Note: This post was originally made on my Tumblr, but I figured I’d share it here as well.
I’ve seen this question bounce around a few times since the announcement of Battle For Azeroth. So I figured I’d do my share of clearing up a misconception.
The Horde and Alliance didn’t team up to fight the Burning Legion.
They tried. Namely the Assault on the Broken Shore.
It ended with the Horde retreating after their Warchief was fatally injured and the Alliance losing another King of Stormwind. After that, the joint efforts collapsed since the Alliance blamed the Horde for the death of Varian, and the Horde has pretty much run out of fucks to give about reaching out to an Alliance that constantly blames them for crap going wrong.
Without the Horde and Alliance willing to work together to fight the Burning Legion, the duty fell instead to the Class Orders to rally their ranks and push back the Legion. Hence why the Armies of Legionfall banner has the symbols of each of the class orders represented on it.
In short, the Alliance and Horde failed at teaming up and fell into their old hatreds while the Class Orders stepped up and joined forces under Khadgar and Illidan to stop the Legion’s invasion and ultimately assault Argus. Hence why the only faction leader present on Argus is Velen – who has a vested non-political interest in reclaiming his homeworld.
I might be wrong in this, but while the heroes of Azeroth who are aligned with the Horde or Alliance have often worked side by side I don’t think there are many times that the Alliance and Horde as factions have been politically united on something. In the Burning Cruside, it was much more of an effort driven by the Scryers and Aldor united as the Shattered Sun Offensive. In Wrath of the Lich King, the Horde and Alliance were still duking it out over Icecrown while the Ebon Blade and the Argent Crusade made headway into infiltrating the Citadel. In Catalcysm, Faction animosity actually grew in the wake of the struggle for resources after the near apocalypse which ultimately came to a head in the Mists of Pandaria. In Warlords of Draenor, the conflict and alliances between groups was much more centered on the native factions of Alternate Draenor with the Horde and Alliance not openly in conflict but just kind of helping things along for the locals, which gave way to the potential team up at the Broken Shore – where it hit the fan and set the stage for the faction war coming in Battle for Azeroth.
Update: Since originally posting this on Tumblr, I was able to think of a few occasions that the Horde and Alliance worked together for one reason or another. The first is the Battle of the Wrathgate where both the Alliance and the Horde fought against the Scourge as an attack on the Lich King’s back door. However, it’s debatable whether this constituted a formal action by the Horde since it was really only Saurfang the Younger’s forces that joined the assault and that the forces of Overlord Agmar where more aligned with the radical tactics of Garrosh Hellscream and likely would have no desire to join an Alliance assault, and the Forsaken of Venomspite… well… they had OTHER plans.
The one event I could think of that was a 100% combined Horde-Alliance effort was the Might of Kalimdor, a unified army made up of the Alliance’s legendary 7th Legion and the Horde’s mighty Kor’kron Guard that fought during the ten-hour Ahn’Qiraj War after the Scarab Wall was opened. This along with the War Effort that bolstered the Might of Kalimdor is probably the most clear cut example of the Horde and Alliance joining forces to confront a potentially world-ending threat (The return of the Qiraji after the War of the Shifting Sands nearly 1000 years before the first arrival of the Orcs).
Considering both times were led by a member of the Saurfang family, and even Varian was able to put his old grudges aside to let the elder Saurfang mourn the loss of his son at Icecrown Citadel, the High Overlord might be a good choice for an ambassadorship.