For much of Azeroth's prehistory, the Old Gods ruled Azeroth along with their Elemental Lieutenants. Deep within Azeroth's core lies a world-soul, a nascent, sleeping Titan that, when awakened, could be extremely powerful. Its echoing dreams reached out to the Titan Aggramar, who then traveled with the rest of the Pantheon to Azeroth. They fought the Old Gods in a global war of ordering.
The trolls are presumed to be Azeroth's first sentient race, not including the titan constructs who would later suffer the Curse of Flesh. From Zandalar, they spread across Kalimdor and formed great empires, most notably the Gurubashi and Amani.
Around this time, the elves appeared in Kalimdor. Using the potent magics of the Well of Eternity, the elves broke troll influence in many regions and established a magocratic empire. At the height of kaldorei power, however, the Burning Legion invaded Azeroth, inciting the War of the Ancients.
Following the war there was a schism among the elves. The night elves started druidism with Malfurion Stormrage as the first Night Elf Druid; the high elves refused to give up arcane magic. These high elves were banished from Kalimdor and settled in the north of Lordaeron, establishing the mighty kingdom of Quel'Thalas.
Thousands of years later, the Burning Legion attempted to destroy Azeroth once more. The extended conflict with alien invaders became known as the First and Second Wars, during which time the Horde established strongholds across Azeroth. The mighty Horde was defeated by an alliance of high elves, dwarves, and humans.
Angered by the Horde's failure, the Burning Legion prepared for yet another invasion. In the east, the Legion's Lich King pawn set loose a plague of undeath on the human kingdom of Lordaeron. This undead Scourge quickly crippled the northern powers, demoralizing the people of the Eastern Kingdoms in preparation for a Legion offensive.
Through the machinations of Medivh, Last Guardian of Tirisfal, leaders of the New Horde and the Alliance were drawn to Kalimdor. There they joined with the night elves to thwart the Legion advance into Hyjal, where the World Tree was vulnerable to demonic attack.
The weakening of the Frozen Throne allowed some of the Scourge to regain their free will. Under the Banshee Queen Sylvanas Windrunner, these Forsaken managed to force the Legion-controlled undead from the ruins of Lordaeron and establish a stronghold beneath its streets.
Following the Third War, the political landscape of Azeroth changed greatly. The night elves joined the Alliance, and the Forsaken joined the New Horde. The remaining citizens of Lordaeron, broken by the Scourge, formed new regimes: the Scarlet Crusade and the Argent Dawn.
After major threats in Silithus were neutralized by Horde and Alliance forces in the Qiraji War-Effort, the power blocs once more formed a tentative alliance in response to the opening of the Dark Portal. The Horde formed an alliance of necessity with the blood elves, while the Alliance warily welcomed the draenei exiles into their ranks.
At war once again, Alliance and Horde heroes now compete for glory throughout the northern lands: aiding the Kirin Tor of Dalaran, protecting allies from corrupted Titanic Keepers of Ulduar, and joining the Argent Crusade in the final push against the Lich King and the Scourge.
Meanwhile, deep within Deepholm, the corrupted Dragon Aspect Deathwing waited, recovering from the wounds of his last battle against Azeroth and biding his time until he tried again to reforge the world in molten fire. His eruption from Deepholm massively damaged the world.
After the ending of Deathwing, the Alliance and the Horde renewed their conflict for the land and resources of the planet. Mysteriously, the previously impenetrable mists covering the hidden continent of Pandaria cleared and both factions attempted to make allies and extend their reach to this newly discovered continent.