What Is a ‘Just War’?

The Catechism of the Catholic Church lays out the conditions for just war in paragraph 2309:

• 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 modem means of destruction weighs very heavily in evaluating this condition.

The Catechism goes on to conclude in that same paragraph:

These are the traditional elements enumerated in what is called the “just war” doctrine. The evaluation of these conditions for moral legitimacy belongs to the prudential judgment of those who have responsibility for the common good.