Vietnam: A Just War?

Many of you have written about deciding to fight or not depending on whether you believed in the war.
Many people evaluate their position against the theory of a "just war."
Check it out here:
http://en.wikipedia.org/wiki/Just_war_theory

What do you think of just war theory--how is it helpful in evaluating a war and the choice to participate?

Take a few minutes to find a site that explains the basics of the Vietnam war and look for the reasons the US particiapted in it. When you find a good site, comment with the URL on this post

Now, was Vietnam a "just war"?
How does that impact your evaluation of Tim O'Brien's actions?