In its nearly four years of rule, the Khmer Rouge saw the deaths of up to 2 million people, but the regime’s chief prison administrator, Duch, told tribunal judges Tuesday many of those were caused by fighting with Vietnam.

Duch’s atrocity crimes trial, which began March 30, has begun to explore the complicated relationship between the Khmer Rouge communist guerrillas and their original supporters, the Vietnamese communists.

The two sides eventually became rivals, and it was fear of Vietnamese infiltration that in part unraveled the Khmer Rouge.

Those accused of spying for the Vietnamese were sent to a prison administered by Duch, where they were tortured into confession and sent to be executed on the outskirts of the capital.

Prosecutors at the UN-backed tribunal say at least 12,380 people died under Duch’s supervision, and the former math teacher, whose real name is Kaing Kek Iev, is facing charges of war crimes, crimes against humanity, torture and murder.

In his testimony Tuesday, Duch outlined a chronic ideological conflict between the communist party of Cambodia, or the Khmer Rouge, and the Vietnamese.

Pol Pot refused to follow the leader of the Vietnamese communists, Le Yon, who wanted to unite former Indochinese countries into a federation, Duch said, which led to armed conflict between the two.

Historian Nayan Chanda told the court Tuesday that the Vietnamese communists had helped the Khmer Rouge until the guerrillas had defeated the government of Gen. Lon Nol.

Pol Pot wanted to be free from control of the Vietnamese, Duch said. “That’s why bloody armed conflict broke out, killing so many people.”