What event pushed the U.S. to adopt a more active military role in Vietnam under President Johnson?

The event that led the U.S. to adopt a more active military role in Vietnam under President Johnson was the Tonkin Gulf Incident. This incident, which occurred in August 1964, involved reported attacks on U.S. naval vessels by North Vietnamese forces in the Gulf of Tonkin. The Johnson administration used these reports as a catalyst to seek congressional approval for a more significant military presence in Vietnam.

In response to the perceived aggression and the desire to prevent the spread of communism in Southeast Asia, Congress passed the Gulf of Tonkin Resolution. This resolution gave President Johnson broad authority to use military force without a formal declaration of war. Thus, the Tonkin Gulf Incident served as a pivotal moment that escalated U.S. involvement in Vietnam, ultimately leading to a substantial commitment of American troops and resources to the conflict.

While the other events listed—such as the Korean War, the Cuban Missile Crisis, and the Watergate Scandal—are significant in their own right, they do not directly pertain to the immediate increase in military engagement in Vietnam during Johnson's presidency.