Dr. Jeffrey Mirus at CatholicCulture.org muses on just war and the obligation of citizens of countries at war. Specifically, how can a citizen assess the justice of a war, especially when he doesn’t have all the facts to make a decision, and what obligations do soldiers have?
In general, then, given his limited responsibilities and the confusion of available information, the ordinary citizen has neither the responsibility to decide whether his country will go to war nor the ability to arrive at the kind of moral certitude which would render either his participation or his refusal sinful. Therefore, if on due reflection and with an honest understanding of his own limited role, he becomes convinced that it would be immoral for him to participate, refusal to do so becomes a perfectly legitimate moral option. Once again, the very uncertainties which ordinarily free him from guilt in obeying the command of the state also free him from guilt if he refuses to fight.
The problem is that, except in certain rare circumstances, it is very difficult to become absolutely convinced of either the evil intent of your government in waging a war or its absolute justice. Are you being lied to? Are they being lied to? What is the truth and who has it? The only certainty in discussions of a just war is the uncertainty and so each person has to make his best guess and judgment about the justice of a war and any particular actions related to it.
In a related vein is this story linked at Fumare about an Army officer—who joined up after 9/11—now refusing to be deployed to Iraq, claiming that the war is immoral.