I was reading LBJ speech regarding why Americans are fighting in Vietnam. Johnson makes it clear that we need to help South Vietnam from the communist in the North; how it is our duty as the United States to help South Vietnam. He states that we are there to keep a promise; the promise? to support South Vietnam just like every President has since 1954.
Johnson says "We fight because we must fight if we are to live in a world where every country can shape its own destiny, and only in such a world will our own freedom be finally secure." Johnson is telling the American public we need to be in South Vietnam fighting because we are not only fighting for their freedom but ours as well.
The Link below will take you to a broadcasting of his speech.