What controversy surrounded the Vietnam War?

Why were we there? What purpose did it serve the United States to be fighting a war on the opposite side of the earth? Why were we fighting a war that we had no intention of winning? Supposedly we were just trying to stop agression from North Vietnam into the territory controlled by our friends in South Vietnam, so we would only fight the "enemy" that came up to the DMZ, but we didn't go after the leaders by attacking the center of their power. We drew a line in the sand and effectively said, "don't cross that line... or else."