war theory today is a composite that has evolved from ideas developed by various religious figures. In the 5th century, St. Augustine discussed in City of God the circumstances under which killing could be justified and empires legitimately expanded. In the 13th century, St. Thomas Aquinas laid out a more elaborate just war doctrine in his Summa Theologica. He wrote that three conditions were necessary to make a war just: it must be ordered by a competent authority; the cause must be just; and the combatants must have "a right intention, so that they intend the advancement of good, or the avoidance of evil.