Failure to unify the country by referendum led to the Vietnam War in 1955. The North Vietnamese People's Army of Vietnam and the South Vietnam-based Việt Cộng guerrilla fought against the military of South Vietnam (by then the Republic of Vietnam) and were backed by their communist allies, mainly China and the Soviet Union. To prevent other countries from becoming communist in Southeast Asia, the United States intervened in the conflict along with other anti-communist forces from South Korea, Australia and Thailand, who heavily supported South Vietnam militarily. The conflict spread to neighboring countries and North Vietnam supported the Pathet Lao in Laos and the Khmer Rouge in Cambodia against their respective US-backed governments. By 1973 the United States and its allies had been forced to withdraw from the war, this left South Vietnam alone and it was swiftly overrun by the superior Northern forces.