Just war theory
In the 13th century Aquinas reflected in detail on peace and war. Aquinas was a Dominican friar and contemplated the teachings of the Bible on peace and war in combination with ideas from Aristotle, Plato, Socrates, Saint Augustine and other philosophers whose writings are part of the Western canon. Aquinas' views on war drew heavily on the Decretum Gratiani, a book the Italian monk Gratian had compiled with passages from the Bible. After its publication in the 12th century, the Decretum Gratiani had been republished with commentary from Pope Innocent IV and the Dominican friar Raymond of Penafort. Other significant influences on Aquinas just war theory were Alexander of Hales and Henry of Segusio.
In Summa Theologica Aquinas asserted that it is not always a sin to wage war, and he set out criteria for a just war. According to Aquinas, three requirements must be met. Firstly, the war must be waged upon the command of a rightful sovereign. Secondly, the war needs to be waged for just cause, on account of some wrong the attacked have committed. Thirdly, warriors must have the right intent, namely to promote good and to avoid evil. Aquinas came to the conclusion that a just war could be offensive and that injustice should not be tolerated so as to avoid war. Nevertheless, Aquinas argued that violence must only be used as a last resort. On the battlefield, violence was only justified to the extent it was necessary. Soldiers needed to avoid cruelty and a just war was limited by the conduct of just combatants. Aquinas argued that it was only in the pursuit of justice, that the good intention of a moral act could justify negative consequences, including the killing of the innocent during a war.

Renaissance and Christian Humanists: Various Renaissance humanists promoted Pacificist views. 

John Colet famously preached a Lenten sermon before Henry VIII, who was preparing for a war, quoting Cicero "Better an unjust peace rather than the justest war."
Erasmus of Rotterdam wrote numerous works on peace which criticized Just War theory as a smokescreen and added extra limitations, notably The Complaint of Peace and the Treatise on War (Dulce bellum inexpertis).
A leading humanist writer after the Reformation was legal theorist Hugo Grotius, whose De jura belli ac pacis re-considered Just War and fighting wars justly.

First World War: At the beginning of the First World War, a group of theologians in Germany published a manifesto that sought to justify the actions of the German government. At the British government's request, Randall Davidson, Archbishop of Canterbury, took the lead in collaborating with a large number of other religious leaders, including some with whom he had differed in the past, to write a rebuttal of the Germans' contentions. Both German and British theologians based themselves on the just war theory, each group seeking to prove that it applied to the war waged by its own side.

Contemporary Catholic doctrine: The just war doctrine of the Catholic Church found in the 1992 Catechism of the Catholic Church, in paragraph 2309, lists four strict conditions for "legitimate defense by military force:"

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 Compendium of the Social Doctrine of the Church elaborates on the just war doctrine in paragraphs 500 to 501, while citing the Charter of the United Nations:

If this responsibility justifies the possession of sufficient means to exercise this right to defense, States still have the obligation to do everything possible "to ensure that the conditions of peace exist, not only within their own territory but throughout the world". It is important to remember that "it is one thing to wage a war of self-defense; it is quite another to seek to impose domination on another nation. The possession of war potential does not justify the use of force for political or military objectives. Nor does the mere fact that war has unfortunately broken out mean that all is fair between the warring parties".
The Charter of the United Nations ... is based on a generalized prohibition of a recourse to force to resolve disputes between States, with the exception of two cases: legitimate defence and measures taken by the Security Council within the area of its responsibilities for maintaining peace. In every case, exercising the right to self-defence must respect "the traditional limits of necessity and proportionality".

Therefore, engaging in a preventive war without clear proof that an attack is imminent cannot fail to raise serious moral and juridical questions. International legitimacy for the use of armed force, on the basis of rigorous assessment and with well-founded motivations, can only be given by the decision of a competent body that identifies specific situations as threats to peace and authorizes an intrusion into the sphere of autonomy usually reserved to a State.
Pope John Paul II in an address to a group of soldiers said the following: Peace, as taught by Sacred Scripture and the experience of men itself, is more than just the absence of war. And the Christian is aware that on earth a human society that is completely and always peaceful is, unfortunately, an utopia and that the ideologies which present it as easily attainable only nourish vain hopes. The cause of peace will not go forward by denying the possibility and the obligation to defend it.

Russian Orthodox Church: The War and Peace section in the Basis of the Social Concept of the Russian Orthodox Church is crucial for understanding the Russian Orthodox Church's attitude towards war. The document offers criteria of distinguishing between an aggressive war, which is unacceptable, and a justified war, attributing the highest moral and sacred value of military acts of bravery to a true believer who participates in a justified war. Additionally, the document considers the just war criteria as developed in Western Christianity to be eligible for Russian Orthodoxy; therefore, the justified war theory in Western theology is also applicable to the Russian Orthodox Church.
In the same document, it is stated that wars have accompanied human history since the fall of man, and according to the gospel, they will continue to accompany it. While recognizing war as evil, the Russian Orthodox Church does not prohibit its members from participating in hostilities if there is the security of their neighbours and the restoration of trampled justice at stake. War is considered to be necessary but undesirable. It is also stated that the Russian Orthodox Church has had profound respect for soldiers who gave their lives to protect the life and security of their neighbours.

Just war tradition: The just war theory, propounded by the medieval Christian philosopher Thomas Aquinas, was developed further by legal scholars in the context of international law. Cardinal Cajetan, the jurist Francisco de Vitoria, the two Jesuit priests Luis de Molina and Francisco Suárez, as well as the humanist Hugo Grotius and the lawyer Luigi Taparelli were most influential in the formation of a just war tradition. The just war tradition, which was well established by the 19th century, found its practical application in the Hague Peace Conferences (1899 and 1907) and in the founding of the League of Nations in 1920. After the United States Congress declared war on Germany in 1917, Cardinal James Gibbons issued a letter that all Catholics were to support the war because "Our Lord Jesus Christ does not stand for peace at any price... If by Pacifism is meant the teaching that the use of force is never justifiable, then, however well meant, it is mistaken, and it is hurtful to the life of our country."
Armed conflicts such as the Spanish Civil War, World War II and the Cold War were, as a matter of course, judged according to the norms (as established in Aquinas' just war theory) by philosophers such as Jacques Maritain, Elizabeth Anscombe and John Finnis. Other scholars such as Robert L. Holmes cited a presumptive moral imperative prohibiting violence against all innocent people as the prima facie basis for questioning whether the norms of just war theories could adequately serve as a rational moral justification for military conflict within the nuclear age.
The first work dedicated specifically to just war was the 15th-century sermon De bellis justis of Stanisław of Skarbimierz (1360–1431), who justified war by the Kingdom of Poland against the Teutonic Knights. Francisco de Vitoria criticized the conquest of America by the Spanish conquistadors on the basis of just-war theory. With Alberico Gentili and Hugo Grotius, just war theory was replaced by international law theory, codified as a set of rules, which today still encompass the points commonly debated, with some modifications.
Just-war theorists combine a moral abhorrence towards war with a readiness to accept that war may sometimes be necessary. The criteria of the just-war tradition act as an aid in determining whether resorting to arms is morally permissible. Just-war theories aim "to distinguish between justifiable and unjustifiable uses of organized armed forces"; they attempt "to conceive of how the use of arms might be restrained, made more humane, and ultimately directed towards the aim of establishing lasting peace and justice". 
The just war tradition addresses the morality of the use of force in two parts: when it is right to resort to armed force (the concern of jus ad bellum) and what is acceptable in using such force (the concern of jus in bello). 
In 1869 the Russian military theorist Genrikh Antonovich Leer theorized on the advantages and potential benefits of war.