Specifically, in 1963, Kennedy gave the green light for the assassination of Ngo Dinh Diem, who was the President of South Vietnam. As a result of Diem's assassination, Prince Norodom Sihanouk, who was the leader of Cambodia, declared Cambodia to be a neutral party in the war between North and South Vietnam. At the same time, Sihanouk kicked American military advisers out of Cambodia. Sihanouk took these actions because he believed that if Kennedy was willing to have an ally like Diem assassinated, he might very well do the same thing to Sihanouk.
As a result of Sihanouk's actions, Cambodia became a safe haven for the Viet Cong and the NVA and the Ho Chi Minh trail, thru which the Viet Cong was supplied, was extended for hundreds of miles south along the eastern border of South Vietnam. This made South Vietnam's war with North Vietnam a much tougher fight and ultimately led to the takeover of South Vietnam by Norht Vietnam in 1975.
eastern border = western border