Final answer:
The U.S. began fighting the Vietnam War in 1964 primarily due to the Gulf of Tonkin incident. The justification for the U.S. involvement in the war based on the Gulf of Tonkin incident remains questionable.
Step-by-step explanation:
The U.S. began fighting the Vietnam War in 1964 primarily due to the Gulf of Tonkin incident. The Gulf of Tonkin Resolution, passed by Congress on August 7, 1964, gave President Lyndon Johnson broad authority to retaliate against North Vietnamese attacks and protect U.S. lives. However, it is important to note that the Gulf of Tonkin incident itself is widely believed to have been based on inaccurate information, as there was no other evidence to support the claim of a North Vietnamese torpedo attack. Therefore, the justification for the U.S. involvement in the war based on the Gulf of Tonkin incident remains questionable.