[CaptainBubba]

The Horde is an alliance of convenience for all but the orcs, tauren, and trolls who are in truth allies. The Undead and Blood Elves hope to get mutual benifit from their alliance with the Horde but have no true allegiance to their allies.

Its not that hard to understand and fits fine with WC3 Lore. Factions and races re-alligned all the time in the game and alliances were formed for what people saw as their joint interests, so why would it not happen now?

Pretty much every dungeon with humanoids is a good example of Not Alliance but also Not Horde.