In early 1967, an insurrection broke out in the area around Samlot in Battambang, 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 revenue-starved 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, Kampong Cham, Kampong Speu, Kampot, and the central province of Kampong Thom. By the end of 1968, unrest was reported in eleven of the country's eighteen provinces. The Khmer Loeu regions of Mondul Kiri (Mondolkiri) Province and Ratanak Kiri Province fell almost entirely under KCP control by the end of the decade.
United States bombing of enemy troop dispositions in Cambodia - particularly in the summer of 1973, when intense aerial bombardment (known as Arclight) was used to halt a Khmer Rouge assault on Phnom Penh -bought time for the Lon Nol government, but did not stem the momentum of the communist forces. United States official documents give a figure of 79,959 sorties by B-52 and F-111 aircraft over the country, during which a total of 539,129 tons of ordnance were dropped, about 350 percent of the tonnage (153,000 tons) dropped on Japan during World War II. Many of the bombs that fell in Cambodia struck relatively uninhabited mountain or forest regions; however, as declassified United States Air Force maps show, others fell over some of the most densely inhabited areas of the country, such as Siemreab Province, Kampong Chhnang Province, and the countryside around Phnom Penh. Deaths from the bombing are extremely difficult to estimate, and figures range from a low of 30,000 to a high of 800,000, with UN and CIA estimates of 600,000 or more and others 100,000. Whatever the real extent of the casualties, the Arclight missions over Cambodia, which were halted in August 15, 1973, by the United States Congress, delivered shattering blows to the structure of life in some of the country's villages, and, according to some critics, drove the Cambodian people into the arms of the Khmer Rouge.
The bombing was by far the most controversial aspect of the United States presence in Cambodia. In his book Sideshow, William Shawcross provides a vivid image of the hellish conditions, especially in the months of January to August 1973, when the Arclight sorties were most intense. He claims that the bombing contributed to the forging of a brutal and single-mindedly fanatical Khmer Rouge movement. However, his arguments have been disputed by several United States officials -including the former ambassador to Cambodia, Emory C. Swank, and the former Air Force commander in Thailand, General John W. Vogt - in an appendix to the second volume of the memoirs of then Secretary of State, Henry Kissinger.