Expert witness Stephen Morris took the stand yesterday to testify on the subject of Vietnam’s 1979 in-vasion of Cambodia, arguing that Vietnam desired to control the communist movements in Laos and Cambodia.

Morris, who was granted unprecedented access to Soviet communication files with the Vietnamese during the Khmer Rouge period, published a book on his research in 1999.

Khmer Rouge leaders Nuon Chea and Khieu Samphan are on trial for various crimes against humanity, including genocide against the Vietnamese, as well as internal purging of their own officers. Defence teams have long argued that the purges were a reaction to an attempt by Vietnam to overthrow the Cambodian government, and Morris testified that Vietnam did indeed have imperial ambitions towards Cambodia.

“The history of Vietnam is a history of a long march south,” Morris said. “Large parts of what is now southern Vietnam used to be part of Cambodia.”

Morris also testified that the Vietnamese wanted to use people who were loyal to them to establish communist parties in other countries. He claimed the attempt to found an Indochinese communist federation was really an attempt by Vietnam to assert its authority in the region.

“The Vietnamese conceived of Indochina as a place where they would be dominant over Laos and Cambodia,” he said.

One of the documents from the Soviet archives was a 1973 communiqué from the Vietnamese to the Soviet ambassador which said Vietnam’s goal was “to replace the reactionary regimes in Saigon, Vientiane, and Phnom Penh with progressive ones”.

While the communist Khmer Rouge regime did take control of Phnom Penh soon after, Morris said Vietnam had less control than anticipated. “The Vietnamese Communist Party tried to control the entire communist movement in Indochina, but lost control of the movement in Cambodia,” Morris testified, adding “Vietnamese communists never fully understood what was going on in Cambodia”.

Morris also heavily criticised fellow Khmer Rouge researcher Ben Kiernan accusing the author of The Pol Pot Regime of having a “political agenda”.

The Australian researcher said it was a “tragic mistake” by the US State Department to grant Kiernan more than $1 million to research the Khmer Rouge. The prosecution has previously used Kiernan’s writings in relation to charges of genocide committed against the Vietnamese and Cham people.