What enabled the drastic increase in US troop deployment to Vietnam between 1964 and 1965?

The Gulf of Tonkin Resolution, passed by Congress in 1964, gave President Lyndon Johnson broad authority to use military force in Vietnam without a formal declaration of war. He used that authority to sharply increase U.S. troop deployments in 1965. The Geneva Accords and French withdrawal occurred earlier, while the USS Maddox incident helped prompt the resolution but did not itself authorize the troop buildup.

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