Some things to keep in mind:

-The US direct 'big war' began in 1965

• The anti-war movement did not really emerge until after 1968

• Since c. 1961, the US had thousands of military advisors in South Vietnam

• The anti-war movement is regarded by some historians as a failure in its specific objective to stop the war immediately. Nixon, after defeating the peace candidate, continued to escalate the war through expanded bombings, the invasion of Cambodia, and increased support for the South Vietnamese army.

To summarize, all wars have political considerations that affect their course ('war is an extension of politics by other means' per Clausewitz) and Vietnam was not unique in that respect. Political pressure only began to really mount after 1972, seven years into the war 'big war' and a dozen years after the introduction of a large scale US advising presence. US and South Vietnamese military measures failed to produce a favorable outcome in that time period.