The Domino Theory, which argued that if one country fell to communist forces, then all of the surrounding countries would follow, was first proposed as policy by the Eisenhower administration. It was, and is still, commonly hypothesized that it applied to Vietnam. John F. Kennedy, then a U.S. senator, said in a speech to the American Friends of Vietnam: "Burma, Thailand, India, Japan, the Philippines and obviously Laos and Cambodia are among those whose security would be threatened if the Red Tide of Communism overflowed into Vietnam."

Rule

A devout Roman Catholic, Diem was fervently anti-communist, nationalist and socially conservative. Historian Luu Doan Huynh notes, however, that "Diem represented narrow and extremist nationalism coupled with autocracy and nepotism." As a wealthy Catholic, Diem was viewed by many ordinary Vietnamese as part of the elite who had helped the French rule Vietnam; Diem had been interior minister in the colonial government. The majority of Vietnamese people were Buddhist, and were alarmed by actions such as his dedication of the country to the Virgin Mary.

Beginning in the summer of 1955, he launched the "Denounce the Communists" campaign, during which communists and other anti-government elements were arrested, imprisoned, tortured, or executed. Diem instituted a policy of death penalty against any activity deemed communist in August 1956. Opponents were labeled Viet Cong ("Vietnamese communist") by the regime to degrade their nationalist credentials. As a measure of the level of political repression, about 12,000 suspected opponents of Diem were killed in the years 1955–1957 and by the end of 1958 an estimated 40,000 political prisoners had been jailed.

In May, Diem undertook a ten-day state visit to the United States. President Eisenhower pledged his continued support. A parade in New York City was held in his honor. Although Diem was openly praised, in private Secretary of State John Foster Dulles conceded that he had been selected because there were no better alternatives.

Robert McNamara wrote that the new American patrons were almost completely ignorant of Vietnamese culture. They knew little of the language or long history of the country. There was a tendency to assign American motives to Vietnamese actions, and Diem warned that it was an illusion to believe that blindly copying Western methods would solve Vietnamese problems.

Insurgency in the South, 1956–1960

The Sino-Soviet split led to a reduction in the influence of the PRC, which had insisted in 1954 that the Viet Minh accept a division of the country. Trường Chinh, North Vietnam's pro-PRC party first secretary, was demoted and Hanoi authorized communists in South Vietnam to begin a low level insurgency in December 1956. This insurgency in the south had begun in response to Diem's Denunciation of Communists campaign, in which thousands of local Viet Minh cadres and supporters had been executed or sent to concentration camps, and was in violation of the Northern Communist party line, which had enjoined them not to start an insurrection, but rather engage in a political campaign, agitating for a free all-Vietnam election in accordance with the Geneva accords.

Ho Chi Minh stated, "Do not engage in military operations; that will lead to defeat. Do not take land from a peasant. Emphasize nationalism rather than communism. Do not antagonize anyone if you can avoid it. Be selective in your violence. If an assassination is necessary, use a knife, not a rifle or grenade. It is too easy to kill innocent bystanders with guns and bombs, and accidental killing of the innocent bystanders will alienate peasants from the revolution. Once an assassination has taken place, make sure peasants know why the killing occurred." This strategy was referred to as "armed propaganda."

Soon afterward, Lê Duẩn, a communist leader who had been working in the South, returned to Hanoi to accept the position of acting first secretary, effectively replacing Trường. Duẩn urged a military line and advocated increased assistance to the insurgency. Four hundred government officials were assassinated in 1957 alone, and the violence gradually increased. While the terror was originally aimed at local government officials, it soon broadened to include other symbols of the status quo, such as schoolteachers, health workers, and agricultural officials. Village chiefs were Diem appointees from outside the villages and were hated by the peasantry for their corruption and abuse.) According to one estimate, 20 percent of South Vietnam's village chiefs had been assassinated by the insurgents by 1958. (The insurgency sought to completely destroy government control in South Vietnam's rural villages and replace it with a shadow government.

In January 1959, the North's Central Committee issued a secret resolution authorizing an "armed struggle". This authorized the southern communists to begin large-scale operations against the South Vietnamese military. North Vietnam supplied troops and supplies in earnest, and the infiltration of men and weapons from the north began along the Ho Chi Minh Trail. In May, South Vietnam enacted Law 10/59, which made political violence punishable by death and property confiscation.

Observing the increasing unpopularity of the Diem regime, on December 12, 1960, Hanoi authorized the creation of the National Liberation Front as a common front controlled by the communist party in the South.

Successive American administrations, as Robert McNamara and others have noted, overestimated the control that Hanoi had over the NLF.^[45] Diem's paranoia, repression, and incompetence progressively angered large segments of the population of South Vietnam. According to a November 1960 report by the head of the U.S. military advisory team, Lieutenant General Lionel C. McGarr, a "significant part" of the population in the south supported the communists. The communists thus had a degree of popular support for their campaign to bring down Diem and reunify the country.