In early 1967, an insurrection broke out in the area around Samlot in Batdambang, a province long noted for the presence of large landowners and great disparities of wealth. Local resentment focused on tax collections and on the decision of the revenuestarved government to expropriate land to build a sugar refinery near Samlot. In January 1967, irate villagers attacked a tax collection brigade--an incident that recalled the 1925 murder of the French resident in the area. With the probable encouragement of local communist cadres, the insurrection quickly spread through the whole region. Sihanouk was on one of his frequent sojourns in France, and Lon Nol, the prime minister, responded harshly. After returning home in March 1967, Sihanouk personally supervised counterinsurgency measures. He later mentioned, in an offhand way, that the effectiveness of the royal armed forces had restored the peace but that approximately 10,000 people had died.

The insurgency was not suppressed completely. It spread rapidly from Batdambang to the southern and to the southwestern provinces of Pouthisat (Pursat), Kampong Chhnang (Kompong Chang), Kampong Cham, Kampong Spoe (Kompong Speu), Kampot, and the central province of Kampong Thum. By the end of 1968, unrest was reported in eleven of the country's eighteen provinces. The Khmer Loeu regions of Mondol Kiri (Mondolkiri) Province and Rotanokiri Province fell almost entirely under KCP control by the end of the decade.

In January 1968, the communists established the Revolutionary Army of Kampuchea. During Sihanouk's last two years in power, the RAK obtained minimal assistance from the North Vietnamese, the Viet Cong, and the Chinese. Although North Vietnam had established a special unit in 1966 to train the Cambodian communists, it was extremely reluctant to alienate Sihanouk at a time when vital supplies were passing through the port of Kampong Saom and along the Ho Chi Minh Trail to the Viet Cong bases along the CambodiaVietnam border. Beijing and Moscow also were providing Sihanouk with arms, many of which were being used against the insurgents. The indifference of the world communist movement to the Cambodian struggle from 1967 to 1969 made a permanent impression on Pol Pot and other Khmer Rouge leaders.