By 1963 the war in Vietnam had spilled over into adjacent neighbors Laos and Cambodia. PAVN and NLF forces operating in Vietnam had allied themselves with the national Communist insurgent movements named Pathet Lao and Khmer Rouge respectively, plunging the whole of what was once French Indochina into civil war. North Vietnamese forces operating against ARVN and US positions in Vietnam began a rigorous campaign to expand the Ho Chi Minh trail across the border into these friendly countries in an attempt to avoid the spoiling interdiction sorties levied against them within their own territories, as well as exploiting their supposed neutrality for safe areas for retreat and reorganization; Communist forces which came under attack in Vietnam could safely exfiltrate to Laos and Cambodia, while also enjoying a measure of popular support in the countryside from the Pathet Lao and Khmer Rouge. It was this frustrating situation which inspired Nixon to refer to the United States presence in Vietnam was a "pitiful, helpless giant," as the Communists could attack allied forces while persistently relying upon neutral ground to escape to, the defenders unable to respond effectively. While US and ARVN forces had the might and logistics to pursue and destroy the insurgents, they could not rightfully cross the border into a country which was officially not involved in the war and so were forced to endure the aggression without a proper response. The "Mini-Tet Offensive" of 1969, in which PAVN forces originating from base areas in southern Cambodia attacked the south of Vietnam, would inspire Nixon to change his policy on the neutral countries: designing and implementing a secretive and illegal bombing campaign against the Khmer country.
On the 18th of March, 1969 Operation Menu began, resulting in a systematic bombing of Cambodia by B-52 Stratofortress aircraft. This covert operation, as the bombing sorties in Vietnam proper, caused consternation in the countryside and inspired a government coup and widespread subsequent military crackdown and murder of Vietnamese people living within the country, many of whom had lived within Cambodia for generations and had no connection with insurgent activities. The United States endorsed Lon Nol, who had deposed the popular Prince Sihanouk as head of state, and in the spirit of the Nixon Doctrine immediately began to ferry in materiel, advisors and munitions in an attempt to modernize the Cambodian army and snuff out Khmer Rouge before they could expand. As a consequence of the murderous and systematic bombing of the countryside, which as similar operations had in Vietnam, failed to cause any damage to the Communist infrastructure but did succeed in contributing to a growing mass grave of innocents, Khmer Rouge ranks expanded and the rural landscape soon slipped into Communist dominion. This myopic failing of US policy was cemented by Hanoi's response in the PAVN "Campaign X," which in April succeeded in capturing most of the countryside east and north of Cambodian capitol Phnom Penh, as well as expanding the Ho Chi Minh trail into the interior of the country, laying the groundwork for a final capture of the city. With Communist lines less than thirty miles from the capitol Nixon was stirred to make public his designs, and to lobby to the public for support for a ground invasion.
Publically Nixon claimed that an invasion into southern Cambodia was necessary to capture the headquarters of all Communist forces in Vietnam, an absurd notion which was not supported by either fabricated or real evidence. We might too readily conclude that if there was no evidence of such a headquarters that Nixon, as the impotent presidents before him, might have wished to avoid international humiliation at the prospect of another country falling to Communism due to failed US efforts and antagonism. This notion is equally without merit, as the B-52 bombing campaigns of the prior months were not yet publicly known, and for all intents and purposes, the US had yet to interfere in Cambodia. The claim that not only a major, but the primary Communist headquarters was located across the Cambodian border, may have instead been a propaganda ploy to lead more urgency to the incursion, as Nixon may have judged that simply arguing that the campaign was needed to end cross-border attacks may have deemed not enough to risk escalation of hostilities. A grand Communist headquarters may have been a precursory version of the "smoking gun," "yellow cake" and "nuclear program" accusations levied against supposed enemies of recent years. While all false and without evidence, these claims would serve as compelling enough arguments to seduce the public's support into endorsing a fantasy. In truth there was no Communist headquarters across the border, but there were regions which the PAVN and the Khmer Rouge exploited as organization and staging areas. We must therefore be reluctantly content to establish the need to relieve cross-border attacks as the logical casus belli for the campaign.
Nixon assumed that his incursion into Cambodia would serve as a legitimizing of his Vietnamization doctrine, by demonstrating that a seemingly primary ARVN force was capable of exercising a major military operation. With the relative success of the campaign, the allied forces capturing large PAVN supply caches and scattering the enemy presence in the South East of Cambodia, Nixon further assumed that Vietnamization was ultimately successful and on the way to ensuring victory in the face of increased US withdrawal within Vietnam itself, that a similar client army could be constructed in Cambodia and could be used to repel the Khmer Rouge there; Nixon believed that the Cambodian Campaign was successful in establishing a "decent interval" in which the ARVN forces could now stand against the Communist aggression by themselves in the face of a complete US withdrawal. The Easter Offensive of 1972, in which the ARVN forces had been successful in delaying a massive PAVN offensive without US ground support further emboldened the illusion that the government of Saigon was capable of victory without US support, serving as an ominous swan song, the last US forces leaving Vietnamese soil in the year to come.
The fatal legacy of the Cambodian campaign was in the devastation of Cambodia and the widening of civil war. While Cambodia had been afflicted by a Communist minority prior to the US bombing and ground incursions, the consternation and destruction caused by the campaign, as well as the misbehavior and corruption of another US-backed coup, exacerbated the situation to a dire state, as the countryside Communists, who once did not interfere too eagerly in local politics and instead merely used depopulated land on the border as staging grounds for combat in South Vietnam, were pledged the full support of Hanoi and the PAVN, and so warred against Phnom Penh with abandon. Cambodia had been plunged into war without consent and sacrificed in order to ensure the "decent interval," millions laid on the altar to avoid humiliation. In this manner the Cambodian Campaign can be evaluated as a grubby extension of Vietnamization, simply another attempt at political damage control, expended at the cost of the death of "the other." Rather than become cognizant of past mistakes and adapt strategy and international relations accordingly, Nixon perpetuated the errors of his priors, at a cost too grave to bear.
 Bernard C. Nalty, Air War Over South Vietnam Washington DC: Air Force History and Museums Program, 1997, p. 127-128.
 Wilfred Deac, Road to the Killing Fields: The Cambodian Civil War of 1970-1975. College Station TX: Texas A&M University, 1997, p. 56-57.
 Samuel Lipsman, Edward Doyle, et al. Fighting for Time. Boston: Boston Publishing Company, 1983, p. 153.
 COSVN or Central Office for South Vietnam, Hanoi's equivalent of MVAC, may have been located in Southern Cambodia but was stretched over hundreds of square kilometers and not at any centralized location.
 William Shawcross, Sideshow. New York: Washington Square Books, 1972, p. 169.
 Colonel Perry Lamy, Barrel Roll, Maxwell Air Force Base AL: Air University Press, 1995, p. 47.