As early as the fifth century, St. Augustine of Hippo was considering the moral consequences of war. He was one of the first people to articulate a philosophical statement on war and justice, known as the Just War doctrine. St. Augustine laid the groundwork for the just war theory but he also did so with caution. Writing in 418 A.D., St. Augustine said: “Peace should be the object of your desire; war should be waged only as a necessity…in order that peace may be obtained. Therefore, even in waging a war, cherish the spirit of a peacemaker, that, by conquering those whom you attack, you may lead them back to the advantages of peace…As violence is used toward him who rebels and resists, so mercy is due to the vanquished or captive.”

The School of Salamanca elaborated on the theory. They reasoned that it ought to be resorted to only when it was necessary in order to prevent an even greater evil. A diplomatic agreement is preferable, even for the more powerful party, before a war is started. Examples of “just war” are:

• In self-defense, as long as there is a reasonable possibility of success. If failure is a foregone conclusion, then it is just a wasteful spilling of blood.
• Preventive war against a tyrant who is about to attack.
• War to punish a guilty enemy.

A war is not legitimate or illegitimate simply based on its original motivation: it must comply with a series of additional requirements:

• It is necessary that the response be commensurate with the evil; use of more violence than is strictly necessary would constitute an unjust war.
• Governing authorities declare war, but their decision is not sufficient cause to begin a war. If the people oppose a war, then it is illegitimate. The people have a right to depose a government that is waging, or is about to wage, an unjust war.
• Once war has begun, there remain moral limits to action. For example, one may not attack innocents or kill hostages.
• It is obligatory to take advantage of all options for dialogue and negotiations before undertaking a war; war is only legitimate as a last resort.

The Catechism states all of the following criteria must be met at the same time in order for a war to be considered just:

• 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.

These requirements are not to be taken lightly. For example, in their pastoral letter, the United States Bishops expanded on these requisites by enumerating the following criteria to which war is a permissible recourse:

• Just Cause: to confront “a real and certain danger” to protect innocent life
• Competent Authority: declared by those with responsibility for public order
• Comparative Justice: Are the values at stake critical enough to override the presumption against war?
• Right Intention: War can only be conducted to satisfy the just cause
• Last Resort: All peaceful alternatives have already been exhausted
• Probability of Success: The outcome cannot be disproportionate or futile
• Proportionality: inflicted damage must be proportionate to the good expectant