The Catechism of the Catholic Church says:
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 (CCC 2313).
“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 2314).
Paragraph 2314 of the CCC quotes the Vatican II document Gaudium et Spes:
Any act of war aimed indiscriminately at the destruction of entire cities of extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation (GS 80.4).
Gaudium et Spes goes on to state:
The unique hazard of modern warfare consists in this: it provides those who possess modem scientific weapons with a kind of occasion for perpetrating just such abominations; moreover, through a certain inexorable chain of events, it can catapult men into the most atrocious decisions. That such may never truly happen in the future, the bishops of the whole world gathered together, beg all men, especially government officials and military leaders, to give unremitting thought to their gigantic responsibility before God and the entire human race (GS 80.5)
At the entrance to the Peace Memorial Museum in the Hiroshima Memorial Peace Park in Hiroshima, Japan, are inscribed these words of John Paul II:
War is the work of man. War is destruction of human life. War is death. To remember the past is to commit oneself to the future. To remember Hiroshima is to abhor nuclear war, to remember Hiroshima is to commit oneself to peace.
Pacem in Terris** (Latin, “Peace on earth”) by Pope John XXIII
Just War Doctrine by Catholic Answers
Karl Keating’s E-Letter (August 3, 2004)