Map of the Communist forces in South Vietnam, 1964, showing areas controlled by the Viet Cong and the Ho Chi Minh trail that brought reinforcements from the North.
“The Sihanouk Trail was a logistical supply system in Cambodia used by the People’s Army of Vietnam (PAVN) and its Viet Cong (VC) guerillas during the Vietnam War (1960–1975). Between 1966 and 1970, this system operated in the same manner and served the same purposes as the much better known Ho Chi Minh Trail (the Truong Son Road to the North Vietnamese) which ran through the southeastern portion of the Kingdom of Laos. The name is of American derivation, since the North Vietnamese considered the system integral to the supply route mentioned above. U.S. attempts to interdict this system began in 1969. Prince Norodom Sihanouk had ruled Cambodia adeptly since he had wrested independence from the French on 9 November 1953. He had accomplished this task by deft political maneuvering between both the left and the right to achieve what no other ruler or political group in Indochina had managed, a relatively bloodless transition to independence. During the next ten years, while the conflicts in neighboring Laos and South Vietnam heated up, Sihanouk managed to sustain his delicate domestic political balance while at the same time maintaining his nation’s neutrality (guaranteed by the 1954 Geneva Conference that ended the First Indochina War). This was no small accomplishment considering that Cambodia was wedged between its perennial enemies: Thailand to the west and the Republic of Vietnam (South Vietnam) to the east, both of whom were increasingly supported by the United States. Sihanouk came to believe that communist triumph in Southeast Asia was inevitable and that Cambodia’s military was incapable of defeating the Democratic Republic of Vietnam (North Vietnam), even with U.S. support. If Cambodia (and his rule) was to survive, he would have to make a bargain with the devil. … In the earliest days of the Vietnam War, North Vietnam supplied the VC in the South by two methods. The first was to extend the Ho Chi Minh Trail southward into the tri-border region of Laos/Cambodia/South Vietnam. The Trail, a labyrinth of paths, roads, river transportation systems and way-stations, was constantly being expanded and improved. It served as a logistical jugular vein, for both men and material, for the North Vietnamese war effort against South Vietnam. The second method was to transport supplies by sea. Estimates of this seaborne traffic ran as high as 70 percent. It was carried out due to the higher volume of material that could be transported by sea, as opposed to the overland route. …”
W – Kingdom of Cambodia (1953–1970)
NY Times: Sihanouk Hails Plan on Cambodian Neutrality (Jan. 16, 1964)
[PDF] Communism in Cambodia – CIA (1972)