The United States even before entering the war in Vietnam as already supporting rebels in South Vietnam and sending military advisors.
The support started way back in 1955 in form of military advisors and by 1959 in the form of money and ammunition.
The aim of the United States was to stop the formation of a new Communist state that would become an ally of the Soviet Union.
In this regard, they were already prepared but initially wanted to stay out of the conflict, instead relying on indirect support.
By the early 1960s, the United States had some 17,000 'advisors' in Vietnam and once it was clear that US-backed groups will not be successful, the United States, officially and openly went to war in 1964.
It is probable that by the 1960s the United States was already making preparation for a war. The Incident at Gulf of Tonkin only provided a perfect pretext for a war.