The entire Peninsula was ravaged by brutal combat when Khadgar, the new leader of the mages of the Empire successfully reopened the portal after three years when before, Medivh had managed to seal it. Utilizing the vast powers of the Ley Lines in Azeroth, Khadgar had managed to puncture a hole in the fabric of space that separated Azeroth from Draenor. The Emperor had prepared for this and had massed a massive force of troops and supplies at the gateway awaiting for the moment of the reopening of the portal for additional conquests.
The Emperor was aware of the fact that the Orcs and the Draenei were two extremely powerful races and knew that if he did not enslave them, then they would oppose the Empire's will. It was during this time that Khadgar began experimenting with demonic magics in an attempt to achieve more power.
The Draenei and the Orcs were caught off guard. The Orc city of Zeth'kur was the first to fall. The Orcs and Draenei fled from Zeth'kur but were under heavy attack from the Empire's forces until they managed to reach the Temple of Telhamat. Luckily by then many of the Draenei and even Alliance refugees had arrived to reinforce them from Shattrath. The Empire's forces held much of the Peninsula for the first few weeks but the unfamiliar territory, and wild beasts such as the enormous Colossi slowed their forces down significantly. To make matters worse, the Draenei were using the Holy Light against them. Though the Empire had paladins of their own, the Draenei were able to utilize more of the Holy Light than the Empire thought possible to restore and boost the forces of Draenor.
The Empire was bringing forces in from Azeroth in such vast amounts that it seemed almost impossible for them to be stopped. With the use of hit and run tactics to force the Empire's battlefield trained armies to constantly reposition themselves, the Draenor forces bought valuable time for their armies to be boosted. The great prophet Velen, with Medivh and the other alliance leaders prayed for assistance from the Holy Light.
As the two vast armies gathered for a valiant battle through the entire peninsula, from the skies came the radiance of the Naaru. Khadgar was present and the Naaru sensed within him the taint of the Burning Legion. They sensed that Khadgar himself had even been contacted by the Eredar, possibly their leaders Archimonde and Kil'jaeden. The Naaru joined the Draenor Armies against the Empire and with their vast powers managed to force the Empire's armies to retreat back into the portal.
The Naaru sealed the gateway between the worlds, but they knew that the Empire was now tainted with the corruption of the Burning Legion. Khadgar managed to escape back to Azeroth and the Naaru feared that Humanity would become the Legion's next pawn. A'dal would work with Medivh, and Velen to re-open a portal to Azeroth, this time to Theramore where the Draenor Forces would return to Azeroth to free humanity from the Burning Legion.