One of the key issues to be raised in the early parts of the trial has been a controversial amnesty and pardon granted to the regime's former foreign minister Ieng Sary.
Presenter: Robert Carmichael
Michael Karnavas, defence lawyer for Ieng Sary; William Smith, deputy international prosecutor; Clair Duffy, observer for Open Society Justice Initiative.
Editor's note: (August 18) In a story about the trial of former Khmer Rouge leaders in Cambodia, an ABC radio story reported that "the collapse of the Soviet Union in 1991 meant that proxy players in the Cold War struggle, such as the Khmer Rouge, ran out of patrons". To be clear, at the height of the cold war, the Cambodian Government was being backed by Vietnam and the USSR, while the Khmer Rouge guerrillas were being backed by the US, Thailand, China, and Great Britain.
CARMICHAEL: In 1996 the Khmer Rouge foreign minister Ieng Sary defected to the Cambodian government. His move precipitated the end of the Khmer Rouge movement.
Ieng Sary's decision was timely -the collapse of the Soviet Union in 1991 meant that proxy players in the Cold War struggle, such as the Khmer Rouge, ran out of patrons. Ieng Sary had seen the writing on the wall.
His defection was sweetened by the Cambodian government. Ieng Sary received a royal pardon for the death penalty that a 1979 tribunal in Phnom Penh had handed down to him in absentia. That tribunal had been established in 1979 by the new Cambodian government just months after it drove the Khmer Rouge from power.
The remnants of Pol Pot's movement had fled west to the Thai border, where they found sanctuary and support during the final decade of the Cold War.
As part of that 1996 deal, Ieng Sary received a second sweetener an amnesty from prosecution under a law that made membership in the Khmer Rouge a criminal offence.
This week the international tribunal where Ieng Sary is on trial with three other surviving leaders of the Khmer Rouge movement for genocide, crimes against humanity and war crimes heard arguments for and against that amnesty.
Ieng Sary's defence team argued that the UN-backed court was essentially a national court, and therefore had to abide by the law that granted his client that amnesty.
Michael Karnavas is Ieng Sary's international defence lawyer:
KARNAVAS: Mr. Ieng Sary has abided by all of the conditions of the amnesty, and the amnesty itself as I have indicated brought fruit the very fruit that it was intended to bring. Peace to Cambodia, because as after that the rest either put down their arms or just gave up".
CARMICHAEL: Ieng Sary's defence team said their client's 1996 defection was contingent upon no further prosecutions - and therefore he should not be on trial.
Karnavas also cited the example of Sierra Leone in support of amnesty. He said the United Nations had not objected to amnesties agreed in that West African state when it established a tribunal for crimes against humanity committed there.
KARNAVAS: And I mention this and I highlight this because it demonstrates that even for the crimes for which Mr Ieng Sary is charged even for these sorts of crimes national jurisdictions are permitted to grant amnesties and there is no international prohibition, or customary international law, prohibiting this.
CARMICHAEL: Unsurprisingly, the prosecution disagrees. Deputy international prosecutor William Smith listed a number of reasons why Ieng Sary's amnesty was invalid, adding that international precedent compelled that the judges set it aside.
In summary, said Smith, the 1996 deal was never intended to provide an amnesty for genocide and other serious crimes.
SMITH: Again for arguments sake, if your honours found it did, this chamber has the discretion to reject such an amnesty on the basis that it did not comply with international treaty obligations and customary international law on the issue. It should also be rejected because of this court's obligation as an internationalized court to uphold principles in treaties and conventions, the purpose of which is to protect humanity.
CARMICHAEL: Clair Duffy, who monitors the trial for the Open Society Justice Initiative, said the prosecution's arguments against the amnesty were compelling.
DUFFY: The full reason that we set up courts like this is to end impunity for international crimes. And from what we've seen many other courts have read these kinds of amnesty provisions down for that very reason.
CARMICHAEL: The UN-backed tribunal estimates that between 1.7 million and 2.2 million people died during the Khmer Rouge's rule of Cambodia between 1975 and 1979.
The three other accused in the dock with Ieng Sary are party ideologue Nuon Chea, known as Brother Number Two; former head of state Khieu Samphan; and the former social affairs minister Ieng Thirith.
This week's hearings are preliminary, and are dealing with procedural issues approving witness lists, for instance, and determining various other aspects of the trial such as the validity of Ieng Sary's amnesty.
Later this year the court will issue its decisions ahead of the start of the trial proper - with witnesses and evidence. The chances are that Ieng Sary will be back in court.
This is Robert Carmichael in Phnom Penh.