Answer: General William Westmoreland
Details: President Lyndon Johnson appointed General William Westmoreland to replace General Paul Harkins as head of the United States Military Assistance Command Vietnam (MACV) in June, 1964.
The buildup of American military presence in Vietnam from 16,000 troops to over 500,000 troops occurred under Westmoreland's leadership, as well as President Johnson's initiative. The ongoing stalemate in Vietnam, in spite of those enormous troop increases, brought about much anti-war sentiment back home in the United States. Plenty of the dissatisfaction was aimed at General Westmoreland. In 1968, President Johnson replaced Westmoreland with General Creighton Abrams as head of MACV.