There is no way to prove this, of course, but it seems safe to say that no war was as closely tied to the popular music of its era than Vietnam.
What we know of as the Vietnam War gradually emerged from War War II and the painful end of the French occupation. Thus, there is no real starting point. Here is an interesting video that features the number one song from each of the years in which the United States was most involved until the bitter end on the embassy roof in Saigon. It was posted by Seth Anderson.
Editor’s Note: I just reread this and perhaps should clarify. My idea is that Vietnam probably was the first war in which the major role of the music was to provide commentary. During earlier wars, music was primarily a source of support and encouragement for the troops. In Vietnam, that role was secondary to criticism of the conflict and the government. That’s a big difference, in my opinion.