Today's church has restated its traditional teaching on conflict in the Catechism promulgated by Pope John Paul II. This teaching does promote peace very strongly but it also condemns genocide, and the violations of international law:
2307 The fifth commandment forbids the intentional destruction of human life. Because of the evils and injustices that accompany all war, the Church insistently urges everyone to prayer and to action so that the divine Goodness may free us from the ancient bondage of war.
2308 All citizens and all governments are obliged to work for the avoidance of war.
However, "as long as the danger of war persists and there is no international authority with the necessary competence and power, governments cannot be denied the right of lawful self-defense, once all peace efforts have failed."
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 modem 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.
2310 Public authorities, in this case, have the right and duty to impose on citizens the obligations necessary for national defense.
Those who are sworn to serve their country in the armed forces are servants of the security and freedom of nations. If they carry out their duty honorably, they truly contribute to the common good of the nation and the maintenance of peace.
2311 Public authorities should make equitable provision for those who for reasons of conscience refuse to bear arms; these are nonetheless obliged to serve the human community in some other way.
2315 The accumulation of arms strikes many as a paradoxically suitable way of deterring potential adversaries from war. They see it as the most effective means of ensuring peace among nations. This method of deterrence gives rise to strong moral reservations. The arms race does not ensure peace. Far from eliminating the causes of war, it risks aggravating them. Spending enormous sums to produce ever new types of weapons impedes efforts to aid needy populations;111 it thwarts the development of peoples. Over-armament multiplies reasons for conflict and increases the danger of escalation.
2316 The production and the sale of arms affect the common good of nations and of the international community. Hence public authorities have the right and duty to regulate them. The short-term pursuit of private or collective interests cannot legitimate undertakings that promote violence and conflict among nations and compromise the international juridical order.
2317 Injustice, excessive economic or social inequalities, envy, distrust, and pride raging among men and nations constantly threaten peace and cause wars. Everything done to overcome these disorders contributes to building up peace and avoiding war...
You may check the above references to the Catholic Catechism at its online site.
Pope John XXIII's encyclical Pacem in Terris (Peace on Earth) would be a valuable resource to explore the inherent rights of every human being and then to ask if those rights are being violated in any conflict.
Pius XII teaches: "That perpetual privilege proper to man, by which every individual has a claim to the protection of his rights, and by which there is assigned to each a definite and particular sphere of rights, immune from all arbitrary attacks, is the logical consequence of the order of justice willed by God." Quoted by Pope John XXIII in Pacem in Terris ."Furthermore, whatever is opposed to life itself, such as any type of murder, genocide... whatever violates the integrity of the human person, such as mutilation, torments inflicted on body or mind, attempts to coerce the will itself; whatever insults human dignity, such as subhuman living conditions, arbitrary imprisonment, deportation, slavery, prostitution, the selling of women and children; as well as disgraceful working conditions...; all these things and others of their like are infamies indeed. They poison human society, but they do more harm to those who practice them than those who suffer from the injury. Moreover, they are a supreme dishonor to the Creator." Pastoral Constitution on the Modern World, Second Vatican Council. (Emphasis added.)
"No foundation... remains for any theory or practice that leads to discrimination between man and the man or people and people, so far as their human dignity and the rights flowing from it are concerned.
The Church reproves, as foreign to the mind of Christ, any discrimination against men or harassment of them because of their race, colour, condition of life, or religion." Declaration on the Relation of the Church to non-Christian Religions. [See this document from the second Vatican Council regarding Islam.]
The idea of a preventive war or preemptive action to avoid evil is regarded as inconsistant with Catholic teaching.
"We join with Pope John Paul in the conviction that war is not "inevitable" and that "war is always a defeat for humanity." This is not a matter of ends, but means. Our bishops' conference continues to question the moral legitimacy of any preemptive, unilateral use of military force to overthrow the government of Iraq. To permit preemptive or preventive uses of military force to overthrow threatening or hostile regimes would create deeply troubling moral and legal precedents. Based on the facts that are known, it is difficult to justify resort to war against Iraq, lacking clear and adequate evidence of an imminent attack of a grave nature or Iraq's involvement in the terrorist attacks of September 11. With the Holy See and many religious leaders throughout the world, we believe that resort to war would not meet the strict conditions in Catholic teaching for the use of military force. " Statement on Iraq, Most Reverend Wilton D. Gregory, President, United States Conference of Catholic Bishops, February 26, 2003. See also the statement of Cardianl Pio Laghi.
CCC1759 "An evil action cannot be justified by reference to a good intention" (cf St. Thomas Aquinas, Dec. praec. 6). the end does not justify the means." And see CCC 1756.