In 1965, as the conflict in Vietnam intensified, the United States
O declared war against North Vietnam.
o decreased the number of US troops in Vietnam.
O escalated its troop commitment to the conflict.
Obegan defending North Vietnam against the Viet cong.

escalated its troop commitment to the conflict.

Explanation:

After the incident of the Gulf of Tonkin in 1964 where two US naval destroyers were attacked by the Vietnamese communist army, president John B Lyndon passed the Gulf of Tonkin Resolution on August 7, 1964.The resolution gave president John B Lyndon the power to take every necessary step to retaliate any armed attack by the Vietnamese communist regime against the military of the United States.Therefore, as the conflict in Vietnam intensified by 1965, the United States had to escalate its involvement in the conflict by increasing its troop commitment.

Democracy and darwinism would be my answer.