, Pen Sovann sometimes opposed Vietnam and sometimes his own Party.

“He also expressed dissatisfaction with . . . the way his military authority was ignored by General Le Duc Anh. Such an attitude was intolerable in the eyes of leadership, so Pen Sovann was taken back to Vietnam to spend the next 10 years under house arrest near Hanoi.”

Pen Souna (centre), the daughter of Pen Sovann, and Yim Sovan pay their respects to Sovann yesterday after the former prime minister passed away on Saturday. Heng Chivoan

Both Sovann and Hun Sen himself have, in years since, attributed the ouster as coming from Hun Sen, but some academics have said that it is unlikely the current premier held such sway in 1981.

“Hun Sen ‘reading the charges’ against Pen Sovann is ludicrous. Hun Sen did not yet have such an exalted position,” wrote historian Michael Vickery in 2005, describing Sovann as one of the least reliable Cambodian political figures for providing accurate accounts of his past.

For his part, Sovann said in his July 2002 interview that it was Hun Sen behind his arrest, and that the reason for his exile to Hanoi – which ended with the October 1991 Paris Peace Accords – was that he stood up to the Vietnamese officials who installed him.

“Hun Sen and Say Phouthang led Vietnamese troops and the A-21 [police unit] to arrest me,” Sovann said. “They surrounded my house with 12 tanks and about 900 troops. They handcuffed me, covered my face with a black cloth, threw me in a car and drove off.

“First, they said I created a free market, which was against communist guidelines. Second, they accused me of discrimination and standing as a nationalist for not wanting Vietnamese to live in Cambodia,” he said. “The third was that I did not respect the orders given by the Vietnamese.”

Returning to Phnom Penh in 1992, Sovann later ran unsuccessfully in the 1998 election as leader of his own party and then for deputy opposition leader of Kem Sokha’s Human Rights Party in 2008.

He finally won a seat at the 2013 election for the Cambodia National Rescue Party, campaigning against Vietnamese influence in between frequent meals of the Vietnamese food he had become accustomed to. However, he would spend much of his electoral mandate resting at his home in Takeo.

As for his legacy, the former premier in his later life remained proud about helping lead the overthrow of the Khmer Rouge but was at times also apologetic for his role in the subsequent occupation of his own country – as well as for the growth of the communism he later rejected.

“Cambodian politics has the head of a chicken and the arse of a duck,” Sovann once said when asked about the future of Cambodia under Hun Sen’s reign. “They speak about democracy and multiple political parties, but they practise communist ways.”