The end of the Khmer Rouge, the gang of zealots who killed at least 1 million people in the four years they ruled Cambodia, was only a matter of time. Mercifully, it seems that time has finally come. Late last month, two of the three surviving leaders of the movement, Mr. Khieu Samphan and Mr. Nuon Chea, defected to the government of Cambodia. Although one Khmer Rouge leader remains in the field — Mr. Ta Mok, the military commander known as “the Butcher”_ for the first time in three decades, Cambodia faces no real threat of internal insurgency. The government in Phnom Penh can now focus on nation-building, free from the distraction of a civil war.

Focus it should, but that does not mean that this grim chapter in Cambodian history is over. While the government of Cambodia devotes its efforts to restoring democracy, human rights and the rule of law, and promoting economic development, the rest of the world must demand an accounting from the survivors of the Khmer Rouge leadership. Mr. Nuon Chea was brother No. 2 in the party, second only to Pol Pot, who died last April; Mr. Khieu Samphan was head of state for the four years the party was in power. These two men deserve an international trial on charges of genocide.

Yet incredibly, upon defecting they were given red-carpet treatment: bouquets of flowers and a luxury hotel. Mr. Hun Sen, the prime minister of Cambodia who no doubt engineered the defections — as he has for other former Khmer Rouge leaders — then said that he was prepared to offer the men amnesty in the name of national reconciliation. That reversed an earlier pledge to hand over Khmer Rouge leaders to an international tribunal.

The willingness to gloss over the past was evident at a press conference given by the two defectors shortly after they arrived in Phnom Penh. During that encounter, they made halfhearted apologies for turning Cambodia into a killing field. Mr. Khieu Samphan urged his compatriots to “let bygones be bygones,” while Mr. Nuon Chea said “Let’s consider that an old issue.”

Never. The savagery the Khmer Rouge unleashed against their own people is unparalleled in human history. In four years, the government is thought to have killed as many as 2 million people out of a nation of some 10 million — 20 percent of its own population. It is impossible to see what defense could even be offered to explain such madness: There was no war, nor any bureaucratic machinery behind which the executioners or their superiors could hide.

Mr. Hun Sen’s ambivalence about a trial is understandable. He himself was a member of the Khmer Rouge (although he has never been connected with any of their crimes) before he moved to the head of the government installed by the Vietnamese when they invaded their neighbor in 1979 and put an end to the genocide. He offered similar amnesties to other Khmer Rouge leaders, including Mr. Ieng Sary, another notorious killer who defected in 1996, received a pardon and now runs his own fiefdom in the gem-rich northwest part of the country, even though he nominally supports the government. Most likely, Mr. Hun Sen sees protection for the former Khmer Rouge leadership as the best way to ensure their support for him in his own domestic power struggles.

The problem for Mr. Hun Sen is that far more is at stake than his political survival. The initial decision to grant a pardon caused an international uproar. King Norodom Sihanouk, who would have to issue any pardon, said that he would not do it. The prospect of a continuing suspension of aid to Cambodia and still more delay before the country gains admission to the Association of Southeast Asian Nations seems to have had an effect. Late last week, Mr. Hun Sen reversed course and said that he supported an international trial after all.

A trial could be embarrassing. Spokesmen for the two Khmer Rouge leaders said that they will discuss the support the party received before and during their years in power. The United States and China are sure to be uncomfortable with that, but it is no reason to flinch.

A trial may reveal more foreign complicity in the Khmer Rouge rise to power than is commonly known. The defendants will try to use such revelations to blur questions of causality and responsibility — which is precisely why a trial is needed. There are almost always matters of degree in such debates, but this is one of those rare cases in which no line needs to be drawn. No matter how much aid the Khmer Rouge killers were given, no one helped them or urged them to commit genocide. There is no balancing that needs to be done. Prima facie, the crimes of the Khmer Rouge qualify as genocide. Only a reckoning is needed.

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