PHNOM PENH, Cambodia — They were leaders of Cambodia’s infamous Khmer Rouge, the fanatical communist movement behind a 1970s reign of terror that transformed this entire Southeast Asian nation into a ruthless slave state — a place where cities were emptied of their inhabitants, religion and schools were banned, and anyone deemed a threat was executed.
When the nightmare ended, in 1979, close to 2 million people were dead — a quarter of Cambodia’s population at the time.
On Thursday, a UN-backed tribunal convicted two of the once all-powerful men who ruled during that era of crimes against humanity in the first and possibly the last verdicts to be issued against the group’s aging, top members.
Although survivors welcomed the decision to impose life sentences against Khieu Samphan, an 83-year-old former head of state, and Nuon Chea, the movement’s 88-year-old chief ideologue, they also say justice has come far too late and is simply not adequate.
‘‘Nothing can compare to the immense suffering they imposed, no sentence can be enough. They belong in hell, not an air-conditioned jail cell,’’ said Youk Chhang, who heads The Documentation Center of Cambodia, which has collected more than a million documents related to Khmer Rouge terror.
‘‘But this gives us hope that we can learn from the past . . . that we can try to prevent this from ever happening again,’’ Chhang said.
There was no visible reaction from either of the accused when the decisions were announced. Nuon Chea, wearing dark sunglasses, was too weak even to stand from his wheelchair. Defense lawyers insisted the case was not over and vowed to appeal within 30 days.
Summarizing the verdict, chief judge Nil Nonn said the defendants were part of ‘‘a joint criminal enterprise’’ that launched ‘‘a widespread and systematic attack against the civilian population’’ after Khmer Rouge guerrillas seized Phnom Penh on April 17, 1975.
The attack took many forms, Nil Nonn said, including ‘‘murder, extermination, enforced disappearances, attacks against human dignity, and political persecution.’’
The case, which lasted about two years, focused on just one of many mass killing sites and the forced exodus of millions of people from Cambodia’s cities and towns, where even hospitals were emptied of patients.
Top Khmer Rouge leader Pol Pot had reset the clock to ‘‘Year Zero.’’ Society was to be ‘‘purified.’’ Money was abolished. Communal kitchens were introduced nationwide. The failed aim: to create an agrarian ‘‘utopia.’’
Most of those who died succumbed to starvation, medical neglect, and overwork. Marked for death were the educated, religious or ethnic minorities, Buddhist monks, and anyone suspected of ties with the former government or who questioned the new rulers.
Khieu Samphan acknowledged mass killings took place. But during the trial he claimed he was just a figurehead with no real authority. He called allegations that he ordered executions a ‘‘fairy tale.’’
Nuon Chea, known as Brother No. 2 because he was Pol Pot’s trusted deputy, also denied responsibility, saying that Vietnamese forces — not the Khmer Rouge — had killed Cambodians en masse.
The hybrid tribunal, composed of Cambodian and international jurists, began operations in 2006. It has been criticized for spending too much — more than $200 million so far — and doing too little.
The court has convicted only one other defendant — prison director Kaing Guek Eav, who was sentenced to life imprisonment in 2011.
The current trial began in 2011 with four senior Khmer Rouge leaders; only two remain. Former foreign minister Ieng Sary died in 2013; his wife, social affairs minister Ieng Thirith, was deemed unfit to stand trial due to dementia.