United States Invades Cambodia

An abortive effort to hasten the end of U.S. involvement in the Vietnam War violated international law, met with outrage and protests in the United States, and plunged Cambodia into two decades of civil war.

Summary of Event

In 1968, when Richard M. Nixon was voted into office on the basis of his promise to bring peace to Vietnam, Cambodia was at peace. Its ruler, Prince Norodom Sihanouk, had successfully maneuvered to keep his country separate from the Vietnam War by allowing the North Vietnamese to use border provinces both as sanctuaries and to channel supplies destined for South Vietnam through Cambodian territory. For Sihanouk, the decision to aid the Vietnamese communists in this manner was one of expediency rather than sympathy. In his eyes, the choices were few; he must either help the communists or accept “American imperialism.” Cambodia;U.S. invasion of
Vietnam War (1959-1975);Cambodia
[kw]United States Invades Cambodia (Apr. 29, 1970)
[kw]Cambodia, United States Invades (Apr. 29, 1970)
Cambodia;U.S. invasion of
Vietnam War (1959-1975);Cambodia
[g]Southeast Asia;Apr. 29, 1970: United States Invades Cambodia[10810]
[g]Cambodia;Apr. 29, 1970: United States Invades Cambodia[10810]
[c]Wars, uprisings, and civil unrest;Apr. 29, 1970: United States Invades Cambodia[10810]
[c]Vietnam War;Apr. 29, 1970: United States Invades Cambodia[10810]
Abrams, Creighton Williams, Jr.
Kissinger, Henry
Lon Nol
Nixon, Richard M.
[p]Nixon, Richard M.;Vietnam War
Sihanouk, Norodom

According to the Nixon doctrine, in the future the United States would provide material support to troops of countries resisting communist aggression but refrain from sending U.S. personnel to the battlefield. The key to Nixon’s plan for ending the war was “Vietnamization,” Vietnamization a program—based on the recently announced Nixon Doctrine Nixon Doctrine —calling for the gradual extrication of U.S. troops and their replacement by Vietnamese. In essence, it was a solution to the U.S. problem of disengaging from the war rather than a solution to the war. In the same way, the prospect of invading Cambodia was viewed only as a means to ease disengagement. That it would actually widen the war and introduce a previously neutral country to the conflict were possibilities that remained secondary considerations.

In April, 1964, U.S. planes, flying from bases in Thailand, strafed two Cambodian villages. Sihanouk soon severed diplomatic relations with the United States. Subsequent border forays by the South Vietnamese army into Cambodia, coordinated with U.S. military advisers, also had little effect on stopping the flow of support from North Vietnam to South Vietnam.

The U.S. military leadership had, for some time, sought permission to invade Cambodia. President Nixon’s immediate predecessor, Lyndon B. Johnson Johnson, Lyndon B.
[p]Johnson, Lyndon B.;Vietnam War , had rejected several requests on the grounds that the impact of such an invasion on the course of the war would be negligible. In February of 1969, however, less than a month after Nixon assumed office, General Creighton W. Abrams, commander of Military Assistance Command, Vietnam Military Assistance Command, Vietnam (MACV), requested that B-52 bombers be used against sanctuaries and supply routes. Nixon, in concurrence with his national security adviser, Henry Kissinger, agreed, and in March, 1969, the bombing of Cambodia Cambodia;U.S. bombing of began.

As the U.S. Constitution Constitution, U.S.;separation of powers specifically holds that only Congress can declare war, this act to widen the war to include a separate sovereign nation may well have been illegal. To prevent the issue of legality from arising, however, Nixon ordered that the bombing be kept secret. To prevent a news leak, he even bypassed the ordinary military chain of command, failing to notify the Pentagon. The domestic outcry over the bombing forced him to order a halt. In this manner, without the knowledge of Congress or the American people, Cambodia was introduced to the war one year prior to the U.S. invasion.

President Richard M. Nixon explains his decision to invade Cambodia to the American people at a press conference on April 30, 1970.

(National Archives)

Although the bombing in itself achieved limited success in interdicting North Vietnamese supply routes and storage areas, it killed more Cambodians than North Vietnamese, had a significant impact on the Cambodian political situation, and was primarily responsible for initiating a series of events that would affect Cambodia’s future for years. First, it pushed the communists out of the border sanctuary areas and deeper into Cambodia. This irritated rightist elements in Sihanouk’s government who, already dissatisfied with his permissiveness in allowing Vietnamese communists access to Cambodian territory, became even more so as they witnessed the communists usurp still more.

Sihanouk, aware of the discord, took measures to allay it. He reopened diplomatic relations with the United States. He informed Washington that he would not object to some attacks on Vietnamese sanctuaries inside Cambodia, but he never agreed to indiscriminate bombing. By not protesting the B-52 raids, which he strongly opposed, he felt he was making a significant concession to these same rightist elements who supported them. Fearing eventual annexation by Vietnam, rightist General Lon Nol, Cambodian armed forces commander, ordered all Vietnamese to leave the country, and anti-Vietnamese demonstrations were organized in Phnom Penh and the provinces along the Vietnamese border.

In March, 1970, as tensions continued to mount within his government, Sihanouk departed Phnom Penh on a diplomatic mission to Moscow and Peking. Again, motivated by the need to settle the unrest among his ministers, he intended to urge both governments to restrain the North Vietnamese from encroaching further into Cambodian territory. However, he had failed to assess accurately how far the crisis in his capital had actually advanced. While still in Moscow, he learned he had been deposed Revolutions and coups;Cambodia
Cambodian coup of 1970 by his pro-U.S. defense minister, Lon Nol. Although there is no evidence that the United States or any other foreign power promoted the coup, it precipitated crucial policy changes on both sides of the Vietnam War.

The struggling Cambodian communist movement Khmer Rouge Khmer Rouge , which previously had been judged by Hanoi to be too small to be effective, was suddenly thrust by Sihanouk’s downfall into a position from which it could make a serious attempt at gaining power. As a result, Vietnamese assistance increased dramatically, and the Khmer Rouge received the support it needed eventually to achieve power. For those among the U.S. leadership who supported an invasion plan, Sihanouk’s downfall was a fortuitous event, since he alone among Cambodia’s leaders had remained strongly opposed. With his removal, all Cambodian opposition to an invasion attempt ended.

On April 29-30, 1970, an invasion was mounted with thirty thousand U.S. and South Vietnamese troops crossing into Cambodia. Secrecy had so pervaded the operation’s planning that no one in Cambodia, including the United States mission and least of all Lon Nol, learned of it until after it occurred. Although Nixon spoke of the invasion as a decisive victory, the military regarded it as having attained a temporary advantage at best. While uncovering enormous stores of supplies, it encountered few enemy troops. In effect, military planners had failed to take into account the communists’ move westward under the impact of the bombing. Thus, while temporarily disrupting the communists’ logistics, the invasion made little impact on their long-term conduct of the war. Pentagon estimates suggested that North Vietnamese plans for an offensive had been set back by no more than a year; in keeping with this assessment, the North Vietnamese, within two months of the withdrawal of United States invasion forces, had reestablished their supply trails and sanctuaries.


Within the United States, the effect of the invasion was devastating. The antiwar movement reacted with intensified demonstrations and student strikes. The death of four students at Kent State University as the result of a confrontation between National Guardsmen and protesters enraged the nation. The extent of the reaction engendered by the invasion surprised President Nixon. Although he defended his action to the American people, his arguments appeared flimsy and misrepresentative.

Claiming that the United States had for five years respected Cambodian neutrality, Nixon neglected to mention the bombing. Declaring that the invasion was intended to destroy the headquarters for the entire communist military operation in South Vietnam, he ignored overwhelming evidence offered by the military proving that no such target existed. Depicting the invasion as a necessary step taken against the North Vietnamese to preclude the possibility of attack on U.S. troops withdrawing from the war, he hid the fact that during the course of negotiations for peace, the North Vietnamese had already offered to refrain from such attacks once a withdrawal date was determined.

Finally, asserting that his decision was crucial to the maintenance of U.S. prestige abroad, Nixon contradicted evidence indicating a substantial fall in U.S. prestige following the invasion. Both internationally and domestically, the feeling prevailed that the president had succeeded only in expanding an already wearisome war.

For Cambodia, the invasion completed the destruction of a tenuous neutrality already severely damaged for more than a year by the bombing campaign. It precipitated an internal war that had not existed before U.S. forces crossed the border and that subsequently enveloped Cambodia in a prolonged conflict between United States-supported anticommunist forces and Vietnam-supported Khmer Rouge insurgents, thereby subjecting the country to still further devastation and eventual communist rule.

In this way, the fate of Cambodia was decided. U.S. policy makers, interested only in exploiting Cambodia’s territory as an adjunct to the Vietnam War, held the welfare of Cambodians and their land in small regard. President Nixon made this clear when in December, 1970, he stated that the Cambodians were “tying down forty thousand North Vietnamese regulars [in Cambodia and] if those North Vietnamese weren’t in Cambodia they’d be over killing Americans.” The Cambodians were thus reduced to acting as surrogate U.S. targets for North Vietnamese guns. The tragedy of the U.S. invasion was that so much was suffered for so little reason.

Cambodia was but a sideshow for Nixon, who failed to take either the subtleties of Indochina or the domestic antiwar movement into account. Sihanouk’s assessment of policy makers in Washington was, in characteristic hyperbole, that “They demoralized America, they lost all of Indochina to the communists, and they created the Khmer Rouge.” When U.S. and Vietnamese troops withdrew from Cambodia, civil war ensued, resulting in the 1975 seizure of power by the Khmer Rouge, who engaged in a widespread policy of genocide within the country and attacked Vietnam to regain lost territory. Cambodia;U.S. invasion of
Vietnam War (1959-1975);Cambodia

Further Reading

  • Caldwell, Malcolm, and Lek Tan. Cambodia in the Southeast Asian War. New York: Monthly Review Press, 1973. A review of contemporary Cambodian history, critical of the U.S. role.
  • Gordon, Bernard K., with Kathryn Young. “The Khmer Republic: That Was the Cambodia That Was.” Asian Survey 2 (January, 1971): 26-40. Prophetically asserts that the U.S. invasion, by bringing Vietnamese into Cambodia, unleashed a Cambodian-Vietnamese enmity that would be difficult to resolve.
  • Grant, Jonathon S., et al. Cambodia: The Widening War in Indochina. New York: Washington Square Press, 1971. Essays by Asian scholars opposed to the war.
  • Kissinger, Henry S. Years of Upheaval. London: Weidenfeld & Nicolson, 1982. Chapter 8, on Indochina, attempts to explain Kissinger’s motives and refute his critics.
  • Shaw, John M. The Cambodian Campaign: The 1970 Offensive and America’s Vietnam War. Lawrence: University Press of Kansas, 2005. Thorough analysis of the U.S. Cambodian campaign, beginning in 1965 with President Johnson’s stewardship of the war and devoting considerable space to strategic and tactical evaluations of the 1970 invasion. Bibliographic references and index.
  • Shawcross, William. Sideshow: Kissinger, Nixon, and the Destruction of Cambodia. Rev. ed. New York: Cooper Square Press, 2002. A thorough account of the way in which U.S. policy upset the delicate balance that held Cambodia together.
  • Sihanouk, Norodom. My War with the CIA. London: Penguin Books, 1973. Blames problems in Cambodia in the 1960’s and 1970’s on the Central Intelligence Agency.

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