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|Previous: Gilneas civil war|
|Concurrent: Elemental War (Elemental Invasion)|
|Alliance–Horde War (Cataclysm)|
|Place: Great Sea, Kalimdor and Eastern Kingdoms|
|Outcome: Undetermined, ongoing|
|Major Battles: Invasion of Gilneas, Battle for Gilneas, Battle for Andorhal|
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Five years after the Battle for Mount Hyjal, the fragile truce between the Horde and Alliance had all but evaporated. In the wake of the Third War and The Founding of Durotar, the Forsaken entered a partnership of convenience with the Horde while the night elves and gnomes emerged from ages of solitude to join the Alliance. For another year the armistice held, though tensions continued to mount as worldwide conflicts drew the two sides closer to all-out war and fighting erupted in strategic battlegrounds. Despite these frequent skirmishes, the Horde and Alliance often set aside their differences and joined forces to face a number of threats. In time the blood elves and draenei led them through the Dark Portal to wage the War in Outland against the Burning Legion.
Following the year-long campaign in Outland, the Horde returned to Azeroth with a new ally in the Mag'har, led by Garrosh Hellscream. Soon, the long-missing Varian Wrynn put an end to 's schemes and returned to reclaim the throne of Stormwind. Attempts at diplomacy between Thrall and Wrynn soon fell apart as a peace conference in Theramore was ambushed by the Twilight's Hammer. Attention quickly turned northward, however, as the looming threat of the bore down on the mortal races of Azeroth.
Garrosh Hellscream and Varian Wrynn, appointed as the supreme commanders of the Horde Expedition and Alliance Vanguard respectively, who had previously butted heads in Theramore, led their forces to Northrend where they waged war against the Scourge. Though fighting a common enemy, tensions between the Horde and Alliance grew swiftly until spilling over after the Battle of Angrathar the Wrathgate. During the resulting Battle for the Undercity, Varian Wrynn, grief-stricken over the loss of , and the state of the former Capital City of Lordaeron, lashed out at Thrall and declared war against the Horde. Hostilities continued to escalate for the remainder of the War against the Lich King, until the Lich King was finally defeated.
Three years have passed since the military forces of the Horde and Alliance first set foot on the shores of Northrend, and they now wage war amidst the disasters wrought by 's return to the world. Thrall has left the Horde to lead the Earthen Ring's efforts in preventing the rift beneath the Maelstrom from tearing apart the world, and has appointed Garrosh Hellscream as Warchief of the Horde in his absence. King Varian Wrynn, convinced that the Horde has remained unchecked for too long, continues to lead the Alliance.
With the weakening of the Scourge after the War against the Lich King, the Alliance forces led by have claimed the western part of Andorhal, and the Forsaken forces led by have claimed the eastern part of the city. The two factions are currently fighting each other for control of the whole city. During the Battle for Andorhal, the Forsaken, their ranks bolstered by the use of the val'kyr to raise their slain foes, push the Alliance out of the city and claim Andorhal as their own, effectively conquering almost all of the old kingdom of Lordareon. All Alliance forces are now hard-pressed, the Forsaken being in the South and the Sin'Dorei in the north.
In Hillsbrad Foothills, The Forsaken destroy the Alliance presence entirely by destoying the damaged city of Southshore. The Forsaken blight used by the attackers devastated the land, driving the alliance back to Alterac for good. The Forsaken also conquer Arathi Highlands, destroying Stromgarde with the help of the risen prince Galen Trollbane. Hammerfall is reinforced and rebuilt as a fortress. The forsaken control now almost all of the lands from Western Plaguelands to Wetlands.
By orders of the new Warchief of the Horde, Garrosh Hellscream, the Forsaken mobilise and move into Gilneas to secure a port for the Horde in southern Lordaeron. The Gilneans, now transformed into worgen, attempt to fight off the invaders as best they can. Eventually the night elves arrive in Gilneas, bringing both vital military aid and the truth behind the worgen curse. Likewise, Orcs reinforce the Forsaken forces, causing an escalation into an all-out battle between the Alliance and the Horde in the region. The Forsaken, against the orders of the Warchief to cease production, unleash their devastating Forsaken Blight against Gilneas. Many of the survivors flee across the sea to Teldrassil, where the night elves grant them sanctuary.
Even with Gilneas falling into the hands of the Forsaken, the fighting between the Gilnean worgens and the Forsaken continues. Bloodfang worgen who had recently allied themselves with the Gilneas Liberation Front, with the help of 7th Legion reinforcements from Stormwind, destroyed three Orc gunships that had arrived.. After that the 7th Ligion and the Bloodfang worgens invaded the city of Gilneas took back the city's districts and drove most of the Forsaken back to Silverpine. Despite this the Forsaken still held the northern parts of the land, and would continue to pose a serious threat to the Worgens. Later, the Forsaken killed many of the Hillsbrad refugees on Fenris Isle and resurrected them, bolstering Forsaken forces and making up for the loss of the three orc gunships.
Druids in the Gilneas Liberation Front used their bear form in guerilla warfare against Horde forces at the Forsaken Front, which resulted in the loss of a few Horde squads. Eventually the Horde forces learn of this tactic, and it becomes ineffective after counter-measures are implemented.
In the Twin Peaks near Grim Batol in the Twilight Highlands, the dwarven Wildhammer clan fight against the orcish Dragonmaw clan, having recently rejoined the Horde, battle for control of this strategic mountainous region. The Twin Peaks are crucial for staging effective offensives against the black dragonflight and the Twilight's Hammer, who currently control the Twilight Highlands. To the south, the humans of Marshtide Watch battle the Stonard orcs in the Swamp of Sorrows. In the end Stonard had fallen in to the hands of the Alliance killing all the armed orcs and letting the citizens flee to the Blasted lands.
In the Southern Barrens Alliance forces have claimed the eastern lands while the Horde have taken the west. The Alliance has constructed new military outpost to solidify their control of their new territory while the Horde has done the same to prevent further Alliance expansion. The central region between the two camps will become a war zone as the factions fight for control.. Bael Modan is destroyed by the advancing Horde army. Eventually, because of one of the Alliance commander's tactical incompetence, the Horde retakes the Barrens, driving the defeated Alliance off for good, devastating anything left standing, and anyone caught in their way.
In the Northern Barrens, the Horde defensive post Mor'shan Rampart is under attack by the Alliance. In turn Northwatch Hold is now under attack by Horde forces. After a crucial battle, the Northwatch Hold and the Alliane fleet defending it are devastated by Horde warships. The survivors retreat back to Stormwind.
In Ashenvale, the Horde has been recently able to conquer more of the ancient forest and have cut down even more trees. This allowed the Horde to wrest control of Silverwind Refuge. Meanwhile Zoram'gar Outpost has its defenses improved and air assaults from the outpost commence on the Night Elf. The village of Astranaar, Maestra's Post, Raynewood Retreat, and surrounding towers have also been destroyed and taken by the Horde.  Despite Silverwind Refuge and other Alliance outposts being conquered by the Horde, Alliance forces led by the Night Elves have taken back certain parts of Ashenvale. Splintertree Post comes under ground assault by the Alliance, all Horde forces at the nearby road have been killed. This victory was short lived, however, as the Horde utterly destroys all the attackers and continues to drive the Night Elves back from the region. 
Off the coast of Kalimdor, on the Lost Isles, and the island off Azshara, the Bilgewater Cartel goblins with aid from stranded Orcs fight against Alliance forces and attempt to establish a new city to replace the loss of Kezan. In Azshara Alliance forces are attacking Valormok, a largely undefended but unimportant Horde outpost in Azshara.
In Darkshore, the Horde allied themselves with the Shatterspear tribe trolls seeking to conquer it for the Horde. Realizing the Horde's plans, the night elves launched an assault on Shatterspear Vale. After a short fight, the night elves have finally taken the village and burned it down, killing every single troll in the village.
The Night Elf saboteurs assaulting Orgrimmar rear gate have been destroyed, thanks to the Goblins building up the area. All Night Elves are also killed on the road to Ashenvale. 
The war even extends to the Great Sea where naval battles are fought for control of key trade routes.
- ↑ Cataclysm Lore Details - World of Raids Forums
- ↑ Quest:The Warchief's Fleet
- ↑ Quest:Rise, Forsaken
- ↑ Quest:A Wolf in Bear's Clothing
- ↑ World of Warcraft: Cataclysm - Features - Twin Peaks Battleground
- ↑ World of Warcraft: The Magazine Issue 2, pg. 46
- ↑ 7,0 7,1 7,2 Frejya. Cataclysm Compilation (7/10) - E. Kingdom 2. Retrieved on 2010-07-11.
- ↑ Quest: Blastranaar!
- ↑ Quest: All Apologies
- ↑ The Story So Far... Part II (2010-12-07).
- ↑ Quest: Operation Fishgut
- ↑ World of Warcraft: Cataclysm - Features - Vashj'ir