In the late 1960s, while preserving relations with China and with North Vietnam, Sihanouk sought to restore a measure of equilibrium by improving Cambodia's ties with the West. This shift in course by the prince represented another adjustment to prevailing conditions in Asia. The Cultural Revolution had made the Chinese very difficult with which to carry on relations. The increasing North Vietnamese presence in Eastern Cambodia was destabilizing Cambodia politically and economically. When the Cambodian left went underground in the late 1960s, Sihanouk made concessions to the right since he did not have any force that he could play against them. Cambodia served as the southern terminus of the Ho Chi Minh Trail, the logistical resupply route of the North Vietnamese and Viet Cong. The use by these forces of sanctuaries in Cambodia put Cambodian was neutrality in jeopardy. China, preoccupied with its Cultural Revolution, did not intercede with Hanoi. On Cambodia's eastern border, even in the face of the communist Tet Offensive in 1968, South Vietnam surprisingly had not collapsed and President Nguyễn Văn Thiệu's government was bringing a measure of stability to that war-ravaged country. As the government in Phnom Penh began to feel keenly the loss of economic and military aid from the United States, which had totaled about US$400 million between 1955 and 1963, it began to have second thoughts about the break with Washington. The unavailability of American equipment and spare parts were exacerbated by the small amount and poor quality of Soviet, Chinese, and French substitutes.