Final answer:
Robert McNamara highlighted that the US sent troops to Vietnam to prevent the spread of communism from North to South Vietnam, aligning with the domino theory during the Cold War.
Step-by-step explanation:
Robert McNamara, who served as the Secretary of Defense under President Lyndon Johnson, pointed out that the reason 'they sent US troops' to Vietnam was b. to prevent the spread of communism from North Vietnam to South Vietnam. This was in line with the domino theory, which suggested that if one country fell under communist control, neighboring countries would follow in a domino effect.
The US intervention in Vietnam was part of a broader strategy during the Cold War to contain communism and support democratic regimes against communist takeovers. Despite the intent to uphold democracy, the US-backed South Vietnamese government struggled with legitimacy and effectiveness, leading to a complicated and prolonged conflict.