What gave Johnson the authority to take any "necessary measures to repel an attack "on American forces

Tonkin Gulf Resolution

Explanation:

The agreement that have President Johnson the authority to take any "necessary measures to repel an attack "on American forces was the Tonkin Gulf Resolution.

This resolution declares that a sitting president had the ability to repel an attack on American soil by invaders. This prompted President Johnson to send 2 US destroyers to make an offensive in Vietnam

i think the answer is : the declaration of rights and grievances

hope i

-lavira : )