What prompted the united states to enter into an all-out war in vietnam?

a. south vietnam's president diem was assassinated.
b. two u. s. destroyers were attacked by the viet cong.
c. north vietnam invaded south vietnam.
d. the viet cong increased its attacks on u. s troops.

The correct answer is B. The event that prompted the United States to enter into an all-out war in Vietnam was that two U.S. destroyers were attacked by the Viet Cong.

Explanation:

On August 2, 1964, the destroyer USS Maddox, on a secret surveillance and electronic espionage mission, entered the Gulf of Tonkin and was attacked by three torpedo boats of the North Vietnamese navy. Two days later, with the USS Turner Joy at his side, the Maddox again reported the presence of enemy torpedoes in their direction.

The incident, which had worldwide repercussions, given the fact that the United States, despite the political, financial and military support it gave to South Vietnam, then at war with the communist north, was not yet in direct open hostilities with the northern communists, was the pretext for the US Congress to sign an act, the Gulf of Tonkin Resolution, giving President Lyndon Johnson legal permission to enter the war. The government of Hanoi made several official declarations that there was only one attack, and that this was caused by the fact that foreign war vessels entered its territorial waters.  

The incident led to the bombing of North Vietnamese bases by fighter jets from two aircraft carriers off the coast, destroying fuel tanks and torpedo-making machinery, marking the first US air strike on military installations in North Vietnamese territory .