 The People 's Army of Vietnam had been utilizing large sections of relatively unpopulated eastern Cambodia as sanctuaries into which they could withdraw from the struggle in South Vietnam to rest and reorganize without being attacked . These base areas were also utilized by the Vietnamese communists to store weapons and other material that had been transported on a large scale into the region on the Sihanouk Trail . PAVN forces had begun moving through Cambodian territory as early as 1963 . In 1966 , Prince <unk> Sihanouk , ruler of Cambodia , convinced of eventual communist victory in Southeast Asia and fearful for the future of his rule , had concluded an agreement with the People 's Republic of China which allowed the establishment of permanent communist bases on Cambodian soil and the use of the Cambodian port of Sihanoukville for resupply . 
