Not only were the reasons for starting the war somewhat dubious, the American military did not have a realistic plan for winning the war. Once the United States became involved and it became increasingly apparent that the war was not being won, the United States simply fell back on its dependence on superior firepower. In an attempt to win a war that it had no idea how to win, the United States dropped more ordinance on Vietnam that they did in all of World War II. And the goal of the war was not to establish a lasting peace but to force the North Vietnamese to accept an American backed corrupt state in the South, and to do this by destroying as much of their infrastructure as possible. In this endeavor, the United States engaged in a strategy of using disproportionate firepower against the North Vietnamese, and since in the 1960's precision bombing was only a distant dream, the United States killed a great number of innocent civilians.

It...

Not only did the United States create a situation where the native people were divided and unhappy, it also created a corrupt, militaristic government in the South which abused it own people. And to justify its wish to engage in a conflict, the United States government feigned an attack by the North Vietnamese in order to gain the authority to wage war. This act alone, one can argue, makes the entire war illegitimate, but America then went on to wage an unjust war by "redressing" a grievance that did not happen, by using a disproportionate amount of force against North Vietnam, by not limiting their attacks to strictly military targets, and killing hundreds of thousands of innocent civilians as a result.
References

Zinn, Howard. A People's History of the United States. New York: Harper, 2010. Print