From 1964, two years after taking control of the Cambodian Communist Party, then called the Workers’ Party of Kampuchea, Saloth Sâr and his close associates, many of whom were former members of the Paris Marxist Circle, were well aware of the party's limited political influence, particularly due to its clandestine nature and its illegality in Cambodia. The party leadership therefore decided to first exile themselves to the rural areas and then seek powerful international allies.
On the ground, while spreading a strong revolutionary, nationalist, and anti-capitalist ideology, the Khmer Rouge began to learn how to manage the population. This involved, among other things, a hardening of security and repressive measures, which would become cornerstone elements of their future governance.
Since 1963, the Khmer Rouge leaders had joined, during their rural exile, strongholds in contested areas of the country, particularly those disputed by the Vietnamese. However, their intent was always to detach themselves from Vietnamese control, as Saloth Sâr considered them, in the long term, to be pro-Sihanouk. During this period, the party's general secretary, accompanied by the leadership committee, met to determine a course of action approving "all forms of struggle" against the Cambodian government under the head of state. After settling in Hanoi between April 1965 and February 1966, the future Pol Pot, sensing that the Vietnamese were hesitant to fully support him, decided to move closer to Maoist China, which he saw as more aligned with his convictions. He then secretly renamed the Workers’ Party of Kampuchea to the Communist Party of Kampuchea (CPK). The deterioration of their relationship with the Vietnamese and the name change had to remain internal to the party. Only the Centre, the Khmer Rouge leadership core, was informed. Many Cambodians were aware of a communist movement in the country, but those who knew its exact structure and organizational chart could be counted on one hand.
An event would further amplify this secretive stance and confirm that it was part of a deliberate strategy. Norodom Sihanouk, who had numerous questions about his government, which he had initially opened to left-wing members at the start of the decade, threatened his leftist ministers—Khieu Samphân, Hou Yuon, and Hu Nim—with summonses to military tribunals. The trio fled the capital in 1967 and joined some of the CPK’s bases, notably in the Cardamom Mountains. Several media outlets speculated that they were killed by Sihanouk's men, and a sense of injustice began to grow in the minds of many young activists, who organized demonstrations in Kampong Cham and Kandal to honor these "martyrs." The CPK then seized this opportunity to publicly announce their affiliation with the party, presenting them as "the three ghosts," the new leaders of the movement, to lend it legitimacy. However, true power still lay in the hands of Saloth Sâr, Nuon Chea, and Son Sen. It was at this time that they began promoting the term "Angkar" (The Organization) exclusively within the CPK.
From this moment on, Saloth Sâr believed it was time to take action on the ground. The first Khmer Rouge insurgencies began in January 1968, gradually joined by some 10,000 Cambodian villagers. From Kampot to Anlong Veng, from the southwest to the north, numerous guerrilla movements broke out, and those previously called the Khmer Rouge became the insurgents. However, the core group had to expand in order to cover more territory, and by the end of the year, insurgents were recorded in 12 of the 19 provinces of the country. Ta Mok and So Phim were assigned to the southwestern and eastern zones, respectively. The two military leaders had to face the scorched earth policy orchestrated by Lon Nol, the new prime minister of Norodom Sihanouk. These destructions of resources and strategic points completely isolated, among other places, these two regions contested by the Khmer Rouge, who had to wait for weeks or even months before receiving any supplies. The lack of resources was rampant, but behind the scenes, the high-ranking party leadership was meticulously orchestrating their plan.
Lon Nol, a sympathizer of the United States, deposed Norodom Sihanouk from his role as head of state on March 18, 1970. Determined to regain his position, Sihanouk went to Beijing and met Phạm Văn Đồng, the prime minister of communist North Vietnam, who asked him to ally with the Khmer Rouge under Saloth Sâr. This request was crucial and reflected the growing importance of the Communist Party of Kampuchea during the Vietnam War. Its leaders, with a strong sense of nationalist identity, still wanted to gradually distance themselves from communist Vietnam. However, the relationship was mutually beneficial. The Khmer Rouge insurgencies were stalling, and resources were lacking in some isolated provinces. The Vietnamese could provide them with resources, weapons, and ammunition. In return, the Khmer Rouge had to guarantee access to the Ho Chi Minh Trail in northeastern Cambodia. This route, which also passed through Laos, had allowed North Vietnamese forces to supply the fighters leading guerrilla operations in the south, an area controlled by the Americans.
A few hours later, on March 23, Sihanouk announced in Beijing the creation of an exile government, the Royal Government of National Union of Kampuchea (GRUNK). Officialized on May 5 of the same year and recognized by China, Cuba, North Vietnam, and North Korea, it would, through subsequent reshuffles, include a host of Khmer Rouge ministers until 1975. At the same time, on March 23, Sihanouk called, in a speech, for all Cambodians to take up arms and wage guerrilla warfare against Lon Nol's regime. The speech had been reviewed and somewhat modified by Chinese Premier Zhou Enlai, a Maoist, and Saloth Sâr, who was also present in Beijing but hidden from Sihanouk’s sight, as he feared him. Sihanouk would simply receive a letter expressing support, but signed by Khieu Samphân, Hou Yuon, and Hu Nim, his three former ministers, who were official leaders of the CPK.
The Khmer Rouge, still operating in the shadows, were becoming increasingly significant in the Southeast Asian geopolitical landscape. Growing financial and logistical support from China allowed them to be more direct and strict in their relations with North Vietnam, which, before 1972, would fully support them. The Vietnamese would not only provide additional resources but also military training to the insurgent sympathizers in the rural areas. They would also fight, under the CPK's leadership, to support the Khmer Rouge in their quest for territorial dominance. By 1972, one-third of the Cambodian population, or 2 million people, was living under the control of the Khmer Rouge.