In the four decades since the Khmer Rouge took power, Cambodia has not been a happy country. But how could it be, when a former Khmer Rouge commander still rules?

In 1998, when Hun Sen manoeuvred himself into the premiership, he was one of the world’s younger leaders. Now 62, he is one of the longest-serving – and shows no intention of stepping aside.

His story is remarkable in so many ways. In 1975, he was among the Khmer Rouge rebels who swept into Phnom Penh and set the country on a deadly, regressive course of agrarian socialism. Anywhere up to a fifth of the population – an estimated 1.7 million, lost their lives either through hunger, exhaustion or in the countless “Killing Fields”.

In 1977, during a purge, Hun Sen fled to neighbouring Vietnam, only to return to Cambodia two years later when Vietnamese forces invaded and he found himself a major figure in the new regime imposed by Hanoi. While the Vietnamese departed in 1987, Hun Sen never left the centre field of politics.

Most in Cambodia, where more than half the population is estimated to be under 25, have never known a time when he was not a central figure. And his grip on power has tightened, not lessened, during the years when the country has described itself as a democracy. Potential rivals within his own party have been given senior positions, alongside members of his family, while opponents such as Sam Rainsy, have been smeared, sidelined and out-manoeuvred. His administration has been often accused of torturing, and even killing, dissidents.

Hun Sen’s cementing of political power has taken place alongside painful efforts to try and bring to justice some of the premier’s former senior colleagues. The Extraordinary Chambers in the Courts of Cambodia was finally established in 2006, to try some of the ageing senior Khmer Rouge figures.

In 2010, Kaing Guek Eav – also known as Comrade Duch, who oversaw the Tuol Sleng interrogation camp in Phnom Penh – was found guilty of genocide and crimes against humanity, and a court sentenced him to 35 years imprisonment. Last summer, Khieu Samphan, the 83-year-old former head of state of the Khmer Rouge, and 88-year-old Nuon Chea, its chief ideologue, were similarly found guilty.

But they are slim pickings. Five KR leaders had originally been brought for trial. But the former foreign minister, Ieng Sary, died in 2013, while his wife, the social affairs minister Ieng Thirith, was deemed unfit to stand trial. The most senior leader, Pol Pot, died in 1998.

Whether there will be any more trials remains unclear. The tribunal has been rocked by allegations that Hun Sen was trying to interfere. He has said that he believes it threatens to tear society apart. However, commentators have suggested privately that he simply wants to protect colleagues. Forty years after the Khmer Rouge seized Cambodia, the impact of their actions continues to reverberate.