PHNOM PENH, Cambodia — The four surviving leaders of the Khmer Rouge went on trial Monday, more than three decades after the collapse of a government that caused the death of as much as one-fourth of the population and left Cambodia a nation of traumatized survivors.

Now frail and fading from the memory of many Cambodians, the three men and one woman are charged with crimes against humanity, war crimes, genocide, homicide and other offenses that occurred when the Khmer Rouge were in power from 1975 to 1979.

The case is the centerpiece of a United Nations-backed tribunal that has lasted five years and cost more than $100 million and is intended finally to lay the past to rest.

The defendants are Khieu Samphan, 79, the nominal head of state; Nuon Chea, 84, described as the Khmer Rouge’s ideologue; Ieng Sary, 85, the foreign minister; and his wife, Ieng Thirith, 79, who was minister of social affairs. All have declared their innocence....