By the 1960s, the Khmer Serei continued to recruit new members from the
Khmer Krom. With the experiences gained during the last decade, they had become conditioned into a well-trained force, both disciplined and well-armed. During this time, Sihanouk's increasing unpredictability resulted in a trickle of young men and women fading into the forest, joining Thanh's cause, possibly causing the Khmer Serei numbers to swell. In 1961, Sihanouk severed diplomatic relations with Bangkok over the 'Thai support' provided to the Khmer Serei. By 1963, Thanh strategically moved the Khmer Serei moved back to South Vietnam for financial and material support from the Saigon government. Moving back to the Mekong Delta area gave Thanh the potential to recruit new members from the large populations of Khmer Kroms living there. In addition, the
U.S. Special Forces and its
Civilian Irregular Defense Group (CIDG) would become its allies, and recruited the Khmer Serei into its ranks. The recruited members maintained their membership to Khmer Serei and were also obligated to bear arms for the Saigon government. This was beneficial on two fronts: firstly, the monetary rewards from joining the CIDG were more regular and higher than what they previously obtained and therefore many were recruited; secondly, the Khmer Serei was able to maintain its identity and building itself as a single cohesive force. Although there is no official documentation that shows the organisational working structure and functional responsibilities of the Khmer Serei movement, there is one cited source derived from personal observation: The movement's military organisation coincided with the structure of the CIDG, that is, the Khmer Serei were organised into platoons, companies and battalions. Whether this was coincidental, or to accommodate the CIDG, is unknown. Therefore, it was possible that their leader, Thanh had indeed maintain tight control over the group but it was structurally led by junior leaders in their respective military units. According to sources, the Khmer Serei headquarters was located at the Cambodian Buddhist compound and supported by field headquarters on the Cambodian border at the South Vietnamese village of Tinh Bien, in close proximity to the Seven Mountains region of the border. With exception of the independent Khmer Serei units along the border, virtually all of the other Khmer Serei personnel were associated with the U.S. Army Special Forces' activities conducted in the Mekong Delta. It is estimated that at least fifty percent of all Khmer Krom CIDG soldiers were also members of the Khmer Serei. At their peak in 1968, the Khmer Serei and related forces were thought to number up to 8000 men. The Khmer Serei participated in a number of general intelligence gathering missions. It is believed that they assisted
Project GAMMA, an intelligence gathering mission on the
North Vietnamese Army (NVA) bases in Cambodia, as well as providing support to Cambodia against the North Vietnamese and
Viet Cong, one of these activities from Project Gamma was Operation Cherry. It was also suggested that the Khmer Serei were involved in making liaison with unnamed units in Cambodia as part of intelligence gathering on the North Vietnamese Army's dispositions and to develop a strategy for the Cambodian Army movements. The Khmer knew the terrain and language well and therefore were invaluable to the mission. According to one U.S. Army commander, the Khmer Serei were good soldiers: They knew the border area, could communicate with the local populations, and could get intelligence about the Viet Cong and what they were doing in their sanctuaries across the border. In addition, member of the Khmer Serei has anti-communist contacts in the Cambodian Army who were sympathetic to any plan to rid Cambodia of the North Vietnamese presence. The information provided would have been crucial to the success of any plan for a military incursion into Cambodia, and it gives an indication of the importance of the Khmer Serei's role in Operation Cherry. ==Sihanouk government's response to the Khmer Serei movement==