How do I reconcile humans and demons?

Question

Humans and demons are one of many races within my fantasy world. These two races gave been going at it for a couple of hundred years.

No one remembers which side started it first. The humans, of course, believe that the demons started it first. The demons believe in the opposite, the humans started the war for the abundant resources within the demons’ lands.

So far, it seems that the religious leaders of the humans wish to continue this war against the demons just because of principle. The demons also wish to continue the war, although their peace faction is gaining support.

Though there were rumors, both sides never killed each other's civilians in cold blood. They follow the rules of war.

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The neutral side that wishes to unite these two is a group of humans equipped with sci-fi level technology. The reason for uniting the both of them (and the rest of this world’s races) is because they need to be stuffed into a spaceship and flung off into outer space. Their planet is going to experience a massive eruption the likes of which its never seen before. It’ll render the planet inhospitable to life.

Answer 1

You have answered your own question already.

When the fight is because of principle, a pragmatic approach can be very effective.

Look at Europe: because of principle (religion, nationalism, etc.) the major powers have fought each other for centuries. After the last two bloody wars where their populations have been decimated, they have realized that being friendly with each other could be a viable option, and so they tried it.

As you state, a huge threat is hanging on their heads on top of the several centuries of carnages already behind their shoulders, the neutral side just need to convince the less hot headed among the leaders of both sides that a truce would be in their best interest. In this way they could then work on convincing the other leaders.