Thirty years later, Cambodia's "killing fields" are still haunting. A Buddhist memorial displays 5,000 haphazardly arranged human skulls - a tiny fraction of the 1.7 million Cambodians butchered by the Khmer Rouge.
While the world must never forget what happened, there is at least the beginning of justice. On Monday, a United Nations-backed tribunal convicted Kaing Guek Eav, known as Duch, of war crimes and crimes against humanity - the first major Khmer Rouge figure to be tried since the regime was overthrown. He has already spent 16 years in prison, and the tribunal sentenced him to another 19 years.
Duch oversaw a notorious prison where more than 14,000 people were tortured and killed. During an eight-month trial, he admitted to many of the charges against him. His defense - he was a "cog in a machine" - is no defense at all.
We understand why many of the victims of the Khmer Rouge and their families were disappointed that he was not given life in prison. He should never taste freedom, but at least he was held to account and he will be 86 years old when his sentence is served.
Four other more senior Khmer Rouge leaders - including Khieu Samphan, the former president - are facing charges of genocide and war crimes. Their trial needs to be pursued expeditiously, and many more need to be held accountable if Cambodia is to see true justice.
The conduct of Duch's trial raises serious concerns about whether that will ever happen. Human rights experts say most of the five international prosecutors on the case did not know the file in detail and only one had senior level trial experience. There were conflicts between the tribunal's international and Cambodian staff, bureaucratic delays, and heavy-handed political interference by the Cambodian government.
It is disturbing that the Cambodian prime minister, Hun Sen, has said the court will not prosecute more suspects than the ones in custody. One has to ask, whom is he trying to protect?
The United Nations, the United States and other donor countries need to insist that the tribunal have the independence to do its job openly, fairly and thoroughly. The Documentation Center of Cambodia, which collects evidence of Khmer Rouge crimes, estimates that five million survivors of the regime are still alive. They deserve to see their tormentors held fully accountable.
