Catechism of the Catholic Church §2312 — excerpt
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."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: GS 79 # 4