Khmer Rouge head of state, Khieu Samphan, and Nuon Chea, right-hand man to the group's late chief, Pol Pot, are among the few surviving top leaders of the brutal communist group that was responsible for some 1.7 million deaths from starvation, exhaustion, disease and execution when it was in power in 1975-79.
It will be the second case for the defendants, who have already been tried for war crimes and crimes against humanity related to forced evacuations and a mass execution, one of many massacres at sites around the country that came to be known as the "killing fields." The verdict in that two-year trial is due next week. If found guilty, the two men could be put in prison for the rest of their lives.
The new trial brings additional charges of genocide, alleging that Pol Pot and other senior leaders intended to wipe out the members of the country's Vietnamese and Muslim Cham ethnic minorities. Hundreds of thousands of ethnic Vietnamese were forced into Vietnam, and virtually all of those remaining were executed. Estimates of the number of Chams killed range from 90,000 to 500,000.
International deputy co-prosecutor William Smith said the team felt there was sufficient evidence to bring such cases.
"It's really significant because genocide is one of the most serious charges in international law," Smith said.
In Wednesday's initial hearing, lawyers and judges will discuss which witnesses and experts will be called, the issue of requests for reparations, and procedural legal objections. The judges expect the actual trial to begin in the last quarter of this year, said Lars Olsen, a tribunal spokesman.
Lyma Nguyen, an international civil party lawyer representing ethnic Vietnamese victims, said the trial represents not only a rare chance to shed light on the suffering caused by the alleged genocidal policies, but also on the long-standing harm they have inflicted until this day.
Those forced to flee retained no documentation proving their Cambodian origins, so when they returned they were plunged into statelessness. Today, hundreds of thousands of ethnic Vietnamese remain undocumented, living on the fringes of society without access to proper schooling, health care, jobs or social services.
The trial also marks the first time rape and forced marriage will be addressed by the court, as offenses considered crimes against humanity.
Court officials and historians had argued that Khmer Rouge policy banning sexual relations among unmarried couples was proof rape could not have been widespread or systematic. But Duong Savorn, head of the gender-based violence team at the Cambodian Defenders Project, a local legal aid group, said his research has suggested that there were "many, many cases" of rape. "I think it's good to put on the record," he said.
After years of legal and political wrangling, the Khmer Rouge tribunal was established in 2006 to bring the ruthless regime to justice some 30 years after their reign of terror. Since then, however, the court has been plagued by corruption, mismanagement, and financial woes. The hybrid structure of the court, in which U.N.-appointed international judges share the bench with Cambodian counterparts, has led to allegations of political interference and repeated deadlocks.
To date, only a single conviction has been obtained by tribunal. It sentenced Kaing Guek Eav, also known as "Duch," the director of the infamous S-21 torture center, to life imprisonment in a case where reams of documentation and a confession made for as smooth a trial as possible.
Of four original top-level defendants, only Nuon Chea and Khieu Samphan remain in the dock to hear next week's verdict and attend the new trial. Ieng Sary, the Khmer Rouge foreign minister, died in March 2013, while his wife, social affairs minister Ieng Thirith, was released in September 2012 after dempentia made her unfit to stand trial.
The first trial of the leaders took two years, and experts say the second trial, which covers more cases and crimes, could take much longer.
"There are so many different crimes and so many things to be proved," said Anne Heindel, co-author of "Hybrid Justice: The Extraordinary Chambers in the Courts of Cambodia." ''It seems extremely likely that one or both of the accused will die before judgment."
Both aging defendants are ailing in health. Khieu Samphan, 83, has been hospitalized for high blood pressure and breathing problems while the health woes of Nuon Chea, 88, led to repeated trial adjournments during the last case.
Smith, the prosecutor, acknowledged that age could pose a problem.
"It's a concern whether we'll reach the end of this next trial," he said. "But the most important point is that these leaders are put on trial for these crimes, and, in a sense, the process is more important than the endpoint - that there is a rule of law and people must be held to account, whether you get to the end or not."