Answer:
The American president that was responsible for the massive military buildup in Vietnam was Lyndon Johnson.
Step-by-step explanation:
On the night of August 4, 1964, President Lyndon B. Johnson, sitting in his office in the White House, before television cameras, declared that two days earlier, the destroyer USS Maddox had been attacked by North Vietnamese torpedo boats, while in international waters, in the Gulf of Tonkin. He also said that a few hours earlier - from that day 4 -, other North Vietnamese torpedoers had attacked, in the gulf waters themselves, the destroyer USS Turner Joy. Finally, he added that neither the Maddox nor the Turner Joy were conducting any hostile activity towards North Vietnam, nor were they participating in intelligence work to support the attacks that the torpedo ships of South Vietnam were carrying out against the coastal facilities of their northern neighbor.
Three days later, the United States Congress passed the Resolution of the Gulf of Tonkin, which authorized the Johnson government to use all the necessary force to punish North Vietnam for that aggression and to intervene, openly, in the war. By mid-1965, there were already hundreds of thousands American soldiers fighting in Vietnam and their warplanes massively bombarded the cities of North Vietnam and rebel positions in the south.