SOMMARIO: 1. Premessa - 2. S. Agostino - 3. S. Tommaso d’Aquino - 4. Francisco de Vitoria - 5. Francisco Suarez - 6. Problemi attuali.

ABSTRACT: At the present time, the complexity of international relations make us to reflect on ideologies that in past centuries, opening the modern history, helped to organize the international life and the relations between peoples and between States. Especially Hugo Grotius, Samuel von Pufendorf and Emmer de Vattel are to be regarded as founders of modern international law, but it would be a serious shortcoming for the scholar the oversight of the work of some spanish theologians and jurists, such as the dominican Francisco de Vitoria and the jesuit Francisco Suarez which, starting from the 16th century, have developed principles that are fundamental results in international law. The universal character of the christian message has had many applications in the social and legal sphere, requires that the human race constitutes one family and that the spirit of christianity is directed toward each institution aimed to prevent disputes between States and between the peoples become violent clashes, destruction or massacres. In ancient period the most important contributions that have enriched the catholic doctrine as well as its teaching in these matters were those of St. Augustine and St. Thomas Aquinas. In augustinian and thomistic doctrine the war must be regarded as a last resort to which it is not used that after having verified the impossibility to safeguard the good right; again, that the legitimate end of the war is not the victory, but the peace in justice, namely that is the restoration lasting public order in which everything is repositioned in the right place. The affirmative answer to the question if you can give a just war is the key element that is remained a fundamental basis in catholic morality. The war can be considered lawful under certain conditions; but it is nevertheless a profound injustice, as a result of the sins of humanity against the virtue of charity. An important part of the teaching of the spanish theologian Francisco de Vitoria, was especially concerned by the problems created in Spain by the conquest and colonization of Latin America. He was considered the true founder of international law from many specialists. He put in question the conviction then widespread that indigenous not enjoyed human rights, i.e. rights brought from civilization. The dominican theologian rejected these views because he was inspired by the natural law, which must regardless of profession of the true religion which regards the guidelines toward the personal spiritual salvation. Reflecting on the thought of the spanish jesuit Suarez, we are devoting, in particular, our attention to the “jus gentium”. The expression is not related to the international relations between peoples and between nations, what happened in the evolution that subsequently took the expression. Originally the expression jus gentium was referring to some of the rules common to all peoples; in some way a natural right, which led to consider the community of nations as forming a great social community, governed by a set of rules, principles, morals, rights, and duties, according to the needs or opportunities or indications of human nature that is the work of a single Creator. This concept has opened the way to the idea of international law understood as complex rules in relations between nations. In the contemporary era the concept of just war must be considered in a different way and must take account of two considerable elements: the lawfulness of the just war should be read today in harmony and with the observance of an “jus cogens” which goes beyond the respect for the territorial integrity and political independence of a State and then the use in war of deadly weapons, even for the civilian population, which upset the reflection on just cause for a war. We believe that this has fostered a rethinking of catholic doctrine and the positions of the Pope John Paul II who undertook very on the theme of peace and war: he had shown himself absolutely contrary to the war and, again, he seemed to want to overcome the doctrine of the just war. There are those who in the catholic world believes that today it is no longer possible to speak of just war and that the war can no longer be regarded as a means of resolving disputes between States. But before humanity some questions remain.