During his presidency, Diệm strongly focused on his central concern: internal security to protect his regime as well as maintain order and social change: staunch anti-subversion and anti-rebellion policies.
After the Bình Xuyên was defeated and the Hòa Hảo, Cao Đài were subdued, Diệm concentrated on his most serious threat: the communists.
Diệm's main measures for internal security were threats, punishment and intimidation.
According to Gabriel Kolko about 12,000 suspected opponents of Diệm were killed between 1955 and 1957 and by the end of 1958 an estimated 40,000 political prisoners had been jailed.

By the end of 1959, Diệm was able to entirely control each family and the communists had to suffer their "darkest period" in their history.
Membership declined by two thirds and they had almost no power in the countryside of South Vietnam.
Diệm's repression extended beyond communists to anti-communist dissidents and anti-corruption whistleblowers.
In 1956, after the "Anti-Communist Denunciation Campaign", Diệm issued Ordinance No.
6, which placed anyone who was considered a threat to the state and public order in jail or house arrest.

Nevertheless, Diệm's hard policies led to fear and resentment in many quarters in South Vietnam and negatively affected his relations with the US in terms of counter-insurgent methods.
On 22 February 1957, when Diệm delivered a speech at an agricultural fair in Buôn Ma Thuột, a communist named Hà Minh Tri attempted to assassinate the president.
He approached Diệm and fired a pistol from close range, but missed, hitting the Secretary for Agrarian Reform's left arm.
The weapon jammed and security overpowered Tri before he was able to fire another shot.
Diệm was unmoved by the incident.
The assassination attempt was the desperate response of the communists to Diệm's relentless anti-communist policies.

As opposition to Diệm's rule in South Vietnam grew, a low-level insurgency began to take shape there in 1957.
Finally, in January 1959, under pressure from southern Viet Cong cadres who were being successfully targeted by Diệm's secret police, Hanoi's Central Committee issued a secret resolution authorizing the use of armed insurgency in the South with supplies and troops from the North.
On 20 December 1960, under instructions from Hanoi, southern communists established the Viet Cong (NLF) in order to overthrow the government of the south.
On 11 November 1960, "a failed coup attempt against President Ngô Đình Diệm of South Vietnam was led by Lieutenant Colonel Vương Văn Đông and Colonel Nguyễn Chánh Thi of the Airborne Division of the ARVN (ARVN)".
There was a further attempt to assassinate Diệm and his family in February 1962 when two air force officers—acting in unison—bombarded the Presidential Palace.

In 1962, the cornerstone of Diệm's counterinsurgency effort – the Strategic Hamlet Program (Vietnamese: Ấp Chiến lược), "the last and most ambitious of Diem's government's nation building schemes", was implemented, calling for the consolidation of 14,000 villages of South Vietnam into 11,000 secure hamlets, each with its own houses, schools, wells, and watchtowers supported by South Vietnamese government.
The hamlets were intended to isolate the National Liberation Front (NLF) from the villages, their source for recruiting soldiers, supplies, and information, and to transform the countryside.
In the end, because of many shortcomings, the Strategic Hamlet Program was not as successful as had been expected and was cancelled after the assassination of Diệm.
However, according to Miller, the program created a remarkable turnabout in Diệm's regime in their war against communism.
During his presidency, Diệm strongly focused on his central concern: internal security to protect his regime as well as maintain order and social change: staunch anti-subversion and anti-rebellion policies. After the Bình Xuyên was defeated and the Hòa Hảo, Cao Đài were subdued, Diệm concentrated on his most serious threat: the communists. Diệm's main measures for internal security were threats, punishment and intimidation. According to Gabriel Kolko about 12,000 suspected opponents of Diệm were killed between 1955 and 1957 and by the end of 1958 an estimated 40,000 political prisoners had been jailed. 
By the end of 1959, Diệm was able to entirely control each family and the communists had to suffer their "darkest period" in their history. Membership declined by two thirds and they had almost no power in the countryside of South Vietnam. Diệm's repression extended beyond communists to anti-communist dissidents and anti-corruption whistleblowers. In 1956, after the "Anti-Communist Denunciation Campaign", Diệm issued Ordinance No. 6, which placed anyone who was considered a threat to the state and public order in jail or house arrest.
Nevertheless, Diệm's hard policies led to fear and resentment in many quarters in South Vietnam and negatively affected his relations with the US in terms of counter-insurgent methods. On 22 February 1957, when Diệm delivered a speech at an agricultural fair in Buôn Ma Thuột, a communist named Hà Minh Tri attempted to assassinate the president. He approached Diệm and fired a pistol from close range, but missed, hitting the Secretary for Agrarian Reform's left arm. The weapon jammed and security overpowered Tri before he was able to fire another shot. Diệm was unmoved by the incident. The assassination attempt was the desperate response of the communists to Diệm's relentless anti-communist policies.
As opposition to Diệm's rule in South Vietnam grew, a low-level insurgency began to take shape there in 1957. Finally, in January 1959, under pressure from southern Viet Cong cadres who were being successfully targeted by Diệm's secret police, Hanoi's Central Committee issued a secret resolution authorizing the use of armed insurgency in the South with supplies and troops from the North. On 20 December 1960, under instructions from Hanoi, southern communists established the Viet Cong (NLF) in order to overthrow the government of the south. On 11 November 1960, "a failed coup attempt against President Ngô Đình Diệm of South Vietnam was led by Lieutenant Colonel Vương Văn Đông and Colonel Nguyễn Chánh Thi of the Airborne Division of the ARVN (ARVN)". There was a further attempt to assassinate Diệm and his family in February 1962 when two air force officers—acting in unison—bombarded the Presidential Palace.
In 1962, the cornerstone of Diệm's counterinsurgency effort – the Strategic Hamlet Program (Vietnamese: Ấp Chiến lược), "the last and most ambitious of Diem's government's nation building schemes", was implemented, calling for the consolidation of 14,000 villages of South Vietnam into 11,000 secure hamlets, each with its own houses, schools, wells, and watchtowers supported by South Vietnamese government. The hamlets were intended to isolate the National Liberation Front (NLF) from the villages, their source for recruiting soldiers, supplies, and information, and to transform the countryside. In the end, because of many shortcomings, the Strategic Hamlet Program was not as successful as had been expected and was cancelled after the assassination of Diệm. However, according to Miller, the program created a remarkable turnabout in Diệm's regime in their war against communism.
Ngo Dinh Diem