What does the Church say about peace and war? 

By: Miscellaneous | Source: Anthology of texts carried out by the Commission of Justice and Peace 

330. Peace is not the mere absence of war, nor is it reduced to the single equilibrium of the opposing forces, nor does it arise from a despotic hegemony, but with all accuracy and propriety it is called the work of righteousness (Is 32, 7). It is the fruit of the order planted in the human society by its Divine Founder, and that the men, thirsty always of more perfect justice, have to carry out. The common good of the human race is governed primarily by the eternal law, but in its specific demands, during the course of time, it is committed to continual changes; that's why peace is never a thing of the whole, but a perpetual chore. Given the fragility of the human will, wounded by sin, the care for peace claims of each one constant self-control and vigilance on the part of the legitimate authority. 

This, however, is not enough. This peace on earth cannot be achieved without securing the good of people and spontaneous communication among men of their riches of intellectual and spiritual order. It is necessary for the firm purpose of respecting other men and people, as well as their dignity, and the passionate exercise of fraternity in order to build peace. Thus, peace is also the fruit of love, which surpasses everything that justice can do. Peace on earth, born from love of neighbor, is the image and effect of the peace of Christ, which comes from God the Father. Indeed, the incarnated Son, Prince of peace, has reconciled with God all men through his cross, and, reconstituting in one single people and one body the unity of mankind, has given death to hatred in his flesh and , after the triumph of his resurrection, has infused the spirit of love into the hearts of men. 

Therefore, it is called insistently the attention of all Christians so that, living with sincerity in charity (Efe 4, 15), join with the peaceful men to implore and establish peace. Moved by the same Spirit, we cannot fail to praise those who, giving up violence in the demand for their rights, resort to the means of defense, which, on the other hand, are within reach even of the weakest, provided that this is possible without injury to the Rights and obligations of others or of society. 
(Gaudium et Spes, N. 78) 

331. Respect and the development of human life demand peace. Peace is not only the absence of war and is not limited to securing the balance of adverse forces. Peace cannot be reached on earth, without the safeguarding of people's property, free communication between human beings, respect for the dignity of people and peoples, the assiduous practice of fraternity. It is the "tranquility of order" (Saint August, of Civ. Dei, IX. 13.1). It is the work of justice and the effect of charity. 
(CIC, N. 2304)

 332. Injustices, excessive inequalities of economic or social order, envy, distrust, and pride, which exist between men and nations, constantly threaten peace and cause wars. All that is done to overcome these disorders contributes to build peace and avoid war: to the extent that men are sinners, threaten them and threaten them until the coming of Christ, the danger of war; to the extent that, united by charity, they overcome sin, violence is also overcome until the word is fulfilled: "From their swords they will forge plows and their pruning spears. No nation will raise the sword against another and no longer be trained for combat" (GS, N. 78; cf. Is 2, 4). 
(CIC, N. 2317) 

333. Non-combatants, wounded soldiers, and prisoners must be respected and treated with humanity. Actions that are deliberately contrary to the rights of people and their universal principles, as well as the provisions that order them, are crimes. Blind obedience is not enough to excuse those who submit to it. Thus, the extermination of a people, of a nation, or an ethnic minority must be condemned as a mortal sin. There is a moral obligation to disobey those decisions that order genocide. 
(CIC, N. 2313)