Why is war counted among the sins opposed to charity?
Because when war is unjust it is one of the greatest evils that can be inflicted on our neighbour.
Is it ever permissible to make war?
Yes, when there is a just cause, and no injustice is committed in the course of the war (XL. 1).
What is meant by a just cause?
By this is meant the hard necessity of making respected even by force of arms the essential rights among men, when these rights have been violated by a foreign nation which refuses to make reparation (XL. 1).
Is it then permitted to make war in this case only?
Yes (XL. 1).
Do those who fight in a just war, and who commit no act of injustice in the course of the war, perform an act of virtue?
Yes, they perform a great act of virtue, since they expose themselves to the greatest of perils for the welfare of their fellow-men or for the good of God.
Next - The Catechism of the Summa - SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - X. OF THE VICES OPPOSED TO CHARITY AND THEIR ACTS: OF HATRED; DISTASTE FOR SPIRITUAL THINGS AND LAZINESS; ENVY; DISCORD; STRIFE; SCHISM; WAR; QUARRELLING (THE DUEL); SEDITION; SCANDAL (I)