On 18 March 1970, Cambodian Head of State King Norodom Sihanouk was removed from power after a vote in the National Assembly. Prime Minister Lon Nol subsequentially invoked emergency powers, effectively becoming the new head of state.
The episode spelt a pivotal turning point in Cambodia’s involvement with North Vietnam and the Second Indochina War. Earlier in the month, large-scale anti-Vietnamese protests erupted in Phnom Penh attacking North Vietnamese embassies while Prince Sihanouk was touring Europe, the Soviet Union and China. Their dissatisfaction stemmed from the increasing presence of troops allied to North Vietnam using Cambodia as a sanctuary.
The appointment of Lon Nol as the new head of state would drastically shape the new State of Cambodia during the first six months after the coup. In short, Lon Nol eventually began the short leadership of the Khmer Republic from 1970-1975, the pro-United States military-led republican government of Cambodia that was formally declared on 9 October 1970.
After Lon Nol’s appointment as head of state, Cambodia became substantially more involved in the Second Indochina War. Arguably behind the initial protests in Phnom Penh, Lon Nol began with issuing an ultimatum to forces allied to North Vietnam to leave Cambodia. Turning Cambodia’s back on North Vietnam in this way led to the People’s Army of Vietnam (PAVN) invasion of Cambodia in April 1970, at the request of Khmer Rouge leader Nuon Chea. This then provoked Lon Nol into military action and began what is considered the start of the Cambodian Civil War. Lon Nol’s US-backed administration went to war with the Khmer Rouge backed by North Vietnam. Thousands of Vietnamese were killed by Lon Nol’s anti-Communist troops and their bodies dumped in the Mekong River. Approximately 300,000 Vietnamese fled the country or were forcibly repatriated.