I'm conceptualising a story involving two heroes who go on an epic journey and return bitter rivals etc. etc. and form their own forces in a sort of civil war.
My issue is that I want them to both be on the "good" side. How can I write a story in which both sides are "good"? I want to somehow make the characters each believe the other is evil, despite either of them being evil.
It's actually not an issue because "history is written by the victors", who often use their dominance to codify the evil of the vanquished. The vanquished may have been evil, but evil is a relative concept based on social mores, (despite what deolators would assert), and so, even engaging in acts that would be considered evil, it's a safe bet that most of the actors did not consider such actions evil.
But the depiction of the vanquished as evil is in no way universal. Certainly this was not the case in the Iliad of Homer.
In the Iliad, the Trojans are not presented as evil. Conversely, they are depicted as noble adversaries, with the subversive element that their dignity is greater than that of the Greeks, certainly Agamemnon, Menelaus and Odysseus. It's not insignificant that the epic ends with the line "And thus was the funeral of Hector, breaker of horses." Hector unquestionably possessed the greatest dignity of all of the heroes of the epic, Greek or Trojan. He neither instigated the war nor desired it, but must fight in it regardless. He is blameless and esteemed, and makes the greatest sacrifice for his country.
I'd say that books where the bad guys are evil is a type of shorthand, a technique that allows the author not to have fully explicate their point of view in simplistic narrative.
Another way to contextualize this is per the advice of skilled actors in portraying villains, and may be condensed into:
Everyone is the hero of their own story
Actors who don't use this technique generally deliver one-dimensional performances.
I'd also use postmodernism in presenting the conflict. The Trojan war is generally held to be a result of the "abduction" of Helen by Paris. The reality is that Troy held an incredibly powerful strategic position, and dominated trade between the Euxine (Black) Sea and the Aegean. If the Trojan war actually occurred, it was almost certainly about economic control of the region.
Even the pretext of Helen is illuminating. WWI has been cast by respected historians as a war fought for no real reason, other than Europe has become a power keg in the wake of the success of Fredrick of Prussia's regimental system. (Critiques of Keegan rely on a romanticized view of warfare as an "extension of politics by other means", but should more properly be cast as an "extension of economic and political hegemony by other means.")
WWI can be viewed as an unfortunate outcome of inciting event in which actors in the war were obligated by treaty to take part.
"Good" and "Evil" are, almost always, very relative terms.
While wars are invariably (if there's some counter-example I cannot think of it right away) fought for economic reasons fighters are usually driven by some compelling inner moral reason.
[I know I'm oversimplifying, but otherwise this answer would become a long treatise.]
Your "heroes" fall, most likely, in the "fighters" category and thus they are motivated by sentiment more than pure greed.
This makes very easy to achieve what you ask; there are very many different "views of the World" that are irreconcilable even with the best intentions.
One example for all: dichotomy between "freedom" and "security". Both are seen as "good things", but different persons will give precedence to one or the other. When it comes to a situation where the two things come to conflict each of us will side according to his/her inclination, but there's no "evil" or "good" side (even if participants would think otherwise).
In this framework each of your heroes could become champion of one of these "world views", perhaps one advocating individual freedom (roaming freely in the prairie hunting wild animals and hoarding cattle) while the other tries to build a more organized and secure country (perhaps with a guild/caste system... and barbed wires limiting the prairie).
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!)
I would look for a compelling moral conundrum that serves as the climax to the epic journey, and the very core of the schism between the two.
All you really need is an act with no clear right/wrong, and I tend to think that something that can be split according to the logical solution and the emotional solution should resonate.
For example: Hero A meets and falls in love with Love Interest Z. At some point, Love Interest Z becomes a host for Demon/Parasite/Entity Y, which has the power to destroy the world. Both Hero A and Hero B realise this. Being an objective outsider, Hero B acts to kill Love Interest Z, nullifying Entity Y and preventing the destruction of the world. Hero A now hates Hero B, believing their must have been a non-fatal solution.
That's probably too narrow a scale, but it might get you thinking. If Love Interest Z is a beloved public figure, than perhaps that's enough to escalate to a civil war. If not, maybe consider Love Interest Z as an entire city of people - quarantined and annihilated for the greater good. Metrocide (is that a thing?) seems pretty evil, but if it's the alternative to omnicide? Well...
IMHO opinion both sides in most wars are evil, because they are willing to fight wars.
In your specific plot where the 2 characters go on a quest and return as bitter rivals, perhaps someone has promised a great reward or rewards to the one who fulfills the quest. After the Quest the reward or rewards may actually be granted to one or the other, or both, nor neither, or to a third party and neither of the 2, but each of the 2 believes he and he alone deserves it. Thus each raises a rebellion to take the reward for himself and to deny it to the other.
The 2 may be friends at the start of the quest but each may be ambitious enough to start sabotaging the other at first in minor ways and later in major ways. Each correctly views the sabotage of the other as treason against the goal of the quest but incorrectly views all his sabotage of the other as justified reactions to the the other's acts of sabotage.
And the first acts of sabotage may be psychological attempts to undermine the confidence of the other, following friendly insults that get less and less friendly, and thus the reader will be unsure when friendly rivalry turned unfriendly and who made the first sabotage attempt.
Each of the 2 main characters may have followers on the quest and the followers may be less ethical than the main characters. Each group of followers may do something that put their leader in the position of having to either let the followers be severely punished or let them get away with wrongdoing.
And those situations will be examples of each of the 2 main characters having to chose between 2 or more evil courses with no good course available. Each of the main characters may see the evil decisions of the other main character as examples of his unworthiness, while defending his own evil decisions as the least evil he could have made under the circumstances.
And if the 2 main characters suffer enough during parts of the quest, their thinking may become impaired and they may each come to hate the other for irrational reasons.
For example, during the Great Sioux War of 1876 General Mackenzie commanded the cavalry in General Crook's fall campaign. Mackenzie surprised the main winter camp of the Northern Cheyenne in November, and the Cheyenne were driven to a rocky mountainside while their camp was destroyed. The Cheyenne were shocked to see Sioux, their allies, among the scouts of the soldiers, and bitterly accused them of betrayal. Unable to dislodge the Cheyenne from their position, Mackenzie sent for the infantry, but the Cheyenne left for a long and bitter march to find the camp of Crazy Horse before the infantry arrived.
Crazy Horse's people welcomed the Cheyenne when they arrived at his camp but eventually they could no longer fed the Cheyenne and themselves. The Cheyenne quarreled with the Sioux and headed for the reservations to make peace, blaming the Sioux for their troubles. In the spring of 1877 General Crook sent messengers to the hostiles offering terms, and listing the forces he would lead against them if they refused, including many northern Cheyenne warriors. Most of the hostiles came to the reservation to make peace.
Meanwhile, back in November 1876, Mackenzie was depressed by his failure to capture the Cheyenne, so much that Colonel Dodge the infantry commander watched him carefully for any sign of attempted suicide. Perhaps the bitter weather kept Mackenzie from thinking any clearer than the Northern Cheyennes.
Personally, First thing that comes to mind about Your question are events from PSX game Suikoden 2, where the Young soldier from one side, in order to stop the war and all the deaths, betrays and kills their lord, defects to other side, gets promoted, kills the king and becomes the ruler himself... however he soon realizes that even he cannot stop the war without his country surrendering, which he cannot do, so he settles the outcome by dueling ( and losing to) his best friend, who is now the leader of opposite side...
so you see, there really is no good or bad side here, for while he did do bad things, they were intended for greater good for all
Hope this helps at least a little
Wars are always fought by two groups who see themselves as good and the other guys as evil, because if you're not the good guys why are you contesting the issue and if you're killing good people you're not the good guys. You need to show that both sides of the conflict are people and have the frailties and failings of people, but also the compassion and the empathy, in order for your audience to appreciate that these are sides that are equally evil and depraved but also equally good and decent, "no man is all of one thing". The characters clearly focus and dwell within themselves on the crimes, perceived or actual, of each other and feel that their former friend and ally has "gone to the dark" so to speak. I'd use the mutual hostility bit stemming from 1. a mutual misunderstanding like the Battle of Koom Valley from Discworld remembered as being where the Dwarves ambushed the Trolls who were ambushing them, when in reality two peace envoys got lost and killed by the nature of the valley itself. Or 2. external interference either deliberate or accidental, this can come in many forms, from something as direct as an assassination to something as innocuous as new trade goods that disrupt the existing balance of power.
I can see why you'd think it's hard to avoid black and white morality in a war story, but there are many alternative outcomes. Perhaps you'd prefer black and gray, black and black, grey and gray (note the use of UK and US spellings), white and grey or white and white. Whichever you want, you might benefit from aping the techniques other authors have used to achieve it.
We can take a look at the X-men arc. You can argue that both sides are "good". They are both trying to achieve the same thing... freedom for mutants to live happily in the world. The difference? One side wanted to co-exist with non-mutants while the other side wanted to kill off all non-mutants. This difference of values causes 2 best friends (professor X and Magneto) to rival and go at each other. The reader though, is left with siding with professor X due to this difference in values even though you can empathize with Magneto's point of view.
As the opening line in the "Sith Creed" from Star Wars goes:
Peace is a lie, There is only passion.
As long as people are passionate about about their ideals that conflict other ideals, there is always going to be conflict. Each side is the "good side" from their relative point of view. You don't need to make someone evil to be a conflicting side, but ultimately one side will be the "bad" side based on the ideals they chose.
A lot of time in Anime, they play around with the idea of having 2 good sides conflict with each other. You as the watcher are thinking... why are these 2 fighting... they both want the same thing and are fighting for the same thing. All it takes is a slight misunderstanding. Someone being in the wrong spot at the wrong time (aka at the scene of a murder hovered over the body of a dead friend as the other friend walks in on the scene) and then that person becomes hated due to circumstance.
This is definitely feasible to do and ultimately one of the easiest ways is to give them both the same end goals but with differing ideals/values/process to achieve. This then would cause conflict between 2 "good" sides without 1 being evil but have both sides hate each other because they think each other's way is not right.
You use "designated heroes and villains" to speak in TVTropes. The "good" side of the war may fight for freedom and tolerance, but they are almost all made up of jerks and thugs. Meanwhile, the "bad side" are sympathetic in so far as why they got into the war... but their actions are horrifying.
I highly recommend Animorphs in which the Racist Xenophobic Military Junta Alien Race were the "Good Aliens" and the Democratic Anti-Autocratic Xenophillic (technically speaking) Alien race were the "Bad Aliens". Of course, you have to realize that the later case, their government permits slavery (brain parasites), but they still enjoy diversity among their population and rank did not correlate to which bodies they could infest. In the former case, the do have a point in that brain puppeting is not right.
Another good exploration is in Deep Space 9, specifically the episode Rocks And Shoals. Here, the physical threat and the intellectual threat are not the same source. The enemy soldiers are a physical threat, but in a situation where they must cooperate, Captain Sisko trusts them more then there political officer, who holds the command of the rest of the troops (Kivan, who is quite open to Sisko that he intends to betray his troops to further his own end). In the society of the enemy, the soldiers are devoutly loyal to the political officers and even though the commanding officer of the troops is well aware Kivan is marching him to his death, he is still going to go through with it because of his loyalty. This culminates in a situation where Sisko's men and the enemy troops are facing off, but the episode takes pains to show that Sisko is not a hero and the enemy troops are not villains... they are both opposing forces in a war that are forced to kill or be killed.
Another good dynamic to look at is the one between Dinobot and Rattrap in Transformers: Beast Wars. Dinobot betrays the Predicons because Megatron was incompetent in his leadership and dishonorable in handling that accusation and goes to the Maximals. Rattrap, probably the most committed member of the team to the Maximals, immediately suspects that Dinobot is up to something and this informs a great deal of their interaction. The crux of this relies on both having fundamentally opposing ideologies over loyalty and morality. Dinobot is devoted to the Predacon Ideas of Honor (For the cause, there must be a clear line we will not cross). Thus, when Megatron is believed to be without Honor, Dinobot believes it is a line he cannot cross and throws in his lot with the Maximals. Meanwhile, Rattrap is devoted to the Maximal Cause (there is nothing I will not do if it enables the Maximals to win the engagement). We do see Rattrap "betray" the Maximals, but in both instances, he believes that by betraying them, the Maximals will gain an advantage in the long run. While the pair have argue, it can be see that both see in the other their ideals without conflict. Rattrap is not proud of his dirty fighting and actions that run counter to Maximal ideology and admires Dinobot for his devotion to moral fortitude ("You may be a slag spewing saurian, but at least we know where you stand"). Meanwhile, Dinobot wishes he could show the same devotion to the Predicons that Rattrap shows to the Maximals... but that would mean compromising what he personally stands for. ("Upwind of you by preference") ("My Destiny is my own. And yet, how ironic: I have no choice. I am a warrior still. Let the battle be joined.")
The best way to do go about this is that each side in this conflicts looks at the other and sees a reflection of their own worst features celebrated as a positive feature. Their arguments should be counter to each other not because they are black and white, but because they are blue and orange. They do not mesh with the logic of the others and rely on positives.
You may want to take an example from wars in history that had no good and bad guys, or even read the story from the "evil" perspective to find what complex motivations could be held.
I think the best approach, if you want both sides to be "good" is to start the war due to some misunderstanding. Imagine, for example, a Russian nuclear missile accidentally hitting a large US city, with the subsequent retaliation. WWI was also started by a series of mistakes in the diplomacy process. If you didn't mind portraing both sides as "evil", then you can have two evil empires fighting each other for world domination and narrate the story from a neutral perspective (or even from that of a resisting minority) Unfortunatelly, your case is a bit more complicated.
Let's try a few example plots:
Researchers from Nation A (a global superpower) discover that the effect of global warming is about to get out of control in the next few years. They show their conclusions to the world and their army starts taking action against all polluting industries, both home and abroad. Workers from Nation B (a very relevant country, but not a world leader like Nation A) lose their jobs, children starve and die from diseases on the streets. When soldiers from Nation A arrive at some given farm well into the borders of B, and want to confiscate their tractors and machinery, Nation B forces show resistence. After a few failed attempts for diplomacy, war breaks out.
Some previously unknown land is discovered (almost) simultaneously by explorers from two (or more!) countries. They both genuinely believe they have the right to settle there. After failing attempts to diplomacy, war breaks out.
There is a region (R) in country (C) that has some cultural particularities (like language, traditions...) Country C has modernized a lot but R is mostly composed of rural areas that are still stuck in the past. In modern times, there is some region R tradition (for neither side to appear as evil, you should choose an issue where not everybody in the real world would agree, and most people would have doubts about it) that is seen by C as immoral, so the government passes a law to ban it. R declares independence.
Let's go back to Nation A and Nation B from plot 1, but there is no global warming here. Instead, Nation A is developing some very powerful artificial intelligence (let's say, to try to work out a cure for some disease). However, Nation B's intelligence gets the false information that Nation A actually intends to use this technology to force the world into submission. The reader never gets to learn the truth, or maybe does only at the very end, but Nation B tries to strike first and launches a desperate attack, or, maybe, sabotage attempts fail and it's A instead who takes military actions
I hope some of these can inspire you into something that suits your case
