Emergency powers were subsequently invoked by the Prime Minister Lon Nol, who became effective head of state, and led ultimately to the removal of Queen Sisowath Kossamak and the proclamation of the Khmer Republic later that year.

[4] Following King Norodom Suramarit's death in 1960, Sihanouk had forced the National Assembly to approve a constitutional amendment that made him Chief of State with no fixed term of office, while Queen Sisowath Kossamak remained a mere ceremonial figure.

[5] In the Cambodian elections of 1966, the usual Sangkum policy of having one candidate in each electoral district was abandoned; there was a huge swing to the right, especially as left-wing deputies had to compete directly with members of the traditional elite, who were able to use their local influence.

In particular, the nationalist and anti-communist sensibilities of Lon Nol and his associates meant that Sihanouk's policy of semi-toleration of Viet Cong and People's Army of Vietnam (PAVN) activity within Cambodian borders was unacceptable; Sihanouk, during his swing to the left in 1963–66, had negotiated a secret arrangement with Hanoi whereby in return for the guaranteed purchase of rice at inflated prices, the port of Sihanoukville was opened for weapons shipments to the Viet Cong.

Some reports indicate Sihanouk's involvement in the preparations, or acquiescence, of the demonstrations, in the hope that they would lead Moscow and Beijing to pressure North Vietnam to reduce its presence in Cambodia.

[11] Lon Nol closed the port of Sihanoukville to the North Vietnamese and issued an impossible ultimatum to them: all PAVN and Viet Cong forces were to withdraw from Cambodian soil within 72 hours (on 15 March) or face military action.

[12][13] When, by the morning of 16 March, it was clear that this demand had not been met, some 30,000 youths gathered outside the National Assembly in Phnom Penh to protest against the Vietnamese presence.

On the same day, the Cambodian Secretary of State for Defence, Colonel Oum Mannorine (Sihanouk's brother-in-law), was scheduled to be questioned by the national legislature on allegations of corruption.

A group of Mannorine's men, under the command of Phnom Penh's Chief of Police Major Buor Horl, attempted to arrest the plotters, but it was too late.

[16] The Prime Minister remained uncertain, with the result that Sirik Matak, accompanied by three army officers, compelled a weeping Lon Nol to sign the necessary documents at gunpoint.

"[24] Kenton Clymer summarizes: "In sum, although the precise degree of American involvement remains murky, most scholars have concluded that, at the very least, some military intelligence agents are culpable.

[21] Samuel R. Thornton, a U.S. Navy Intelligence specialist, told journalist Seymour Hersh that he "gained intimate knowledge of coup preparations as early as late 1968.

"[21] According to Thornton, the highest levels of the U.S. government approved a plot "to insert a US-trained assassination team disguised as Viet Cong insurgents into Phnom Penh to kill Prince Sihanouk as a pretext for revolution."

However, per Thornton, Lon Nol rejected the assassination plot as "criminal insanity" but agreed to the insertion of U.S.-trained forces and U.S. military support in an event of a coup.

"[25] Similarly, Forrest B. Lindley, a U.S. Army Special Forces captain operating near the Cambodian border, stated that "I was told there would be a change of government in Cambodia.

[30] These events marked the start of the Cambodian Civil War, pitting Lon Nol's regime backed by US air power against the Khmer Rouge and North Vietnam.