On the other hand, his relations with neighboring South Vietnam went from bad to worse. As early as December 1954, Sihanouk and South Vietnamese President Diem were squabbling over money and border matters. In March 1956, South Vietnam stopped boat traffic on the Mekong River — an act which effectively isolated Phnom Penh from outside trade, since the port of Sihanoukville was still unbuilt. 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