Template:War
The Second Invasion of 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 to the Burning Legion's invasion was the defeat of Kazzak the Supreme at the Throne of Kil'jaeden.