PHNOM PENH — British historian Philip Short took the stand for the second day at the U.N.-backed Khmer Rouge tribunal Tuesday, as he continued to describe the relationship between Vietnamese communists and their Cambodian counterpart.

Short, the 68-year-old author of “Pol Pot: Anatomy of a Nightmare,” told the court Tuesday that the Vietnamese had an “undeniable” interest in the Khmer Rouge, providing support and training for the communist insurgency in its early days.

Short is testifying in the atrocity crimes trial of Khmer Rouge leaders Nuon Chea and Khieu Samphan. Much of his testimony on Tuesday was centered around the relationship between the regime and Vietnam, which would eventually become its enemy....