Originally Posted by
Falrinn
Warcraft doesn't have to refer to the war between the factions, and more often then not it hasn't in WoW.
Burning Crusade: Main enemies were the Illidari and the Burning Legion. When the factions did interact, they tended to cooperate more then they fought.
Wrath of the Lich King: Faction war was featured in some spots, but in importance to the overall story came in fourth behind the war against the Lich King, the forces of Yogg-Saron, and the Nexus War.
Cataclylsm: Faction war showed up a lot in 1-60 content, but after that only really appeared in the Twilight Highlands. None of the patches featured the faction conflict.
MoP: Faction war was obstinately a central theme, but in practice we spent far more time fighting the Mantid, Mogu, and Sha. 5.1 was probably the only patch that featured a straight Horde vs. Alliance fight as a main theme. 5.2 had it a little, but in the much broader context of defeating the Thunder King. From 5.3 onward the Alliance and Horde were working together against an increasingly tyrannical Garrosh.
In practice most PvP content is tacked on for gameplay purposes and only vaguely justified by the story. Take Tol Barad for example, we spent the whole expansion fighting over the island on the vague idea that it's location was strategic for the war effort.