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