Two things: first, let me copy/paste something I said that you may have missed in my admittedly long spiel:
I would be fine with that as the basis for a war, because the undead have apparently gone batshit crazy (or were crazy to being with ;-)) and are being evil, making it reasonable for the Alliance to intervene.
Secondly, I am ALL about the war in World of Warcraft. You name it, war against Deathwing, war against the naga, war against the Scourge, war against the Burning Legion... there are plenty of monsters and villains to go to war against, and I am THERE. There's no point in warring against guys who are just as noble and heroic as you, and there's much nastier things out there waiting to kill us. Those are the things I'm eager to fight. So no, no Peacecraft for me, make no mistake.
A foe ripping the world asunder? Like Deathwing? Seems like a good time to get priorities straight. ;-) And while the Horde isn't exactly allies with the Alliance, they ARE allies with Theramore (Jaina and Thrall are particularly close personal friends... and frankly, if they don't hook up as the game draws to a close, I'll be really sad ;_
Unfortunately, game mechanics reduces Theramore to another Alli town that aggros Horde, despite it making more sense as a neutral town.
No, I'm going to get together with my neighbour and find a way to make more bread. We can accomplish so much more by working together than working alone. That's what WC3, the Shattered Sun Offensive and the Argent Crusade/Knights of the Ebon Blade are all about.
I would grant you this if the war was being pushed solely as a war of convenience and resources. If the general feeling was, "Look, you guys might be cool and all, but there just is not enough to go around," I'd be upset (for the above reason) but it would at least make sense. But they seem to be getting all RAWR KILL THE EVIL ALLI/HORDE PIGS, which makes no sense. For two expansions, Horde and Alliance have worked in close proximity. They KNOW that the other side are not a bunch of mustache twirling villains (well, Undead aside... they're pretty darned villainous). It makes no sense to still dislike each other when they surely recognize by now that the other side are not monsters.
Again, excepting Undead. I acknowledge that peace between the undead and the Alliance is probably slightly less likely than Deathwing appearing at the SW auction house and announcing, "OH I SAY DEAR BOY, may I have some Sulfuron Ingots?" Actually, that would be one of the only other ways I could see the war making sense, with less focus on Varian's "RAWR I HATE ORCS DESPITE THIS ONE BEING PERFECTLY NICE" and more on, "These undead are EVIL, and no matter how much I respect Thrall's loyalty in standing behind them... yeah, no, not happening, die in a fire."
Again, where the hell is all this World of Peacecraft coming from? THERE'S A BIG FARKING DRAGON DESTROYING THE WORLD. An Alliance and Horde army rising up against him? Now THAT is a war that will kick ass! It would be like the Wrathgate times eleven. Screw peace; a war like that would be amazing! LOKTAR OGAR!