He initially refused to commit to the plan; to convince him, Sirik Matak – who appears to have had a coup in mind from the start – played him a tape-recorded press conference from Paris, in which Sihanouk threatened to execute them both on his return to Phnom Penh.

When Haig told him that American ground forces would not be used to assist the Cambodian army, and that (in accordance with the Nixon Doctrine) a programme of aid would be given instead, Lon Nol openly wept.

Sihanouk, in the meantime, had formed GRUNK, a Beijing-based government-in-exile incorporating the communists and dedicated to the Republic's overthrow; he declared Lon Nol to be a "complete idiot" and characterised Sirik Matak as "nasty, perfidious, a lousy bastard".

Reorganising as the Khmer National Armed Forces (FANK), the republican military had grown to around 150,000 men as early as the end of 1970, mainly through voluntary enlistment as Lon Nol sought to capitalise on a wave of anti-Vietnamese sentiment.

The Joint Chiefs insisted on massive expansion of the FANK to over 200,000 men, despite concerns at the severe negative effect this would have on Cambodia's economy, while the Military Equipment Delivery Team, led by General Theodore C. Mataxis, demanded the 'Americanisation' of the army's French-influenced internal structures, in spite of the chaos this caused in the supply chain.

Almost from the start, the Republic was plagued by many of the same political divisions and infighting that had marked Sihanouk's regime; primary among these was a damaging power struggle between Lon Nol and Sirik Matak.

Sirik Matak had been acting Prime Minister during the Republic's first year, when Lon Nol's health had been extremely poor, but had engendered considerable resentment due to his administrative style and royal connections; there was also growing frustration amongst young, urban Cambodians at the continued corruption and inefficiency of the regime.

[19] Lon Nol used the crisis to oust the Head of State, Cheng Heng, and took over the role himself, appointing the veteran anti-Sihanouk nationalist Son Ngoc Thanh as Prime Minister.

[23] Thanh – whose last political act was to ban Sirik Matak's newspaper – was then forced to resign, going back into South Vietnamese exile, and was replaced by the moderate leftist Hang Thun Hak.

[24] While the Khmer Republic's government was being weakened by infighting, North Vietnamese forces – who had previously carried out much of the fighting against the FANK, as in Operation Chenla I and II – gradually and deliberately scaled back their presence within Cambodian borders, leaving mainly logistical and support staff.

The Paris Peace Accords of early 1973 seemed to offer a temporary respite from the civil war; Lon Nol declared a unilateral ceasefire, despite the FANK's very weak position on the ground.

[25] Accordingly, on 24 April, Lon Nol announced that the National Assembly would be suspended, and that a Political Council formed of himself, Sirik Matak, Cheng Heng, and In Tam, would effectively rule by decree.

Communist forces came within shelling distance of Phnom Penh, and captured the former royal capital of Oudong in March: they 'evacuated' its population – shooting government officials and teachers – and destroyed or burnt much of the town.

The Khmer Rouge had by this point surrounded the capital, whose population had been vastly increased by refugees from the fighting; Lon Nol, who was extremely superstitious, ordered that consecrated sand be spread around the city from helicopters to protect it.

A plan proposed by Étienne Manac'h, the French Ambassador to China, in which Sihanouk would return to Cambodia as the head of a national unity government (leading to the likely immediate defection of a large proportion of the Khmer Rouge's peasant soldiers), failed to materialise.

While Sirik Matak, Long Boret, Lon Non and several other politicians remained in the capital in an attempt to negotiate a ceasefire, Phnom Penh fell to the Khmer Rouge on 17 April.