Whatever goodwill Saigon got by re¬opening the Mekong some months later dissolved when Diem began harboring Sihanouk's bugbear, Son Ngoc Thanh. In 1957, Thanh organized the so-called "Khmer Serei," a group of partisans — mostly ethnic Cambodians from South Vietnam — which intermittedlyharrassed the Sihanouk government for the next thirteen years