At a political level, many conflicts are simply contests for resources. For each side it is good that their wives and children should have enough to eat. Since there is not enough food available to feed both tribes, each feels justified in fighting to feed their families. In the age of extraordinary abundance in which we live today, wars for resources seem barbarous, but in the state of perpetual scarcity in which humanity has lived for most of its existence, wars were often a matter of he who wins gets to eat. Indeed, the development of states can be framed essentially as an arrangement for mutual defence of resources.
But in literary terms, this is not really about politics at all. There is a kind of literature which appeals to our inherent tribalism. There are the good guys (with whom we identify) and the bad guys. The story appeals to us by confirming that the good guys (us) are really good, that the bad guys (them) are really bad, and that the good guys (us) always win. This inherent human desire for easy moral justification and assurance of victory is strong and present in people of every political stripe. Donald Trump may not understand much about politics, but he understood this, and that was sufficient to get him elected. That story is extremely powerful and extremely popular, and when you depart from it you make your road to reader acceptance much more difficult.
Nonetheless, there is a branch of literature that does not follow this model. It is the branch that attempts to look at the human condition as it really is. One of the most basic things that writers in this tradition realize is that no one thinks of themselves as being on the side of evil. Everyone believes their cause is just. However selfish and cruel their behavior may appear to the rest of the world, they have fully justified their aims and their methods for themselves. They may, of course, become disillusioned with their country, their leaders, and their friends, but when they do, they feel justified in their decision to abandon them.
And when you step aside from the fray, you will often see that two people competing for the same thing are often much of a muchness. They both feel justified in pursuit of their goal, and neither may be particularly venal or cruel in their pursuit of it. And yet they will certainly take delight in the misfortunes of their rival, and will certainly be tempted, at least, to play dirty tricks on them. Neither is a saint; neither is a devil; both are sinners.
And that is an interesting thing to write and to read about, but it is a very different kind of story from the we-good-them-bad story. It does not simply have a different plot, it is interested in different things and appeals to a different audience.
So, a story of a war does not require a good or an evil side, but the focus of such a book must be different, it must focus not on the justification of Us and the vilification of Them, but on an honest and penetrating examination of what it is really like to be a human being in time of war.
A very fine example of this kind of book is All Quiet on the Western Front. The author, Erich Maria Remarque said of it "This book is to be neither an accusation nor a confession, and least of all an adventure, for death is not an adventure to those who stand face to face with it. It will try simply to tell of a generation of men who, even though they may have escaped (its) shells, were destroyed by the war." (Thanks Wikipedia!)