What is Just War Doctrine?

The Catholic Church is the first to testify to the evils and injustices that accompany all war.

However, the Church also recognizes that as long as the danger of war persists, governments cannot be denied the right of lawful self-defense, once all peace efforts have failed.

As such, the Church crystallized thousands of years of learning into the "Just War" Doctrine to inform the decisions and actions of nations around the world.

CCC 2309 states:

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.)

Almost every conflict in history fails to meet the conditions listed above. But the Catechism goes on to also discuss the limits for engaging once in a war:

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."
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. 

"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. 

- CCC 2312-2314

And here, any remaining wars that might have passed the criteria for entering into war, lose their status for a "just war" in how they were carried out.

To be clear, not a single war in history could be labeled as a "just war".

Keep Pursuing,

Additional Resources

Catholic Answers explores the "Just War" Doctrine in more detail