Former Khmer Rouge commander Chuon Thy yesterday vehemently denied that his unit encroached on Vietnamese territory in testimony before the Khmer Rouge tribunal, claiming Democratic Kampuchea soldiers only defended Cambodia’s border.

Thy served as one of the commanders of Division 340, created by the Khmer Rouge specifically to defend the border in Svay Rieng province in 1978, prior to the Vietnamese invasion. Others, most recently expert witness and historian Stephen Morris, have accused the Khmer Rouge of harassing the Vietnamese with cross-border raids long before Vietnam decided to invade.

“We were in constant combat,” Thy said yesterday, claiming Cambodian and Vietnamese forces were “attacking one another on a daily basis”, while maintaining that Cambodian troops only fought to protect their territory.

Lysak read from a Vietnamese report claiming Vietnamese forces took captives from the witness’ unit after a Cambodian incursion across the border.

“That is false, we did not enter Vietnam territory; we contained them inside Kampuchean territory,” the witness protested.

Lysak then pressed the witness, asking how the Vietnamese could know the number of his division. The witness acknowledged that his division was “newly established” in 1978 and the number was only known within the military.

Khmer Rouge leaders Khieu Samphan and Nuon Chea are on trial for various crimes against humanity, including alleged atrocities committed against ethnic Vietnamese.

The defence for Nuon Chea has argued that the existence of a legitimate military threat from Vietnam is a key element of its defence, but Chea defender Victor Koppe yesterday argued that alleged crimes committed during excursions into Vietnam are supposed to be outside the scope of this trial.

“Incursions into Vietnam as a crime is not within the scope, but is still relevant in understanding the armed conflict,” prosecuting lawyer Dale Lysak countered. Koppe then claimed he was unprepared to deal with this line of questioning, a statement Lysak called “absurd”.

“Mr Koppe himself has repeatedly tried to tender evidence that it was Vietnam that was initiating conflict,” he said.

The prosecution later asked the witness if he knew of any members of his division who had been arrested and sent to Phnom Penh.

When the witness said he did not, Lysak revealed a list of 16 names of men from Thy’s division who were sent to S-21, including the division secretary. The witness, who testified in 2013 that “people were happy” during the regime, said he did not recognise any of the names and denied having heard of S-21 at that time.