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

the award is named for an american naval historian and theorist, rear admiral alfred thayer mahan, united states navy who, through his writing, provided stimulus and guidance to those who share in the defense of the nation.

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