When the USA officially joined
The fact that the United States of America interfered increasingly in the internal affairs of South Vietnam was generally known. The more and more advisors shored up this as well. The president John F. Kennedy (1960-63) raised their number to 16000 by the year 1963, and most of them were military advisors.
In South Vietnam the number of the Vietcong fellow traveler got higher and higher and the USA couldn't see this without doing anything. The initial astonishment of the States is shown by the subsequent worlds of an American politician: 'Nobody could think that if Ho Chi Minh won the election, he will soon appear with his troops.' Moreover, the Vietnamese Democratic Republic (DRV) supported the Vietcong's' guidelines (the Soviets did so as well). The politicians of the USA wanted to start the war, but they had to find a credible reason why they do so.
However, in August, 1964 everything came to their way because on 2nd and 4th August a North Vietnamese torpedo-sloop attacked the American battleship named Maddox. The problem was that they fired once in Vietnamese sea, but the second time in international water where they attacked an other ship too. This made it the US-government possible to start the war officially. The president announced that two American battleships had been attacked and they had to protect their advisors. Therefore the presence of the US-Army in South Vietnam is essential. The president Lyndon B. Johnson (1964-68) got dictatorship for the retaliation for the Tonkin Gulf incident from the Congress.
Expeditions were started mainly during the arid seasons, but most of them weren't successful because the Soviets gained a small ascendancy in the Cold War and could support the army of the DRV better. At the beginning of the year 1965 the US launched the air offensive 'Rolling Thunder' to destroy the strategic points of the North. In spite of the aim they attack public buildings like schools, hospitals as well.