South Vietnamese Coup (1963)
The South Vietnamese Coup (1963) was a violent overthrow of Ngo Dinh Diem regime in South Vietnam that happened during the first phase of the Vietnam War and concluded the 1963 Buddhist Crisis. The Buddhist Crisis brought much unwanted international attention to the repressive regime of Ngo and ultimately forced the Kennedy Administration to authorize a CIA-backed "regime change" during which President Ngo and other members of his family and administration were killed.
Nhu escalates; U.S. government conveys last warnings to Diem
On August 21, Ngo Dinh Nhu used the authority of martial law to carry out massive raids on Buddhist pagodas and portrayed those attacks as the responsibility of the ARVN and the CIA. However, the raids were not by ARVN forces at all, but by paramilitary police and Vietnamese Special Forces under the direct command of Le Quang Tung. This prompted key U.S. decisions, expressed in "Cable 243" from Assistant Secretary of State Roger Hilsman, conveying the Administration position to Ambassador Lodge.  Hilsman's cable included the points:
- "US Government cannot tolerate situation when such power lies in Nhu's hands. Diem must be given the chance to rid himself of Nhu and his coterie and replace them with the best military and polite personalities available
- If, in spite of all your efforts, Diem remains obdurate and refuses, we must face the possibility that Diem himself cannot be preserved.
Hilsman made it clear that the Administration was insistent that the blame be placed on Nhu, that the ARVN be cleansed of taint, and the Nhus must go. The Nhu-must-go message was presented again and again, by different emissaries, with no results.
McNamara suggested sending an investigator, and, on September 6, MG Victor Krulak, JCS Special Assistant for Counterinsurgency and Special Activities. Joseph Mendenhall, a career diplomat, with him.  their report of the Krulak-Mendenhall mission generated a legendary response from the President.
You two did visit the same country, didn't you?
On September 12, the National Security Council told Ambassador Lodge to reopen "tough" negotiations with Diem... Robert Kennedy speculated that if the war can be won neither with Diem nor in the event of a disruptive coup, a U.S. a U.S. disengagement should be considered.  Robert Kennedy's perspective is representative of the Kennedy administration's position of not making a highly visible U.S. commitment to South Vietnam, in contrast to the situation where the Johnson administration kept escalating, for fear of the political repercussions of "losing".
A brief positive note came when South Vietnam ended martial law on the 16th. The next day, the NSC decided to escalate pressure on Diem, and not agree to Diem and Nhu staying as a team. Both for fact-finding and pressure, the NSC decided to send Secretary McNamara and General Taylor there on the "fact-gathering" McNamara-Taylor mission. In their first meeting with Ambassador Lodge and General Harkins, on the 25th, it was immediately obvious the Embassy and MACV had quite different perceptions.
After more meetings, the two senior officials returned and briefed the NSC on October 2nd. Their report was a military-civilian compromise, confirming military progress but warning of political instability. Greater GVN effort, they believed, was needed in both clear-and-hold operations and securing strategic hamlets. They recommended pressuring Diem by withdrawing 1,000 troops, selectively withholding general aid, and ending all support for the Vietnamese Special Forces (i.e., that carried out the pagoda raids). While McNamara and Taylor recommend against a coup, they did recommend identifying and cultivating alternative leadership. Kennedy approved their recommendations.
On the same day, CIA officer Conein "accidentally" met General Don, who tells him that a coup is nearly ready; the corps commander for the Saigon area had not yet committed. They schedule another meeting for the 5th.
At that meeting, Conein, with Lodge's approval, met with Gen. Duong Van Minh, the potential coup leader. He mentioned three possible plans; one involved assassination. Conein was noncommittal, but, when he reported back to Lodge, Lodge asked Washington that Conein "be authorized to say that the U.S. will not thwart a coup, that we are willing to review plans, and that we will continue support to a successor regime."
Diem overthrown and killed
On October 30, Lodge said the U.S. was powerless to stop the coup, while GEN Harkins strongly disagreed. In a November 1 meeting, even as the coup leaders were mobilizing troops, Lodge, and Admiral Felt (commander, United States Pacific Command) met with Diem; Diem called Lodge to the side, and suggest they talk about options, after Felt left. At this point, the coup was actually in progress. At 1:45PM, Don formally notified Harkins' operations officer. At 4 PM, the generals called and asked Diem and Nhu to surrender; they refused; the coup leadership made a public announcement at 4:30. When it was announced Diem called Lodge, expressed concern for Diem's safety.
At 5, the generals called Diem again, put Col. Le Quang Tung, LLDB commander, on the phone, who told Diem that he was a prisoner of the coup leaders. After the call ended, the coup leaders executed Tung. Diem and Nhu stayed on the phones, searching for supporters, and left the Presidential Palace around 8 PM.
In something on an irony given that Diem had ordered, in 1956, that Chinese residents, proud of their heritage, "Vietnamize", a Chinese merchant, in the ethnic Cholon district, gave them refuge. The coup plotters, not knowing the brothers were gone, bombarded the palace all night. Between 6:20 and 6:30 AM, Diem called Don, and offered to surrender without telling Don their location. Diem told the palace guard to surrender. The attack force commander learned Diem's location, received permission to arrest them, and, after finding them briefly escaped to a Catholic church,takes them into custody at 6:50. On the way back to headquarters, the two brothers are shot and killed; the details remain unclear, other than they were in the back of an armored personnel carrier, and were actually slain by a relatively low-ranking aide to a Vietnamese general; the aide later died in combat.The U.S. was aware of the planned coup, and had decided, at a policy level, that Diem could not prevail. They did not warn Diem, and CIA officer Lucien Conein essentially told the coup plotters that the U.S. would not intervene. Both the Embassy and Washington seemed genuinely shocked, however, that Diem had been killed; Ambassador Henry Cabot Lodge Jr. had offered refuge to Diem. Ironically, President John F. Kennedy, who had authorized the U.S. position and did not expect Diem's death, was himself assassinated in the same month. Lyndon Baines Johnson became U.S. commander-in-chief, and Johnson's approach to the war was different than Kennedy's. 
- John Prados, ed., State-Saigon Cable 243, August 24, 1963, JFK and the Diem Coup, vol. George Washington University Electronic Briefing Book No. 101
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- John Prados, ed., Department of State, John M. Dunn, Memorandum for the Record, November 1, 1963, JFK and the Diem Coup, vol. George Washington University Electronic Briefing Book No. 101
- McMaster, H. R. (1997), Dereliction of Duty: Johnson, McNamara, the Joint Chiefs of Staff, and the Lies That Led to Vietnam, Harpercollins