For the first time, two leaders of Pol Pot's brutal Khmer Rouge regime have been found guilty of genocide.
Nuon Chea, 92, was Pol Pot's deputy, and Khieu Samphan, 87, was the Cambodian regime's head of state.
They were on trial at the UN-backed tribunal on charges of genocide against Cham Muslims and ethnic Vietnamese.
The guilty verdict is the first official acknowledgement that what the regime did was in fact genocide as defined under international law.
The two men - already serving life sentences for crimes against humanity - have again been sentenced to life.
They are two of only three people ever convicted by the court in Phnom Penh.
Up to two million people are thought to have died under the brief but brutal Khmer Rouge regime between 1975 and 1979.
Many of them succumbed to starvation and overwork, or were executed as enemies of the state.
Who were the Khmer Rouge?
The Khmer Rouge were radical Maoists who formed a regime that ruled Cambodia from 1975 to 1979, led by Saloth Sar who was better known as Pol Pot.
The regime - founded by French-educated intellectuals - sought to create a self-reliant, agrarian society: cities were emptied and their residents forced to work on rural co-operatives. Many were worked to death while others starved as the economy imploded.
During the four violent years they were in power, the Khmer Rouge also killed all those perceived to be enemies - intellectuals, minorities, former government officials - and their families.
That included people from ethnic minorities, but largely consisted of ethnic Khmer people.
Why is the genocide verdict significant?
The Khmer Rouge's crimes have long been referred to as the "Cambodian genocide", but academics and journalists have debated for years as to whether what they did amounts to that crime.
Although Cham Muslims and ethnic Vietnamese died in large numbers, the UN Convention on Genocide speaks of "intent to destroy, in whole or in part, a national, ethnic, racial or religious group".
So prosecutors at the tribunal tried to prove that the Khmer Rouge specifically tried to do that to these groups - something experts including Pol Pot biographer Philip Short say they did not.
During the trial, a 1978 speech from Pol Pot was cited in which he said that there was "not one seed" of Vietnamese to be found in Cambodia. And historians say that indeed a community of a few hundred thousand was reduced to zero by deportations or killings.
Apart from being targeted in mass executions, Cham victims have said they were banned from following their religion and forced to eat pork under the regime.
The verdict today may not end the debate completely, but victims groups have long waited for this symbol of justice.
Why is this tribunal controversial?
Officially called the Extraordinary Chambers in the Courts of Cambodia, this could be the tribunal's final decision.
Established in 2006 with both Cambodian and international judges, it has so far only convicted three people for the atrocities of the Khmer Rouge regime at a cost of $300m (£232m).
In 2010 it convicted Kaing Guek Eav, also known as Duch, who was in charge of the infamous Tuol Sleng torture centre and prison in Phnom Penh.
Former Khmer Rouge foreign minister Ieng Sary was a co-defendant with Khieu Samphan and Nuon Chea but died before judges delivered a verdict in the first of the two sub-trials in 2014. His wife Ieng Thirith, the regime's social affairs minister and the fourth co-defendant, was ruled mentally unfit to stand trial and died in 2015.
Although there are cases against four other Khmer Rouge members, Cambodian Prime Minister Hun Sen has been vocal about his opposition to the tribunal starting any new trials and there is little chance this will happen.
A former mid-level member of the Khmer Rouge regime himself, he says his people want to move on and that further prosecutions could lead to violence.
The Khmer Rouge waged an insurgency after they were toppled from power, although thousands defected to the government in the 1990s before the group disbanded completely in 1999. There are parts of the country where victims and perpetrators live side by side in villages.
But many Cambodians pay little attention to the tribunal, and young people in particular are keen for their country to be known for something other than the "killing fields".