The humans of Azeroth descend from the vrykul, Titan-forged beings transformed by the Curse of Flesh. Their descendants founded the first human realms. The empire of Arathor once united the tribes against the trolls, but later divided into independent kingdoms including Stormwind, Lordaeron, Kul Tiras, and Dalaran.
The orcs lived on Draenor in clans and followed shamanic traditions. The draenei—refugees from Argus whom Kil’jaeden had hunted for millennia—settled on the same world. Through them, the demon found a new people to use in his revenge. He deceived Ner’zhul and later made Gul’dan the chief instrument of the fel.
The clans were forged into the Horde. Many orcs drank the blood of Mannoroth, gaining strength at the price of fury and dependence; exposure to fel magic turned their skin green. War against the draenei devastated Draenor. When the dying world could offer no more, Gul’dan sought another. His alliance with Medivh opened the Dark Portal to Azeroth.
Blizzard’s official history emphasizes Kil’jaeden’s deception and Mannoroth’s blood as the instruments of the orcs’ enslavement. The First War was not a chance collision between two peoples, but the conclusion of a carefully prepared demonic plan.
Official Blizzard source: Official World of Warcraft source