Separatist movements

Khmer nationalist Son Ngoc Thanh (1908–77) was a Khmer krom, born in Trà Vinh, Vietnam. Thanh was active in the independence movement for Cambodia. With Japanese support he became the prime minister of Cambodia in March 1945 but was then quickly ousted with the return of the French later that year. Widely supported by the Khmer Krom during the First Indochina War, Thanh's role faded in Vietnam after 1954 as he became more embroiled with politics in Cambodia proper, forming an opposition movement against Prince Sihanouk.
During the Vietnam War and direct American involvement between 1964 and 1974, the Khmer Krom were recruited by the US military to serve in MIKE Force. The force fought on the side of South Vietnam against the Viet Cong but in time the militia regrouped as the "Front for the struggle of Kampuchea Krom" (French: Front de Lutte du Kampuchea Krom). Headed by a Khmer Krom Buddhist monk, Samouk Sen, the group was nicknamed the "White Scarves" (Khmer: Kangsaing Sar; Vietnamese: Can Sen So) and allied itself with FULRO against South Vietnam.
After the Fall of Saigon in 1975 and the Communist take-over of all of Vietnam, the Kampuchea Krom militia found itself embattled with North Vietnamese Army. Many of the fighters fled to Khmer Rouge controlled Cambodia (Democratic Kampuchea) hoping to find a safe haven to launch their operations inside Vietnam. The "White Scarves" arrived in Kiri Vong District in 1976, making overture to the Khmer Rouge and appealing to the leader Khieu Samphan directly for assistance. The force was disarmed and welcomed initially. Subsequent orders from the Khmer Rouge leadership however had Samouk Sen arrested, taken to Phnom Penh, tortured, and killed. His force of 67 Khmer Krom fighters were all massacred. During the following months, some 2,000 "White Scarves" fighters crossing into Cambodia were systematically killed by the Khmer Rouge.
In the late 1970s, the Khmer Rouge regime attacked Vietnam in an attempt to reconquer the areas which were formerly part of the Khmer Empire, but this military adventure was a total disaster and precipitated the invasion of Cambodia by the Vietnamese army and subsequent downfall of the Khmer Rouge, with Vietnam occupying Cambodia.