Great Alliance

The Great Alliance is also the term used for the alliance between the Horde, the Human Expedition and night elves at the Battle of Mount Hyjal.

Birth of the alliance
After being guided to Kalimdor by Medivh, the Orcish Horde led by Thrall and the Alliance forces led by Jaina Proudmoore had come into conflict with each other and with the night elves of Ashenvale, led by Malfurion Stormrage and Tyrande Whisperwind. Acting as a peace broker, Medivh was able to unite the warring factions to defend the World Tree, Nordrassil, from the Burning Legion's assault, the army itself was forged and organized by Malfurion Stormrage, who led them as one force against the Burning Legion.

Legacy
Modern Azeroth is what it is now because of this alliance, if not for it's existence to stop the Burning Legion on it's assault, the planet would've been dominated by demons, also, the Burning Legion suffered massive losses on their defeat and most of the main command of the legion was gone, leaving it leaderless. The political scene inside the Alliance and the Horde suffered great influence of the Great Alliance as well, such as the Night Elves becoming members of the Alliance after being exposed to the outside world once again, the Orcish Horde and the humans from Theramore forged a non-aggression pact, and so on.

=Trivia=
 * The Great Alliance of Lordaeron was another name for the Alliance during the Second War, also known as the Alliance of Lordaeron.

Reference list
Gran Alianza