At the beginning of the First War, the Orcish Horde spilled in to Azeroth through the Dark Portal from some unknown land. By sheer numbers, cunning skill, and savage brutality, they defeated the separated human kingdoms of the south and destroyed the city of Stormwind, winning the First War.
The Orcs followed the remanants of Stormwind north towards Lordaeron and the Dwarven lands. Fighting off the fledling Alliance forces, the Orcs maintained control of the Dwarven lands to the east and pushed towards Lordaeron. The successes of the Horde in the early days of the Second War were largely attributable to their ability to gather allies. The Forest Trolls, Ogres, and Goblins fight along side the Orcish Horde.
The Horde was led by Warchief Ogrim Doomhammer and his supporters Gul'Dan and Zuluhed the Whacked. After Doomhammer was captured and the Dark Portal was destroyed by Khadgar, the morale of the Horde was shattered and they retreated to their mountain fortresses of Grim Batol and Khaz Modan effectively locking the Alliance and Horde remanants into a stalemate.