Complexities of War
The Lord of the Rings is, in large part, a war story. As such, it tells us a lot about Tolkien's attitude toward war. The only writing that might tell us more is his letters, especially those written during World War II. That attitude is as complex as his approach to good and evil. He loathed Hitler, but couldn't understand his fellow countrymen who gloated over the suffering of the German people after the war. He considered the fight against Hitler to be a necessary evil, but saw the continuing war in the Pacific after VE Day as a battle for American imperialism in which Britain shouldn't be involved. His antipathy toward "the machine" colored the way he felt about Christopher's service in the RAF; he saw bombers as the steeds of the Nazgûl and said that Christopher was living "among the orcs." True to his complex beliefs about good and evil, he firmly believed that there were some orcs on "our" side as well as "their" side, and that both sides were capable of committing evil deeds. He also shows us some of this complexity in his fiction, for example, "our" Rohirrim's treatment of the hill people, and Sam's meditation on the humanity of the Southron who falls near him.
After the Quest, Tolkien gives us a pacifist Frodo, but says in a letter that this was a personal choice by Frodo and not meant to be a pattern for everyone else (my own belief on this is that Frodo had experienced the futility of violent power so graphically that he saw no reason to use it). I think it's interesting that Frodo's pacifism was added in the book's revision, which tells me it's something Tolkien gave conscious thought to. Originally, Frodo slays Sharkey (not yet Saruman, but the head ruffian) in a sword duel after returning to the Shire. In the book as published, the duel becomes an exorcism and Frodo specifically refuses to kill Saruman - even after Saruman attempts to kill him - a refusal that leads directly to Saruman's statement that Frodo has 'grown much'. I do see this as an example of the book becoming more "Catholic and religious" in its revision. Although most Catholics aren't pacifists, the Church does recognize that stance as valid so showing us one character who takes it rounds things out. The change in the story also emphasizes that the battle is centrally spiritual.
Catholicism has the concept of a "just war". The way it's looked at has changed over the centuries and some stands taken in the Middle Ages wouldn't be taken today. As we'll talk about more in the section on inspiration, Catholic teaching says that the Holy Spirit continues to instruct us, so there's no contradiction in interpretations that change over time. (For example, the fact that the Church backed the Crusades doesn't mean she couldn't validly speak out against the Iraq war.)
The just war theory agrees that it can take only one side to make a war, as Éowyn says to the Warden of the Houses of Healing, if that one side puts others in the position of defending themselves - or of protecting those who aren't capable of self defense. When it comes to war in LotR, we see the forces of good following this dictum. In no case do they start a war of aggression; even the Scouring of the Shire is undertaken to defend the hobbits against invaders from outside. As complex as Tolkien's approach to good and evil is, seeking illegitimate power over others is invariably shown as negative.
The just war concept demands that the presumption is always for peace. Criteria have to be met to allow for the possibility of war. The following description of the criteria is the clearest I've found and is taken from The Challenge of Peace: God's Promise and Our Response, a pastoral letter written by the Catholic bishops of the United States in 1983. The bishops didn't come up with these ideas in 1983; they are part of the traditional Catholic thinking about just war. The following is all taken directly from the letter, with my own comments in brackets:
a) Just Cause: War is permissible only to confront "a real and certain danger," i.e., to protect innocent life, to preserve conditions necessary for decent human existence, and to secure basic human rights. [Note that the bishops use i.e. ("that is") rather than e.g. ("for example"). This is a complete list of permissible reasons.]
b) Competent Authority: In the Catholic tradition the right to use force has always been joined to the common good; war must be declared by those with responsibility for public order, not by private groups or individuals... Historically, the just-war tradition has been open to a "just revolution" position, recognizing that an oppressive government may lose its claim to legitimacy. [In LotR, we see this in the acts of kings deciding to go to war, as well as the Thain having the right to take control in the Shire.]
c) Comparative Justice: [This addresses] ...questions concerning the comparative justice of the positions of respective adversaries or enemies. In essence: which side is sufficiently "right" in a dispute, and are the values at stake critical enough to override the presumption against war?
d) Right Intention: Right intention is related to just cause -- war can be legitimately intended only for the reasons set forth above as a just cause. During the conflict, right intention means pursuit of peace and reconciliation, including avoiding unnecessarily destructive acts or imposing unreasonable conditions (e.g., unconditional surrender). [I think this latter point is seen in the treatment of those who surrender to the Lords of the West after the battle at the Black Gate.]
e) Last Resort: For resort to war to be justified, all peaceful alternatives must have been exhausted.
f) Probability of Success: This is a difficult criterion to apply, but its purpose is to prevent irrational resort to force or hopeless resistance when the outcome of either will clearly be disproportionate or futile. [For those who've seen the LotR-RotK movie, this will no doubt bring to mind Gimli's, "Almost certain death. Small chance of success. What are we waiting for?" As is made abundantly clear in the book and, I think, even in the movie, the success being hoped for is giving Frodo a chance to complete his quest, not defeating the forces of Mordor in open battle. The latter would certainly have been futile.]
g) Proportionality: ...the damage to be inflicted and the costs incurred by war must be proportionate to the good expected by taking up arms. Nor should judgments concerning proportionality be limited to the temporal order without regard to a spiritual dimension in terms of "damage," "cost," and "the good expected."
In summary, I think Tolkien's approach to war, as seen in the decisions of his characters on the side of good, is abundantly in line with traditional Catholic thinking on the matter.