Catholic Church Teaching on Just War Doctrine:
III. SAFEGUARDING PEACE
Peace
2302 By recalling the commandment, “You shall not kill,” [Mt. 5:21] our Lord asked for peace of heart and denounced murderous anger and hatred as immoral.
Anger is a desire for revenge. “To desire vengeance in order to do evil to someone who should be punished is illicit,” but it is praiseworthy to impose restitution “to correct vices and maintain justice.” [St. Thomas Aquinas, ST II-II q158, a1 ad3] If anger reaches the point of a deliberate desire to kill or seriously wound a neighbor, it is gravely against charity; it is a mortal sin. The Lord says, “Everyone who is angry with his brother shall be liable to judgment.” [Mt. 5:22]
2303 Deliberate hatred is contrary to charity. Hatred of the  neighbor is a sin when one deliberately wishes him evil. Hatred of the neighbor is a grave sin when one deliberately desires him grave harm.  “But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven.” [Mt. 5:44-45]
2304 Respect for and development of human life require peace.  Peace is not merely the absence of war, and it is not limited to maintaining a balance of powers between adversaries. Peace cannot be attained on earth without safeguarding the goods of persons, free communication among men, respect for the dignity of persons and peoples, and the assiduous practice of fraternity. Peace is “the tranquility of order.” [St. Augustine, City of God 19, 13,1]  Peace is the work of justice  and the effect of charity. [Cf. Is. 32:17; cf. Vatican II, Gaudium et spes #78, 1-2]
2305 Earthly peace is the image and fruit of the peace of Christ, the messianic “Prince of Peace.” [Is. 9:5] By the blood of his Cross, “in his own person he killed the hostility,” [Eph. 2:16; cf. Col. 1:20-22] he reconciled men with God  and made his Church the sacrament of the unity of the human race and of its union with God. “He is our peace.” [Eph. 2:14] He has declared: “Blessed are the peacemakers.” [Mt. 5:9]
2306 Those who renounce violence and bloodshed and, in order to safeguard human rights, make use of those means of defense available to the weakest, bear witness to evangelical charity, Â provided they do so without harming the rights and obligations of other men and societies. They bear legitimate witness to the gravity of the physical and moral risks of recourse to violence, with all its destruction and death. [Cf. Vatican II, Gaudium et spes 78, 5]
Avoiding war
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. [Cf. Vatican II, Gaudium et spes 81, 4] 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.” [Cf. Vatican II, Gaudium et spes 79, 4]
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 modern means of destruction weighs very heavily in evaluating this condition.
These are the traditional elements lasting, grave, and certain; enumerated in what is called the “just war” doctrine. The evaluation of these conditions for moral legitimacy belongs to the prudential judgement 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.[Cf. Vatican II, Gaudium et spes 79, 5]
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. [Cf. Vatican II, Gaudium et spes 79, 3] 2312 The Church and human reason both assert the permanent validity of the moral law during armed conflict. “The mere fact that war has regrettably broken out does not mean that everything becomes licit between the warring parties.” [Cf. Vatican II, Gaudium et spes 79, 4]
2313 Non-combatants, wounded soldiers, and prisoners must be respected and treated humanely.
Actions deliberately contrary to the law of nations and to its universal principles are crimes, as are the orders that command such actions. Blind obedience does not suffice to excuse those who carry them out. Thus the extermination of a people, nation, or ethnic minority must be condemned as a mortal sin. One is morally  bound to resist orders that command genocide.
2314 “Every act of war directed to the indiscriminate destruction of whole cities or vast areas with their inhabitants is a crime against God and man, which merits firm and unequivocal condemnation.” [Cf. Vatican II, Gaudium et spes 80, 3]A danger of modern warfare is that it provides the opportunity to those who possess modern scientific weapons – especially atomic, biological, or chemical weapons – to commit such crimes.
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; [Pope Paul VI, Populorum Progressio 53] 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:
Insofar as men are sinners, the threat of war hangs over them and will so continue until Christ comes again; but insofar as they can vanquish sin by coming together in charity, violence itself will be vanquished and these words will be fulfilled: “they shall beat their swords into plowshares, and their spears into pruning hooks; nation shall not lift up sword against nation, neither shall they learn war any more.” [Cf. Vatican II, Gaudium et spes 78, 6; cf. Is. 2:4]
About Iran and nuclear weapons:
They’ve been at war with us since 1979. The IEDs that have killed so many soldiers, they are manufactured in Iran. Iran is not any other country. It is a country that is ruled by the equivalent of al Qaida on top of this country. They are a radical theocracy. The principle virtue of the Islamic Republic of Iran according to President Ahmadinejad is not freedom or opportunity, it’s martyrdom. The idea, Ron, that mutual-assured destruction, like the policy during the cold war with the Soviet Union, would work on Iran when their principle virtue is martyrdom, mutual-assured destruction with respect to Iran would not be any kind of idea of preventing a war, it would be an inducement to war. This is what their objective is, their objective is to in fact to create a calamity. This is what their theology teaches. They believe that it is their mission to take on the West. They don’t hate us because [of] what we do or the policies we have; they hate us because of who we are and what we believe in. And we need to make sure that they do not have a nuclear weapon, and we need to be working with the state of Israel right now. We need to use covert activities. And we need to plan a strike against their facilities and say to them that if you do not open up those facilities and close them down, we will close them down for you. ~ Rick Santorum F.ox News Debate Souix City Iowa 15 December 2011
RON PAUL: I am running with the American people because I have a much better policy. There are a lot of people at the UN who say that Iran will not have a weapon. It is no different than 2003. There are war propagandas going on and it’s another Iraq. We are going to overreact and bomb Iran. The head of Israeli security says it would make no sense to take Iran out because they might be having a weapon. There is no evidence that they have a bomb. If we lived through the cold war we need to sit back and think and not jump the gun. That’s how we got involved in the useless war in Iraq and lost so much. All we are doing is promoting their desire to have one. They are surrounded for geopolitical reasons and they have a desire to get a lot more respect. Look what we did to Libya. We talked them out of nuclear weapons and then we killed them. We have to get things in a proper context and we don’t need another war.
CAVUTO: Iran is talking of closing Strait of Hormuz.
RON PAUL: All we talk about is when the West is going to bomb Iran. So they are saying that if we bomb them we will close the Strait. It would be an economic calamity to take all the oil out of Europe. We need to approach this differently. We need to use a little diplomacy once in a while. F.ox News Debate Souix City Iowa 15 December 2011
Just War Doctrine retrieved from
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F.ox News Debate Souix City Iowa 16 December 2012