The struggle was exacerbated by the influence and actions of the allies of the two warring sides. North Vietnamese Army involvement was designed to protect its Base Areas and sanctuaries in eastern Cambodia, without which the prosecution of its military effort in South Vietnam would have been more difficult. The U.S. was motivated by the need to buy time for its withdrawal from Southeast Asia and to protect its ally, South Vietnam. American, South and North Vietnamese forces directly participated (at one time or another) in the fighting. The central government was mainly assisted by the application of massive U.S. aerial bombing campaigns and direct material and financial aid.
After five years of savage fighting that brought about massive casualties, the destruction of the economy, the starvation of the population, and grievous atrocities, the Republican government was defeated on 17 April 1975 when the victorious Khmer Rouge proclaimed the establishment of Democratic Kampuchea. This conflict, although an indigenous civil war, was considered to be part of the larger Vietnam War (1960–1975) that also consumed the neighboring Kingdom of Laos, South Vietnam, and North Vietnam.
By the late 1960s, Sihanouk's delicate domestic and foreign policy balancing act was beginning to go awry. In 1966, an agreement was struck between the prince and the Chinese, allowing the presence of large-scale People's Army of Vietnam (PAVN) and NLF troop deployments and logistical bases in the eastern border regions. He had also agreed to allow the use of the port of Sihanoukville by communist-flagged vessels delivering supplies and materiel to support the PAVN/NLF military effort in Vietnam. These concessions made a sham of Cambodia's neutrality, which had been guaranteed by the Geneva Conference of 1954.
Sihanouk was convinced that the PRC, not the U.S., would eventually control the Indochinese Peninsula and that "our interests are best served by dealing with the camp that one day will dominate the whole of Asia – and coming to terms before its victory – in order to obtain the best terms possible.
During the same year, however, he allowed his pro-American minister of defense, General Lon Nol, to crack down on leftist activities, crushing the Prachea Chon by accusing its members of subversion and subservience to Hanoi. Simultaneously, Sihanouk lost the support of Cambodia's conservatives as a result of his failure to come to grips with the deteriorating economic situation (exacerbated by the loss of rice exports, most of which went to the PAVN/NLF) and with the growing communist military presence.
On 11 September, Cambodia held its first open election. Through manipulation and harassment (and to Sihanouk's surprise) the conservatives won 75 percent of the seats in the National Assembly. Lon Nol was chosen by the right as prime minister and, as his deputy, they named Sirik Matak, an ultraconservative member of the Sisowath branch of the royal clan and long-time enemy of Sihanouk. In addition to these developments and the clash of interests among Phnom Penh's politicized elite, social tensions created a favorable environment for the growth of a domestic communist insurgency in the rural areas.
Simultaneously, Sihanouk ordered the arrest of Chinese middlemen involved in the illegal rice trade, thereby raising government revenues and placating the conservatives. Lon Nol was forced to resign, and, in a typical move, the prince named new leftists to the government to balance the conservatives. The immediate crisis had passed, but it engendered two tragic consequences. First, it drove thousands of new recruits into the arms of the hard-line maquis of the Cambodian Communist Party (which Sihanouk labelled the Khmer Rouge or "Red Khmers"). Second, for the peasantry, the name of Lon Nol became associated with ruthless repression throughout Cambodia.
On 17 January 1968, the Khmer Rouge launched their first offensive. It was aimed more at gathering weapons and spreading propaganda than in seizing territory since, at that time, the adherents of the insurgency numbered no more than 4–5,000. During the same month, the communists established the Revolutionary Army of Kampuchea as the military wing of the party. As early as the end of the Battambang revolt, Sihanouk had begun to reevaluate his relationship with the communists. His earlier agreement with the Chinese had availed him nothing. They had not only failed to restrain the North Vietnamese, but they had actually involved themselves (through the Khmer Rouge) in active subversion within his country. At the suggestion of Lon Nol (who had returned to the cabinet as defense minister in November, 1968) and other conservative politicians, on 11 May 1969, the prince welcomed the restoration of normal diplomatic relations with the U.S. and created a new Government of National Salvation with Lon Nol as his prime minister. He did so "in order to play a new card, since the Asian communists are already attacking us before the end of the Vietnam War. Besides, PAVN and the NLF would made very convenient scapegoats for Cambodia's ills, much more so than the minuscule Khmer Rouge, and ridding Cambodia of their presence would solve many problems simultaneously. The Americans took advantage of this same opportunity to solve some of their own problems in Southeast Asia.
Sihanouk, hearing of the turmoil, headed for Moscow and Beijing in order to demand that the patrons of PAVN and the NLF exert more control over their clients. On 18 March 1970, Lon Nol requested that the National Assembly vote on the future of the prince's leadership of the nation. Sihanouk was ousted from power by a vote of 92-0. Heng Cheng became president of the National Assembly, while Prime Minister Lon Nol was granted emergency powers. Sirik Matak retained his post as deputy prime minister. The new government emphasized that the transfer of power had been totally legal and constitutional, and it received the recognition of most foreign governments. There have been, and continue to be, accusations that the U.S. government played some role in the overthrow of Sihanouk, but conclusive evidence has never been found to support them.
The majority of middle-class and educated Khmers had grown weary of the prince and welcomed the change of government. They were joined by the military, for whom the prospect of the return of American military and financial aid was a cause for celebration. Within days of his deposition, Sihanouk, now in Beijing, broadcast an appeal to the people to resist the usurpers. Demonstrations and riots occurred (mainly in areas contiguous to PAVN/NLF controlled areas), but no nationwide groundswell threatened the government. In one incident at Kompong Cham on 29 March, however, an enraged crowd killed Lon Nol's brother, Lon Nil, tore out his liver, and cooked and ate it. An estimated 40,000 peasants then began to march on the capital to demand Sihanouk's reinstatement. They were dispersed, with many casualties, by contingents of the armed forces.
Lon Nol hoped to use the Vietnamese as hostages against PAVN/NLF activities, and the military set about rounding them up into detention camps. That was when the killing began. In towns and villages all over Cambodia, soldiers and civilians sought out their Vietnamese neighbors in order to murder them. On 15 April, the bodies of 800 Vietnamese floated down the Mekong River and into South Vietnam.
The South Vietnamese, North Vietnamese, and the NLF harshly denounced these horrendous actions. Significantly, no Cambodians – not even those of the Buddhist community – condemned the killings. In his apology to the Saigon government, Lon Nol stated that
"it was difficult to distinguish between Vietnamese citizens who were Viet Cong and those who were not. So it is quite normal that the reaction of Cambodian troops, who feel themselves betrayed, is difficult to control
The prince then allied himself with the Khmer Rouge, the North Vietnamese, the Laotian Pathet Lao, and the NLF, throwing his personal prestige behind the communists. On 5 May, the actual establishment of FUNK and of the Government Royal d'Union Nationale du Kampuchea or GRUNK (Royal Government of National Union of Kampuchea), was proclaimed. Sihanouk assumed the post of head of state, appointing Penn Nouth, one of his most loyal supporters, as prime minister.
Khieu Samphan was designated deputy prime minister, minister of defense, and commander in chief of the GRUNK armed forces (though actual military operations were directed by Pol Pot). Hu Nim became minister of information, and Hou Yuon assumed multiple responsibilities as minister of the interior, communal reforms, and cooperatives. GRUNK claimed that it was not a government-in-exile since Khieu Samphan and the insurgents remained inside Cambodia. Sihanouk and his loyalists remained in China, although the prince did make a visit to the "liberated areas" of Cambodia, including Angkor Wat, in March 1973. These visits were used mainly for propaganda purposes and had no real influence on political affairs.
For Sihanouk, this proved to be a short-sighted marriage of convenience that was spurred on by his thirst for revenge against those who had betrayed him. For the Khmer Rouge, it was a means to greatly expand the appeal of their movement. Peasants, motivated by loyalty to the monarchy, gradually rallied to the FUNK cause. The personal appeal of Sihanouk, the overall better behavior of the communist troops, and widespread allied aerial bombardment facilitated recruitment. This task was made even easier for the communists after 9 October 1970, when Lon Nol abolished the loosely federalist monarchy and proclaimed the establishment of a centralized Khmer Republic.
As combat operations quickly revealed, the two sides were badly mismatched. Government troops, were now renamed the Forces Armees Nationales Khemeres or FANK (Khmer National Armed Forces) and thousands of young urban Cambodians flocked to join it in the months following the removal of Sihanouk. With the surge of recruits, however, FANK expanded well beyond its capacity to absorb the new men. Later, given the press of tactical operations and the need to replace combat casualties, there was insufficient time to impart needed skills to individuals or to units, and lack of training remained the bane of FANK's existence until its collapse.
During the period 1974–1975, FANK forces officially grew from 100,000 to approximately 250,000 men, but probably only numbered around 180,000 due to payroll padding by their officers and due to desertions. U.S. military aid (ammunition, supplies, and equipment) was funneled to FANK through the Military Equipment Delivery Team, Cambodia (MEDTC). Authorized a total of 113 officers and men, the team arrived in Phnom Penh in 1971. The attitude of the Nixon administration could be summed up by the advice given by Henry Kissinger to the first head of the liaison team, Colonel Jonathan Ladd: "Don't think of victory; just keep it alive.
There were other problems. The officer corps of FANK was generally corrupt and greedy. The inclusion of "ghost" soldiers allowed massive payroll padding; ration allowances were kept by the officers while their men starved; and the sale of arms and ammunition on the black market (or to the enemy) was commonplace. Worse, the tactical ineptitude among FANK officers was as common as their greed. Lon Nol frequently bypassed the general staff and directed operations down to battalion-level while also forbidding any real coordination between the army, navy, and air force.
The common soldiers fought bravely at first, but they were saddled with low pay (with which they had to purchase their own food and medical care), ammunition shortages, and mixed equipment. Due to the pay system, there were no allotments for their families, who were, therefore, forced to follow their husbands/sons into the battle zones. These problems (exacerbated by continuously declining morale) only increased over time.
At the beginning of 1974, the Cambodian army inventory included 241,630 rifles, 7,079 machine guns, 2,726 mortars, 20,481 grenade launchers, 304 recoilless rifles, 289 howitzers, 202 APCs, and 4,316 trucks. The Khmer navy had 171 vessels; the Khmer air force had 211 aircraft, including 64 North American T-28s, 14 Douglas AC-47 gunships and 44 helicopters. American embassy military personnel – who were only supposed to coordinate the arms aid program – sometimes found themselves involved in prohibited advisory and combat tasks.
Initially arrayed against an armed force of such limited capability was arguably the best light infantry army in the world at the time – the People's Army of Vietnam. When their forces were supplanted, it was by the tough, rigidly indoctrinated peasant army of the Khmer Rouge with its core of seasoned leaders, who now received the full support of Hanoi. Khmer Rouge forces, which had been reorganized at an Indochinese summit held in Conghua, China in April 1970, would grow from 12–15,000 in 1970 to 35–40,000 by 1972, when the so-called "Khmerization" of the conflict took place and combat operations against the Republic were handed over completely to the insurgents.
The development of these forces took place in three stages. 1970 to 1972 was a period of organization and recruitment, during which Khmer Rouge units served as auxiliaries to PAVN. From 1972 to mid-1974, the insurgents formed units of battalion and regimental size. It was during this period that the Khmer Rouge began to break away from Sihanouk and his supporters and the collectivization of agriculture was begun in the liberated areas. Division-sized units were being fielded by 1974–1975, when the party was on its own and began the radical transformation of the country.
With the fall of Sihanouk, Hanoi became alarmed at the prospect of a pro-Western regime that might allow the Americans to establish a military presence on their western flank. To prevent that from happening, they began transferring their military installations away from the border regions to locations deeper within Cambodian territory. A new command center was established at the city of Kratié and the timing of the move was propitious. President Nixon was of the opinion that:
"We need a bold move in Cambodia to show that we stand with Lon Nol...something symbolic...for the only Cambodian regime that had the guts to take a pro-Western and pro-American stand."
Extensive logistical installations and large amounts of supplies were found and destroyed, but as reporting from the American command in Saigon disclosed, still larger amounts of material had already been moved deeper into the countryside. According to Republican General Sak Sutsakhan, the withdrawal of U.S. forces, after only a 30-day campaign, created "a void so great on the allied side that neither the Cambodian nor the South Vietnamese armies were ever able to fill it.
On the day the incursion was launched, the North Vietnamese reacted by launching an offensive (Campaign X) of its own against FANK forces in order to protect and expand their Base Areas and logistical system. By June, three months after the removal of Sihanouk, they had swept government forces from the entire northeastern third of the country. After defeating those forces, the North Vietnamese turned the newly won territories over to the local insurgents. The Khmer Rouge also established liberated areas in the south and the southwestern parts of the country, where they operated independently of the North Vietnamese.
During the night of 21 January 1971, a force of 100 PAVN/NLF commandos attacked Pochentong airfield, the main base of the Republican Air Force. In this one action, the raiders destroyed almost the entire inventory of government aircraft, including all of its fighter planes. This may have been a blessing in disguise, however, since the air force was composed of old (even obsolete) Soviet aircraft. The Americans soon replaced the airplanes with more advanced models. The attack did, however, stall a proposed FANK offensive. Two weeks later, Lon Nol suffered a stroke and was evacuated to Hawaii for treatment. It had been a mild stroke, however, and the general recovered quickly, returning to Cambodia after only two months.
It was not until 20 August that FANK launched Operation Chenla II, its first offensive of the year. The objective of the campaign was to clear Route 6 of enemy forces and thereby reopen communications with Kompong Thom, the Republic's second largest city, which had been isolated from the capital for more than a year. The operation was initially successful, and the city was relieved. The PAVN and Khmer Rouge counterattacked in November and December, annihilating government forces in the process. There was never an accurate count of the losses, but the estimate was "on the order of ten battalions of personnel and equipment lost plus the equipment of an additional ten battalions. The strategic result of the failure of Chenla II was that the offensive initiative passed completely into the hands of PAVN and the Khmer Rouge.
The main U.S. contribution to the FANK effort came in the form of the bombers and tactical aircraft of the U.S. Air Force. When President Nixon launched the incursion in 1970, American and South Vietnamese troops operated under an umbrella of air cover that was designated Operation Freedom Deal. When those troops were withdrawn, the air operation continued, ostensibly to interdict PAVN/NLF troop movements and logistics. In reality (and unknown to the American Congress and public), they were utilized to provide tactical air support to FANK. As a former U.S. military officer in Phnom Penh reported, "the areas around the Mekong River were so full of bomb craters from B-52 strikes that, by 1973, they looked like the valleys of the moon.
On 10 March 1972, just before the newly-renamed Constituent Assembly was to approve a revised constitution, Lon Nol announced that he was suspending the deliberations. He then forced Cheng Heng, the chief of state since Sihanouk's deposition, to surrender his authority to him. On the second anniversary of the coup, Lon Nol relinquished his authority as chief of state, but retained his position as prime minister and defense minister.
On 4 June, Lon Nol was elected as the first president of the Khmer Republic in a blatantly rigged election. As per the new constitution (ratified on 30 April), political parties formed in the new nation, quickly becoming a source of political factionalism. General Sutsakhan stated: "the seeds of democratization, which had been thrown into the wind with such goodwill by the Khmer leaders, returned for the Khmer Republic nothing but a poor harvest.
In January 1973, hope sprang into the breasts of the Republic's government, army, and population when the Paris Peace Accord was signed, ending the conflict (for the time being) in South Vietnam and Laos. On 29 January, Lon Nol proclaimed a unilateral cease-fire throughout the nation. All U.S. bombing operations were halted in hopes of securing a chance for peace. It was not to be. The Khmer Rouge simply ignored the proclamation and carried on fighting. By March, heavy casualties, desertions, and low recruitment had forced Lon Nol to introduce conscription and, in April, insurgent forces launched an offensive that pushed into the suburbs of the capital. The U.S. Air Force responded by launching an intense bombing operation that forced the communists back into the countryside after being decimated by the air strikes.
By the last day of Operation Freedom Deal (15 August 1973), 250,000 tons of bombs had been dropped on the Khmer Republic, 82,000 tons of which had been released in the last 45 days of the operation. Since the inception of Operation Menu in 1969, the U.S. Air Force had dropped 539,129 tons of ordnance on Cambodia/Khmer Republic.
Reports of the brutal policies of the organization soon made their way to Phnom Penh and into the population foretelling a violent madness that was about to consume the nation. There were tales of the forced relocations of entire villages, of the summary execution of any who disobeyed or even asked questions, the forbidding of religious practices, of monks who were defrocked or murdered, and where traditional sexual and marital habits were foresworn. War was one thing, the offhand manner in which the Khmer Rouge dealt out death, so contrary to the Khmer character, was quite another. Reports of these atrocities began to surface during the same period in which North Vietnamese troops were withdrawing from the Cambodian battlefields. This was no coincidence. The concentration of the PAVN effort on South Vietnam allowed the Khmer Rouge to apply their doctrine and policies without restraint for the first time.
The Khmer Rouge leadership was almost completely unknown by the public. They were referred to by their fellow countrymen as peap prey – the forest army. Previously, the very existence of the communist party as a component of GRUNK had been hidden. Within the "liberated zones" it was simply referred to as "Angka" – the organization. During 1973, the communist party fell under the control of its most fanatical members, Pol Pot and Son Sen, who believed that "Cambodia was to go through a total social revolution and that everything that had preceded it was anathema and must be destroyed."
Also hidden from scrutiny was the growing antagonism between the Khmer Rouge and their North Vietnamese allies. The radical leadership of the party could never escape the suspicion that Hanoi had designs on building an Indochinese federation with the North Vietnamese as its master. The Khmer Rouge were ideologically tied to the Chinese, while North Vietnam's chief supporters, the Soviet Union, still recognized the Lon Nol government as legitimate. After the signing of the Paris Peace Accords, PAVN cut off the supply of arms to the Khmer Rouge, hoping to force them into a cease-fire. When the Americans were freed by the signing of the accords to turn their air power completely on the Khmer Rouge, this too was blamed on Hanoi. During the year, these suspicions and attitudes led the party leadership to carry out purges within their ranks. Most of the Hanoi-trained members were then executed on the orders of Pol Pot.
As time passed, the need of the Khmer Rouge for the sinecure of Prince Sihanouk lessened. The organization demonstrated to the people of the liberated areas in no uncertain terms that open expressions of support for Sihanouk would result in their liquidation. Although the prince still enjoyed the protection of the Chinese, when he made public appearances overseas to publicize the GRUNK cause, he was treated with almost open contempt by Ministers Ieng Sary and Khieu Samphan. In June, the prince told Italian journalist Oriana Fallaci that when "they [the Khmer Rouge] have sucked me dry, they will spit me out like a cherry stone.
By the end of 1973, Sihanouk loyalists had been purged from all of GRUNK's ministries and all of the prince's supporters within the insurgent ranks were also eliminated. Shortly after Christmas, as the insurgents were gearing up for their final offensive, Sihanouk spoke with the French diplomat Etienne Manac'h. He said that his hopes for a moderate socialism akin to Yugoslavia's must now be totally dismissed. Stalinist Albania he said, would be the model.
Phnom Penh, which had a pre-war population of around 600,000 was overwhelmed by refugees (who continued to flood in from the steadily collapsing defense perimeter), growing to a size of around two million. These helpless and desperate civilians had no jobs and little in the way of food, shelter, or medical care. Their condition (and the government's) only worsened when Khmer Rouge forces gradually gained control of the banks of the Mekong. From the riverbanks, their mines and gunfire steadily reduced the river convoys bringing relief supplies of food, fuel, and ammunition to the slowly starving city (90 percent of the Republic's supplies moved by means of the convoys) from South Vietnam. After the river was effectively blocked in early February, the U.S. began an airlift of supplies. This became increasingly risky, however, due to communist rocket and artillery fire, which constantly rained down on the airfields and city.
Desperate, yet determined, units of Republican soldiers, many of whom had run out of ammunition, dug in around the capital and fought until they were overrun as the Khmer Rouge advanced. By the last week of March 1975, approximately 40,000 communist troops had surrounded the capital and began preparing to deliver the coup de grace to about half as many Republican forces.
Lon Nol resigned and left the country on 1 April, hoping that a negotiated settlement might still be possible if he was absent from the political scene. Saukam Khoy became acting president of a government that had less than three weeks to live. Last-minute efforts on the part of the U.S. to arrange a peace agreement involving Sihanouk ended in failure. When a vote in the U.S. Congress for a resumption of American air support failed, panic and a sense of doom pervaded the capital. The situation was best described by General Sak Sutsakhan (now FANK chief of staff):
"The picture of the Khmer Republic which came to mind at that time was one of a sick man who survived only by outside means and that, in its condition, the administration of medication, however efficient it might be, was probably of no further value.
On 12 April, concluding that all was lost (and without notifying the Khmer government), the U.S. evacuated its embassy personnel by helicopter during Operation Eagle Pull. The 276 evacuees included U.S. Ambassador John Gunther Dean, other American diplomatic personnel, the acting president of Cambodia, senior Khmer Republic government officials and their families, and members of the news media. In all, 82 U.S., 159 Cambodian, and 35 third-country nationals were evacuated. Although invited by Ambassador Dean to join the evacuation (and much to the American's surprise), Sirik Matak, Long Boret, Lon Non (Lon Nol's brother), and most members of Lon Nol's cabinet declined the offer. All of them chose to share the fate of their people and were later executed by the Khmer Rouge.
After the Americans (and the president) had departed, a seven-member Supreme Committee, headed by General Sak Sutsakhan, assumed authority over the collapsing Republic. By 15 April, the last solid defenses of the city were overcome by the communists. In the early morning hours of 17 April, the committee decided to move the seat of government to Oddar Meanchay Province in the northwest. Around 10:00, the voice of General Mey Si Chan of the FANK general staff broadcast on the radio, ordering all FANK forces to cease firing, since "negotiations were in progress" for the surrender of Phnom Penh. The war was over but the terrible dreams of the Khmer Rouge were about to come to fruition in the newly-proclaimed Democratic Kampuchea. Khmer Rouge troops immediately began to forcibly empty the capital city, driving the population into the countryside and killing thousands in the process. The Year Zero had begun.