Henry Cabot Lodge was U.S. Ambassador to South Vietnam from 1963 to 1964 and again from 1965 to 1967. Before holding that position, he was Senator from Massachusetts and Ambassador to the United Nations with Eisenhower. Kennedy sent him as his envoy to Vietnam in a shrewd political maneuver, Lodge being one of the most respected Republicans of his time. Lodge's support was fundamental for the success of the coup against Diem.

Biography of Henry Cabot Lodge
From the United States Senate Archives
http://bioguide.congress.gov/scripts/biodisplay.pl?index=L000394

Phone Conversation Between Ngo Dinh Diem and Henry Cabot Lodge, November 1, 1963
From the Vietnam Wars Page at Vassar College
http://vietnam.vassar.edu/~vietnam/doc8.html

Under instructions of "not thwarting" General Minh's coup, Lodge coldly responds to Diem's call for help. While offering U.S. assistance to ensure Diem's personal safety, Lodge refuses to offer any support for the survival of the regime. Diem and his brother Ngo Dinh Nhu were assassinated shortly after.