General Discussion

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 modern means of destruction weighs very heavily in evaluating this condition.