Here's something to think about. It's from the official Catechism of the Catholic Church; the numbers are the paragraphs.

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.

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."

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." 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.