The war started when Sylvanas Windrunner, a renegade of the elvish kingdoms of Heraklon, broke off from the rest of her people and the Alliance, united the persecuted barbarian tribes of the world, and brought them all under the banner of the Uuruk. Facing off against each other, the Heraklonian Alliance and the Uuruk waged war on each other for the right to rule the world and all its peoples. Originally hoping to use the Uuruk as leverage for her own power acquisition, she later found sympathy with their cause, and became heavily devoted to her new people. She appointed an ambitious yaungol warlord named Kai as the General, or "Fist of the Uuruk".
Later, shortly after the arrival of the Second Blue Team to the moon, the Uuruk began suffering great losses due to the powerful outsiders. Driven to desparation, Warlord Kai made contact with a supernatural being who ordered him to unearth a piece of ancient technology, which turned out to be an Anannaki energy conduit. After the artifact was unearthed, Kai again began to communicate with his god, who was in fact Kil'jaeden, lord of the Burning Legion. Kil'jaeden, using the identity of "The Savior Spirit", ordered Kai to kill Sylvanas, sieze control of the Uuruk, and use the ancient energy conduit to open a Dark Portal through which Kil'jaeden's forces could enter the world. Kai agreed, thirsty for power and most of all, victory. He tricked Sylvanas into coming to the artifact, where he attempted to kill her. She managed to escape though, and left the Uuruk forever.
Nevertheless, Kai took control of the Uuruk with his loyalists and allies within the Uuruk council. They joined forces with the savage ogres of the Gorian Empire, and together, harnessed the energy of the Anannaki conduit to open a great Portal through which, despite the Alliance and Blue Team's attempts to stop them, the Burning Legion was able to pour through, thus beginning the Great Devastation.