Which was not a justification for the increase in U. S. involvement in Vietnam?

Question 2 options:

to keep North Vietnam aligned with the United States

to fight communism

to avoid the domino effect of communism's spread in the region

to respond to attacks on U. S. troops in neutral territory

The item in the list which was NOT a justification for increased US involvement in Vietnam:

To keep North Vietnam aligned with the United States.

Explanation:

North Vietnam never was aligned with the United States. The US was aligned with and fought on behalf of South Vietnam. North Vietnam was controlled by a communist government and was seeking to unify Vietnam under communist control.

US foreign policy in those years was one focused on fighting communism. Domino theory was a major idea in US thinking at the time. Domino theory proposed that the spread of communism in Vietnam would result in other countries in the region falling like dominoes to the influence of communism.

The event that sparked a major increase of US involvement in Vietnam was the allegation of attacks on US naval ships in neutral waters in the Gulf of Tonkin. The Gulf of Tonkin Resolution was a measure passed by US Congress that allowed the US President to make military actions, like increase troops, without formal declaration of war. The resolution was passed by Congress in August, 1964, after alleged attacks on two US naval ships in the Gulf of Tonkin. The key wording in the resolution said:

Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, that the Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.

That resolution served as a blank check for President Lyndon Johnson to send troops to whatever extent he deemed necessary in pursuance of the war. Between 1964 and the end of Johnson's presidency in 1969, US troop levels in Vietnam increased from around 20,000 to over 500,000.