I might like to add you specify only that nukes are not a factor in this war, not that they are useless or no longer function.
Have you considered an environmental reason why any country detonating a nuke anywhere on the planet would have global cataclysmic consequences? You may prefer a less hand-wavium explanation which might be a topic of another question, should you choose to go down this route. But the idea that weapons of such destruction may cause an already collapsing environment to turn up the violence in natural disaster side effects and harm their allies could result in turn coating or unrecoverable political ties, not to mention general habitat and natural disaster fallout that could make living in many large areas impossible.
Not to incite some kind of debate about what nukes do to the weather patterns we currently have, but perhaps the scientific data behind unstable weather patterns globally were explored when idiotic notions such as nuking a hurricane were puked out by some fool, revealing how destructive the side effects of such an act would be. And despite that warning, said insipid fool somehow manages to overturn all authorities and actually does it, which causes the extreme backlash as warned, forever ruining the stability of large regions already prone to weather disaster, causing greater storms, and so on... until many years later where these storms are a normal thing, hugely devastating, your war starts to boil up, and an act of NATO or some other convention like the Geneva one which is often ignored set upon us standards to ... disarm... or some such... nukes with recent history as a guide that shows the planet as a whole cannot sustain any further detonations. Here's where your hand-wavium comes into place.
Yeah, we already have agreements of some kind, and yeah people claim they have dismantled some of the nuclear stockpile. But a lot of that could be thought of like an international policy deal that keeps economic stability in place for everyone, as well as makes what seems like a good impression on the inhabitants of the world. Never before has it become so evident that using them would bring upon the planet a global assurance of immanent death. The existence of fallout proved already that nuclear weapons leave a dent in the world none of us alive now will live to see the end of. Careless use, like what started the climate disaster era, have convinced everyone that it is clear these weapons cannot possibly be used to the end of peace. They will destroy us all if anyone uses them. And so your hand-wavium is that this unstable global condition is bad, but how bad and the precise details of all the whys and what abouts don't really get explored. Only that it is clear that use of nuclear weapons, and even the threats of nuclear disaster like Fukushima Dai-ichi eventually going full on melt down pose a threat of such significance that even in a state of war, everyone involved knows that using a nuke anywhere would be an act of suicide. Violent winds, storms, enormous shifts in conflicting temperature heat zones, and even such things like contaminated industry and global trade meaning everyone on the planet could be exposed to lethal levels of radiation simply because one nuke dispersing fallout amid a turbulent environment means everyone alive gets hit with the same bomb no matter where it is dropped.
So people refuse to accept it as a viable option, even in the worst of wars. Threatening such use causes social panic, catastrophe, riots, protests, and possibly even erupts smaller civil wars. Stock markets go nuts over the impending destruction and presence of large corporations involved in global trade. Destabilization is guaranteed. It is no longer a utility to demonstrate power. Possession and use of nuclear devices is now considered a top tier threat to the existence of humanity, and the warlords of the planet know this. As a result, they have not agreed upon anything in the form of a convention. But it is just known that to use one would be ending the war for everyone with no victories.
What use is winning a war if there is no planet left to live on?
And thus, nukes are not a factor in you war. Simply because in your world the warlords are convinced it would mean the end of everyone, which is not anyone's goal.