Catechism number 2309

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.

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.