http://www.vatican.va/archive/ccc_css/archive/catechism/p3s2c2a5.htm#2307 Catechism of the catholic Church
PART THREE
LIFE IN CHRIST
SECTION TWO
THE TEN COMMANDMENTS
CHAPTER TWO
"YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF"
ARTICLE 5
THE FIFTH COMMANDMENT
III. SAFEGUARDING PEACE
Peace
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.105
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."106
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.107