On March 18, 1970 Cambodia's National Assembly voted unanimously to remove Prince Norodom Sihanouk as head of state while he was abroad, the coup that installed Prime Minister Lon Nol, abolished the monarchy, created the Khmer Republic, and pushed Cambodia deeper into the Vietnam War and toward the catastrophe of the Khmer Rouge. Sihanouk, who had led Cambodia since independence, had angered nationalist and anti-communist factions by tolerating North Vietnamese and Viet Cong use of Cambodian territory. In early March, while Sihanouk was on a trip to France, the Soviet Union, and China, anti-Vietnamese protests, encouraged by Lon Nol and his ally Prince Sirik Matak, escalated into the sacking of the North Vietnamese embassy.
On March 16 to 17, Sihanouk loyalists including his brother-in-law Oum Mannorine were arrested, and Sirik Matak reportedly compelled a hesitant, weeping Lon Nol to sign the coup documents at gunpoint. On March 18 the army surrounded Phnom Penh and the Assembly invoked the constitution to withdraw confidence in Sihanouk, transferring power to Lon Nol. Sihanouk, in Beijing, aligned with the Khmer Rouge; the war that followed ended in the Khmer Rouge takeover of 1975.
Researched Details
Researched
Independent research, added record by record. Hover any label for what it means.
Target identified in Coup Atlas research: Prince Norodom Sihanouk, head of state, deposed by the National Assembly vote of March 18, 1970 while he was abroad; he did not return and went into exile in China, and was not physically harmed.
RESEARCHED NAME
Key figures
Norodom Sihanouk (head of state deposed while abroad)
Lon Nol (prime minister who became effective head of state)
Sirik Matak (prince and co-architect of the coup)
Cheng Heng (appointed interim head of state)
Sources
The 1970 Cambodian coup was the removal of the Cambodian Chief of State Prince Norodom Sihanouk after a vote in the National Assembly on 18 March 1970, after which emergency powers were invoked by Prime Minister Lon Nol, who became effective head of state, leading to the removal of Queen Sisowath Kossamak and the proclamation of the Khmer Republic, and it is generally seen as a turning point in the Cambodian Civil War.
On 18 March the army took up positions around the capital and the National Assembly voted unanimously to invoke Article 122 of the constitution withdrawing confidence in Sihanouk, after Sirik Matak, accompanied by three army officers, had compelled a weeping Lon Nol to sign the necessary documents at gunpoint, and after Sihanouk loyalists including Oum Mannorine had been placed under arrest.
On the morning of March 18, 1970 the National Assembly was hastily convened and voted unanimously to depose Sihanouk as head of state, Lon Nol was granted emergency powers, and Cheng Heng was appointed interim head of state, while Queen Sisowath Kossamak was forced to leave the royal palace and later went into exile in China.
SOURCE REVIEWEDCountry Studies, Cambodia: The March 1970 Coup d'Etatopen source ↗
While Sihanouk was on a diplomatic trip to France, the Soviet Union, and China, anti-Vietnamese protests erupted, with demonstrators sacking the North Vietnamese and Viet Cong embassies on March 11, and the coup through a parliamentary vote on March 18 brought an end to Sihanouk's reign as Chief of State and set the stage for the Khmer Republic, escalating Cambodia's involvement in the Vietnam War.
SOURCE REVIEWEDWorld History Edu, 1970 Cambodian coup d'etatopen source ↗
Editorial Note
Coding note: the dataset labels this a palace coup, and it was accomplished through a constitutional-form parliamentary vote (Article 122) rather than open military assault, though the army surrounded the capital and Sirik Matak reportedly coerced Lon Nol at gunpoint; Sihanouk, deposed while abroad, went into exile in China, matching the fled the country fate. Significance flagged by sources: this is widely regarded as a turning point that deepened the Cambodian Civil War and ultimately opened the way to the Khmer Rouge takeover of 1975; Sihanouk aligned with the Khmer Rouge in exile. US and South Vietnamese foreknowledge or encouragement is discussed in sources and noted as contested, not asserted.
Cross-Reference
Datasets agree. At least two of the three reference datasets we check against carry this event, and none of them disagrees about its date or its result.
Identifiers for finding this same event in the other datasets:
Coup Atlas research identifies the target as: Prince Norodom Sihanouk, head of state, deposed by the National Assembly vote of March 18, 1970 while he was abroad; he did not return and went into exile in China, and was not physically harmed..
Baseline coding: Incumbent executive removed from power. Recorded leader fate: deposed; fled the country. The baseline file does not name individual targets; researched names are kept in a separate layer.
Baseline Dataset Citation
Cline Center Coup d’État Project Dataset, v2.2.2
Peyton, Buddy, Joseph Bajjalieh, Michael Martin, and Andrea Gerald. 2026. Cline Center Coup d’État Project Dataset. Cline Center for Advanced Social Research. V.2.2.2. February 17. University of Illinois Urbana-Champaign. doi: 10.13012/B2IDB-9651987_V10
“Pragmatic Cambodian Leader : Lon Nol,” March 19, 1970
“Sihanouk Reported Out in a Coup by His Premier; Cambodia Airports Shut,” by Henry Kamm, March 19, 1970
“3 SIHANOUK AIDES SAID NOT TO EXIST Pnompenh Believes Prince Executed Them in ’67,” August 22, 1970
This is the citation trail carried by the baseline dataset, grouped by outlet and rejoined from the line-wrapped original. Coup Atlas has not independently re-checked every document; read the original before citing a specific claim. URLs the import damaged beyond safe repair are shown as plain text and listed in citations-report.md. How this list is built →