What legislative act started the policy of militarily aiding Vietnam?

The Gulf of Tonkin Resolution is the correct answer because it was a significant piece of legislation passed by Congress in 1964, which essentially authorized President Lyndon B. Johnson to use military force in Vietnam without an official declaration of war. This resolution followed alleged incidents involving U.S. naval vessels in the Gulf of Tonkin and provided the legal basis for escalating U.S. involvement in the Vietnam War. Its passage marked a turning point in American foreign policy, as it enabled increased military support and operations in Vietnam, ultimately leading to a significant commitment of U.S. troops in the subsequent years.

In contrast, other options represent different contexts or outcomes. The War Powers Act, enacted in 1973, aimed to check the president's power by requiring congressional approval for military actions, which is not related to the initial involvement in Vietnam. The Paris Peace Accords, signed in 1973, were aimed at establishing peace and ending U.S. involvement in Vietnam, rather than starting it. The Vietnam Act does not refer to any prominent legislation that initiated military aid in Vietnam, making it an invalid choice. The correct answer encapsulates the beginning of military escalation in Vietnam, reflecting a crucial moment in U.S. history.