We are also the Alliance, if you prefer. Bit less punchy a title, I suppose, and "We are the Horde/Alliance" or "Alliance/Horde" didn't work at all for me.
What I mean by that statement is a touch difficult to express but I'll give it a shot. Basically, after five years of World of Warcraft, and 15 years of the Warcraft franchise in general, it can be easy to get caught up in the personalities and lore figures and lose sight of the fact that Garrosh Hellscream is not the Horde, that Varian Wyrnn is not the Alliance. The Horde and the Alliance are, effectively, people on your server playing the same game as you. They're people in the dungeon finder, the people you team up with in battlegrounds and fight in the arenas, they're your guildmates in raids and the people you PUG with. The story is one of the reasons I still play WoW, of course. I love the lore, I pore over announcements and scrape up all I can find about zones like Ulduar, I actually get giddy at big reveals for Gilneas. But in the end, what makes the game worth playing are the people we play it with, and it's those people who make up the Horde and the Alliance, not the NPCs.