In July of 1966 Lyndon B. Johnson said,
We are not trying to wipe out North Vietnam. We are not trying to change their government. We are not trying to establish permanent bases in South Vietnam...We are there because we are trying to make the Communists of North Vietname stop shooting at their neighbors..to demonstrate that guerilla warfare, inspired by one nation against another nation can never succeed....We must keep on until the Communists in North Vietnam realize the price of agression is too high--and either agree to a peaceful settlement or to stop their fighting.
This was Johnson's argument for entering Vietnam as he played upon the Americans' fears that Communism was a universal threat.
In August of 1964 Johnson said the North Vietnamese forces had twice attacked American destroyers in the Gulf of Tonkin. Known today as the Gulf of Tonkin Incident, this led to the open involvement of the U.S. in the Vietnam Conflict.
Before his death, Robert S. McNamara, former Secretary of Defense under President Johnson, admitted that the Gulf of Tonkin Incident was completely orchestrated in order to persuade Congress to agree to war. After this criminal admission, McNamara begged forgiveness for the senseless deaths of 50,000+ American soldiers, not to mention the countless disabled and mentally damaged soldiers.
Besides the false pretenses given as justification for entering Vietnam, a very strong argument against the Vietnam Conflict--interestingly, it was never named "a war" while it was going on--was the fact that this was really a civil war within the country of Vietnam, and by this fact, there was no reason for the involvement of the United States.
This interference of the United States into the civil war of Vietnam initiated the perception of many countries of America as an imperialistic interference.