I forget who said this but: "Nations think twice before making aggression when there is mutual fear."
Irreconcileable differences don't exist said the hunter to the deer.
Mutual Misunderstanding: this is to be expected. Let me use ethnic groups in this case. No matter how much time one person from one ethnic group (A) spends with another (B), there will still be things that they (A) will never understand, and vice versa.
Don't jump to conclusions & use false info: Info sources are not always correct, and info is always subject to perception and interpretation.
Econ pressure: I've noticed that many times after wars are over, economic prosperity ensues. War brings about a need to become more technologically advanced than your enemy, and eventually this technology filters down to the civilian sector. Nearly all fields get technological advancements during times of war. And the economy responds to the massive government spending like the heart to epinephrine.
For your last statement, you must realize that some people are dead set in their ways. You may offer them the world and they could still tell you to well, do you know what to yourself. I grant you if people weren't so damned determined to keep their ways as they are, there would be less war.
I'm not trying to put a halo over my head, as I will never be a pacifist. Sure peace is great, but I like to mix it up.