When you speak of an unjust war, then, let’s look at the church’s standard for what is a just war.
Our wisdom on this comes primarily from St. Thomas Aquinas. You can find it in your Catechism of the Catholic Church in section 2309, which states this:
The strict conditions for legitimate defense by military force require rigorous consideration. The gravity of such a decision makes it subject to rigorous conditions of moral legitimacy. At one and the same time:
• The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave and certain;
• All other means of putting an end to it must have been shown to be impractical or ineffective;
• There must be serious prospects of success;
• The use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition.
The evaluation of these conditions for moral legitimacy belongs to the prudential judgment of those who have responsibility for the common good.
What we are to do with this information, then, is place it in our hearts, listen for the church to make a declaration about whether the war is just or not, and then pray for the wisdom to follow it.
If the church has not spoken, you will need to follow your own conscience about whether or not you believe the war in question to be unjust or not.
People who join the military do so for numerous reasons: some require the vast financial assistance available to them if they join; others seek training in fields that will provide them with a lifetime of skills which will feed them and their families. Other people join to grow in discipline and or “team concept.” There will also be those noble persons who join the military out of a sense of duty and gratitude.
Whatever the reasons, if someone joins the military during a time of “unjust war” in order to provide assistance in a non-killing manner, I believe that to be morally acceptable. For instance, priests who serve as chaplains in the military do not necessarily need to agree with the war to believe that their help is desperately needed. Medical doctors, or those in training to be doctors, may feel that they are called to offer their skills to soldiers in a war that they believe to be unjust or that the church has deemed unjust.
Public authorities should make equitable provision for those who for reasons of conscience refuse to bear arms; these are nonetheless obliged to serve the human community in some other way.
So, that much is clear. We know that if an individual feels called to be a conscientious objector, he or she is not just allowed to follow that prompting but is actually required to do so.
Let’s pray and work for peace.
Enjoy another day in God’s presence!
– Father Joseph Krupp