"Legitimate defense cannot only be a right but a grave duty for someone responsible for another’s life (CCC 2265). Preserving the common good of society requires rendering the aggressor unable to inflict harm (CCC. 2266). 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:

1. The damage inflicted by the aggressor on the nation or community of nations must be lasting, grave and certain;

2. All other means of putting an end to it must have been shown to be impractical or ineffective;

3. There must be serious prospects of success; and,

4. 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."


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