Nuon Chea, former president Khieu Samphan and ex-foreign minister Ieng Sary are charged with genocide, war crimes and crimes against humanity for their part in Khmer Rouge rule, under which as many as 2.2 million Cambodians died between 1975-79.
A fourth defendant, social affairs minister Ieng Thirith, was judged unfit to stand trial due to dementia.
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"My position in the revolution was to serve the interests of the nation and people," he said.
"I had to leave my family behind to liberate my motherland from colonialism and aggression, and oppression by the thieves who wished to steal our land and wipe Cambodia off the face of the Earth."
He was referring to Vietnam, said Reuters, who he accused of attempting to "swallow Cambodia" to this day.
Nuon Chea also suggested that "unruly elements" within the Khmer Rouge were responsible for atrocities committed under the regime, an argument which prosecutor Andrew Cayley refuted.
He told the court the crimes were the result of "an organised plan developed by the accused and other leaders and systematically implemented" by Khmer Rouge commanders.
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In opening statements Monday, the prosecution described a "narrative of horror" under the regime, the Phnom Penh Post reported:
"The accused turned Cambodia into a massive slave camp, reducing an entire nation to a system of brutality that defies belief," [prosecutor] Chea Leang told the Khmer Rouge tribunal Trial Chamber yesterday.
"One in four people did not survive."
The defense will make its opening statements Wednesday and testimony is due to begin on December 5, the Guardian said.
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