Contrary to the song by Edwin Starr, war is actually good for a lot of things. There are both good and bad aspects to a society affected by the fires of battle that might be hidden away, or never noticed with all the violent and destruction that is in plain sight.
Maoyuu Maou Yuusha is one of those anime that demonstrated the point that war, along with many other things, can't be put into two simple boxes of good and bad. Never is there a sharp contrast of black and white, it is gradually bended together into "50 shades of gray", not to be confused with the book.
Good Parts of War
Going into detail, from just the first episode of Maoyuu Maou Yuusha, we get a good amount of the positive aspects that comes from war; things like unifying a nation, by having a common enemy. This is very much an ironic statement, because you separate a non-specific group of people in order to bring together a better defined section of people; essentially, we divide to bring together.
Another result of war is there are more peaceful pockets of land that are never touched with the chaos of battle. Yet another irony of war times, you have this isolated, tranquil, and flourishing place, all the while being surrounded by the echoes of clashing swords.
This kind of nuance view on war isn't only exclusive to Maoyuu Maou Yuusha, I've seen a not as explicit version in Shinsekai Yori. This being further on in the series, there might be spoils; so if you don't want to be spoiled, skip the next section entirely.
Similar Case
Having finished episode 22 and waiting for the next episode as of writing this, the queerats and human are at the beginnings of a war. The culture of the "us and them" mentality, was establish way before queerats began their attack against the humans; so the humans villages were fairly peaceful while treating the queerats like dirt.
Having the queerats as an unequal species by default, put on a different angle of why the queerats would start any kind of war. Instead of winning and getting slaves from the losing side, like the fight between humans and demon in Maoyuu Maou Yuusha, queerats are trying to gain their freedom all the while imprisoning their own queens and human babies, so they can be used as fiend tools for the queerats victory.
Conquest
All this irony of inside of war, really shows the complexity of the subject of not having just two sides. You can't rightfully say that war is inherently evil, regardless of the amount people that think it is. It also isn't something that is good overall, but these kind of subjects are interesting to me whether it being inside an anime or in real life.
Sometimes we do need war in order to get to the goal of peace and from time to time, things might get worse by ending a war. Like most things this world, there isn't just black and white, we have to look closer and see all the grays in between.