Final answer:
The U.S. sent ground troops to Vietnam in 1965 to counteract the ineffective bombing campaign, support the struggling South Vietnamese army, and prevent the spread of communism in Southeast Asia following General Westmoreland's request and President Johnson's approval.
Step-by-step explanation:
The United States escalated its involvement in the Vietnam conflict and ultimately sent ground troops in 1965 due to several factors. The Operation Rolling Thunder bombardment campaign proved ineffective against the North Vietnamese. U.S. military advisors and CIA efforts to support South Vietnam were not yielding the desired results, and the South Vietnamese army appeared incapable of effectively combating the Viet Cong. General William Westmoreland, the U.S. commander in South Vietnam, called for an increase in U.S. involvement to prevent the fall of South Vietnam to communism, a concept which President Lyndon Johnson supported, fearing the domino effect in Southeast Asia. Consequently, U.S. ground troops were sent to engage directly with the Viet Cong and North Vietnamese Army to demonstrate U.S resolve and assist the South Vietnamese forces.