The quote below comes from the tonkin gulf resolution:

whereas the united states is assisting the peoples of southeast asia to protest their freedom and has no territorial, military or political ambitions in that area, but desires only that these people should be left in peace to work out their destinies in their own congress approves and supports the determination of the president, as commander in chief, to take all necessary measures to repel any armed attack against the forces of the united states and to prevent further aggression.

source:

what led congress to pass this resolution?

evidence of chinese support for north vietnam
a successful viet cong offensive against saigon
reports of north vietnamese attacks on u. s. ships
the election of a communist government in south vietnam

Correct

reports of North Vietnamese attacks on U.S. ships

Further details:

The Gulf of Tonkin Resolution was a measure that allowed the US President to make military actions, like increase troops, without formal declaration of war.  It led to huge escalation of US involvement in the Vietnam War.

The Gulf of Tonkin Resolution, passed by Congress in August, 1964, after alleged attacks on two US naval ships in the Gulf of Tonkin.  The key wording in the resolution said:

Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, that the Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.

That resolution served as a blank check for President Johnson to send troops to whatever extent he deemed necessary in pursuance of the war.  Between 1964 and the end of Johnson's presidency in 1969, US troop levels in Vietnam increased from around 20,000 to over 500,000.

Of course it could have been avoided. a great many scholars argue it happened because of truman's use of fear to gain support. his predecessor fdr was the most popular and charismatic president ever. truman was a machine politician chosen as the least offensive choice around for vp. had henry wallace remained vp and then president, the cold war might not have happened. or had dewey been defeated in the primaries by robert taft, and then taft defeated truman in 48, the cold war also might not have happened. taft was somewhat isolationist, opposing nato and the korean war. wallace believed truman to be unnecessarily provocative in his dealings with the soviets.of course for strong anti communists, they claim the soviets would see any sign of weakness and try to take over the world. but this is a conspiracy theory not backed by evidence. most communist movements were local and not controlled by moscow. stalin had just lost over 20 million people and was exhausted and paranoid. his number one fear was a rearmed germany allied with the us, and truman made his nightmare real with the marshall plan. truman's blunders are at least half responsible for the cold war and neither wallace nor taft would have made them.on top of that, stalin was dying and in poor health. his successor khrushchev was a reformer who ended the forced labor camps, set many political prisoners free, criticized stalin, and declined to build a powerful soviet navy to confront the us. stalin actually proposed a neutral united germany. a paranoid american gov't turned him down, guaranteeing a continued cold war for decades more.