2304 Respect for and development of human life require peace. Peace is not merely the absence of war, and it is not limited to maintaining a balance of powers between adversaries. Peace cannot be attained on earth without safeguarding the goods of persons, free communication among men, respect for the dignity of persons and peoples, and the assiduous practice of fraternity. Peace is "the tranquillity of order." Peace is the work of justice and the effect of charity.
...
2308 All citizens and all governments are obliged to work for the avoidance of war.
However, "as long as the danger of war persists and there is no international authority with the necessary competence and power, governments cannot be denied the right of lawful self-defense, once all peace efforts have failed."
2309 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.
These are the traditional elements enumerated in what is called the "just war" doctrine.
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2313 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.
2314 "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.
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2317 Injustice, excessive economic or social inequalities, envy, distrust, and pride raging among men and nations constantly threaten peace and cause wars. Everything done to overcome these disorders contributes to building up peace and avoiding war.
https://www.vatican.va/archive/ENG0015/__P81.HTM

Steven Heine is professor of religious studies and history and director of Asian studies at Florida International University. Just a few of his many books and writings, as author or editor, include From Chinese Chan to Japanese Zen: A Remarkable Century of Transmission and Transformation (Oxford); Zen and Material Culture (Oxford); Chan Rhetoric of Uncertainty in the Blue Cliff Record: Sharpening the Sword at the Dragon's Gate (Oxford); Zen Koans (Hawaii); Like Cats and Dogs: Contesting the Mu Kōan in Zen Buddhism (Oxford); Dōgen and Sōtō Zen: New Perspectives (Oxford); Dōgen: Textual and Historical Studies (Oxford); Zen Skin, Zen Marrow: Will the Real Zen Buddhism Please Stand Up? (Oxford); Did Dōgen Go to China? What He Wrote and When He Wrote It (Oxford); Opening a Mountain: Kōans of Zen Masters (Oxford); Shifting Shape, Shaping Text: Philosophy and Folklore in the Fox Kōan (Hawaii); The Zen Poetry of Dōgen: Verses From the Mountain of Eternal Peace (Tuttle); Dōgen and the Kōan Tradition: A Tale of Two Shōbōgenzō Texts (SUNY); Existential and Ontological Dimensions of Time in Heidegger and Dōgen (SUNY); and The Zen Canon: Studies of Classic Zen Texts (Oxford) and more.

Dr. Heine's new book centers on Wansong Xingxiu (1166–1246), a pivotal figure in Caodong Chan/Sōtō Zen development, and a participant in the compilation of two influential Caodong/Sōtō Koan collections: Tongxuan's 100 Chan Questions (the main focus of Dr. Heine's book) which represents a crucial, although lesser-known, phase of Caodong Chan history centered on northern China, and the Shoyoroku (Book of Serenity) which is better known today and widely cherished by Sōtō Zen folks. Although Wansong lived during the time Dogen was in China, they likely never met in person because in very distant parts of China, and Dogen may not have had direct knowledge of either Koan collection during his lifetime. However, in his talk, Professor Heine will examine the historical and spiritual irony connecting these two pivotal figures in the development of Caodong/Sōtō thought and literature, illustrating his points with several key examples from their respective works.