A war of aggression is intrinsically immoral. In the tragic case where such a war breaks out, leaders of the State that has been attacked have the right and the duty to organize a defence even using the force of arms.1049Cf. Catechism of the Catholic Church, 2265. To be licit, the use of force must correspond to certain strict conditions: “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”.1050Catechism of the Catholic Church, 2309. If this responsibility justifies the possession of sufficient means to exercise this right to defence, States still have the obligation to do everything possible “to ensure that the conditions of peace exist, not only within their own territory but throughout the world”.1051Pontifical Council for Justice and Peace, The International Arms Trade. An ethical reflection (1 May 1994), ch. 1, 6: Libreria Editrice Vaticana, Vatican City 1994, p. 13. It is important to remember that “it is one thing to wage a war of self-defence; it is quite another to seek to impose domination on another nation. The possession of war potential does not justify the use of force for political or military objectives. Nor does the mere fact that war has unfortunately broken out mean that all is fair between the warring parties”.1052Second Vatican Ecumenical Council, Pastoral Constitution Gaudium et Spes, 79: AAS 58 (1966), 1103.
The Charter of the United Nations, born from the tragedy of the Second World War with the intention of preserving future generations from the scourge of war, is based on a generalized prohibition of a recourse to force to resolve disputes between States, with the exception of two cases: legitimate defence and measures taken by the Security Council within the area of its responsibilities for maintaining peace. In every case, exercising the right to self-defence must respect “the traditional limits of necessity and proportionality”.1053John Paul II, Message for the 2004 World Day of Peace, 6: AAS 96 (2004), 117. Therefore, engaging in a preventive war without clear proof that an attack is imminent cannot fail to raise serious moral and juridical questions. International legitimacy for the use of armed force, on the basis of rigorous assessment and with well-founded motivations, can only be given by the decision of a competent body that identifies specific situations as threats to peace and authorizes an intrusion into the sphere of autonomy usually reserved to a State.