On March 10 to 11, 1972 Marshal Lon Nol carried out a self-coup within Cambodia's Khmer Republic, ousting the interim head of state Cheng Heng, assuming the presidency himself, and sidelining his fellow coup-maker Prince Sisowath Sirik Matak, concentrating power after the March 1970 overthrow of Sihanouk. Since the 1970 coup, the republic had been governed by an uneasy leadership including the interim head of state Cheng Heng, Prime Minister Lon Nol, and Deputy Prime Minister Sirik Matak. Lon Nol, whose health and temper had declined after a February 1971 stroke, moved with his ambitious brother Lon Non to eliminate rivals.
After Sirik Matak dismissed a dissident academic, Lon Non organized student demonstrations demanding Sirik Matak's removal; Sirik Matak resigned and was placed under effective house arrest for his protection. Lon Nol used the crisis to remove the head of state Cheng Heng and take the office himself on March 10 to 11, 1972, appointing the veteran nationalist Son Ngoc Thanh as prime minister. Lon Nol soon staged rigged presidential elections and ruled increasingly alone until the Khmer Rouge captured Phnom Penh in April 1975.
Researched Details
Researched
Independent research, added record by record. Hover any label for what it means.
Target identified in Coup Atlas research: Interim head of state Cheng Heng, ousted when Lon Nol assumed the presidency on March 10 to 11, 1972 (with Sirik Matak also forced out and placed under house arrest); no one was physically harmed at this stage.
RESEARCHED NAME
Key figures
Lon Nol (prime minister who took the presidency in the self-coup)
Cheng Heng (interim head of state ousted)
Sirik Matak (deputy prime minister forced out and put under house arrest)
Lon Non (Lon Nol's brother who organized the demonstrations)
Sources
By 18 March 1972 Lon Nol and his brother had driven Sirik Matak from power; after Sirik Matak sacked a dissident academic, Lon Non organised student demonstrations calling for his removal, Sirik Matak resigned and was placed under effective house arrest, and Lon Nol used the crisis to oust the Head of State Cheng Heng and take over the role himself, appointing Son Ngoc Thanh as Prime Minister.
Lon Nol served as President of the Khmer Republic from 10 March 1972, succeeding Cheng Heng, having been Prime Minister until 11 March 1972, with the leadership change marking his assumption of the head-of-state role in the republic.
Lon Nol's rule became increasingly erratic and authoritarian after his February 1971 stroke; he appointed himself Marshal in April 1971, in October 1971 suspended the National Assembly saying he would no longer play the game of democracy in wartime, and backed by his ambitious brother Lon Non succeeded in reducing the influence of Sirik Matak, In Tam, and the other coup leaders, becoming President of the Khmer Republic on 10 March 1972.
Later in 1972 Lon Nol announced presidential elections, which despite an inevitable and rigged victory for him revealed considerable dissatisfaction with the government, as it is likely In Tam would have won a fair vote, and the political situation continued to unravel with opposition parties refusing to contest the September National Assembly elections.
Coding note: the dataset labels this an auto coup because the sitting prime minister Lon Nol overturned the republic's leadership arrangement from within, ousting the interim head of state Cheng Heng and taking the presidency himself while forcing out his rival Sirik Matak; no one was physically harmed at this stage (Sirik Matak was placed under house arrest, ostensibly for protection), matching the dataset fate. Date nuance: sources give Lon Nol's assumption of the presidency as March 10 to 11, 1972 (the dataset's date is March 11); the wider consolidation ran to March 18. Arc link: this connects to the March 18, 1970 coup against Sihanouk (enriched in batch 22) that created the Khmer Republic; the 1972 self-coup concentrated power in Lon Nol. Tragic arc forward: the republic fell to the Khmer Rouge in April 1975, after which Sirik Matak and other officials were executed, a later event.
Cross-Reference
Only one dataset carries it. One of the three reference datasets we check against carries this event; the others do not list it. That is not a contradiction — the datasets cover different periods and use different thresholds — but it means the record has had less outside corroboration.
Identifiers for finding this same event in the other datasets:
Coup Atlas research identifies the target as: Interim head of state Cheng Heng, ousted when Lon Nol assumed the presidency on March 10 to 11, 1972 (with Sirik Matak also forced out and placed under house arrest); no one was physically harmed at this stage..
Baseline coding: Incumbent executive removed from power. Recorded leader fate: deposed; not physically harmed. 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
⚑ This citation establishes the coded baseline record; use the claim-level sources above for the researched narrative.
Baseline Source-Document Entries
Boston Globe
“Lon Nol’s Coup Surprised Closes Associates,” March 12, 1972
South China Morning Post
“Lon Nol Takes Over Top Post,” March 14, 1972
Washington Post, Times Herald
“Takeover by Lon Nol Surprises Cambodians: Lon Nol Takeover Stuns Cambodia, Delays Assembly Role,” March 12, 1972
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 →