War in Outland

The War in Outland was not a singular event but a series of conflicts carried out by multiple belligerents. The course of these events are detailed in World of Warcraft: The Burning Crusade.

Prelude
Doomlord Kazzak discovered an unknown relic and used its power to re-open the Dark Portal. In his stead he left fellow doomguard Highlord Kruul to sow discord across the major cities of Azeroth. Several waves of contigents from the Burning Legion attempted to breach the portal from Draenor's side, but were kept at bay by the soldiers and mages from Nethergarde Keep, supported by reinforcements from the ever-vigilant Argent Dawn and mercenary troops from across Azeroth. Horde and Alliance forces under Watch Commander Relthorn Netherwane and Warlord Dar'toon arrived later in the battle and were able to successfully repell the Legion's forces.

Conjoined Invasion of Outland
After repelling the Legion's forces, Netherwane and Dar'toon dispatched contigents under Commander Duron and Lieutenant General Orion in a conjoined effort to claim the Stair of Destiny and establish footholds in Hellfire Peninsula. The Alliance was able to reinforce their already established base at Honor Hold, still under the leadership of Danath Trollbane after two decades, while the Horde forces under Thrall's trusted advisor Nazgrel established the new settlement of Thrallmar.

Disruption of the Legion Invasion
After the initial azerothian victory at the Stair of Destiny, the Legion attempted to retake the Portal in order to cease the flow of troops into Outland and resume their invasion of Azeroth. Legion teleporters set behind the enemy lines provided a constant flow of troops to throw themselves at the Stair of Destiny. A nameless Pit Commander organized an army of Infernal Siegebreakers, Fel Soldiers and Wrath Masters in an attempt to push back the azerothian forces. The joint-faction army, inspired by the champions Justinius the Harbinger and Melgromm Highmountain fought a tireless battle to keep the Portal safe. The Legion invasion was finally broken when Alliance and Horde forces under Forward Commander Kingston and Forward Commander To'arch managed to take down the Fel Reavers that were protecting the Legion's front teleporters and proceeded to destroy the portals with decisive air-raids employing technology developed by Area 52. Bombing raids were carried out on the crucial Murketh and Shaadraz gateways and the forge camps of the Abyssal Shelf. Efforts to disrupt Legion factories at Forge Camps: Mageddon and Rage were also carried out by Horde and Alliance troops.

Later on, during the assault on Invasion Point: Annihilator, Warbringer Arix'Amal was killed and the portal shut down.

The final nail in the coffin for the Burning Legion's invasion was the defeat of Kazzak the Supreme at the Throne of Kil'jaeden.

The Conflict with Illidan
With the Legion threat difused, the Alliance-Horde entente became less of a united, cohesive force. Both factions, however, turned to the threat of Illidan Stormrage, who was the hegemonous power in Outland, ruling from his seat at the Black Temple of Karabor. This conflict was not conducted as an organised war effort, but rather as a series of smaller-scale conflicts across Outland, against fel orcs, blood elves, demons and naga loyal to Illidan. Only the forces of Shattrath City offered any form of organised attack; Xi'ri, a naaru general under the command of A'dal offered to push back Illidan's troops at the gates of the Black Temple with a joint Aldor-Scryer force. This provided an elaborate distraction for the liberated Maiev Shadowsong, Illidan's tireless jailor and Akama, Illidan's once-ally, to enter the Temple.

Adventurers, with the help of the Ashtongue Deathsworns, proceeded to bring down the high command of Illidan's vast empire, and even Illidan himself, from the inside.

The Battle for Quel'Danas
The final stage of the conflict on Outland would transpire outside of Draenor itself, on the Isle of Quel'Danas. Kael'thas Sunstrider, once Illidan's most trusted advisor, aligned himself and those of his still loyal subjects with the Burning Legion.

After extracting enormous amounts of arcane energy from Netherstorm via hijacked naaru-technology, Kael'thas was confronted and seemingly killed at Tempest Keep. The Prince, however, managed to survive, albeit as a wretched mana-hungry fiend. Using the great amount of energy taken from Outland and the captured naaru, M'uru, Kael'thas triumphantly restored the shattered Sunwell for the most abominable purpose; the summoning of Kil'jaeden into Azeroth.

A united effort of the Aldor, Scryer and the Blood Knight order, the Shattered Sun Offensive, successfully repelled the Burning Legion from Quel'Danas. With the Legion driven back from the Isle, the proud defenders of Azeroth invaded the Magisters' Terrace and at long last killed the Sun King. These brave adventures then turned their attention to stopping Kil'jaeden's summoning into Azeroth. Within the Sunwell Plateau the demon lord was in the process of being completely summoned and only with the sacrifice of Anveena and the support from the Blue Dragonflight was he banished from Azeroth.

Aftermath
Using the spark of the fallen naaru M'uru the Draenei leader Velen reignited the Sunwell, explaining:
 * "In time, the light and hope within will rebirth more than this mere fount of power. Mayhap, they will rebirth the soul of a nation."

This strongly indicates that the Sunwell has indeed been restored using the energies of M'uru, returning the blood elves' "fount of power". The Sunwell's new energies are possibly derived from M'uru's powers of the Light rather than arcane magic.

Furthermore with Illidan dead the elder sage Akama swears to fill the halls of the Black Temple with Light once again.

Legacy
The aftermath of the invasion, due to the conflicts primarily staying on Outland, left the politics of Azeroth for the most part unchanged. During this time however, Varian Wrynn reclaimed his throne as King of Stormwind, and the warchief Thrall found and appointed the son of his close friend Grom, Garrosh Hellscream as his advisor.

The blue dragon Tyrygosa, while on Outland discovered and researched the eggs of the Netherwing dragonflight, prized tools in Illidan's war machine. After the war, she took some of the eggs back to the Nexus where she cured her flight's patriarch, Malygos, of his insanity. In turn, Malygos would declare war on all magic users for their rampant misuse of magic over his long exile.

And the battle for Quel'Danas, while primarily a organized battle against the Burning Legion, saw the first organized movement of Scourge forces since Naxxaramas appeared over the Plaguelands. If the Lich King subconsciously sent the Scourge there himself to halt the Legion's advance, or an unseen necrolord believing he was doing his what was best for his master is unknown. However, soon after the crisis, the Lich King himself awoke from his long dream and attacked Stormwind, Orgrimmar, and then declared war on the living, intending to wipe out all life on Azeroth.