Long War

"We were made to conquer the galaxy, not to rot here in Hell and die upon our brothers' blades."

– Ezekyle Abaddon

It's called the Long War because it truly is loooooooooooooonnnnnnnnnngggggg. That is not taking the mickey either. It has been going on for 10,000 years and see no signs of abating. Well, sort of anyway.

It is the ongoing conflict within Warhammer 40k between the forces of the Chaos Traitor Legions who turned to Chaos during the Horus Heresy and the mess of a super empire that is the Imperium. According to the traitors the war is still on going (hence long), but as far as the Imperium is concerned they won when they killed the traitor leader, routed his forces and pushed them back to the Eye of Terror, though even before the opening of the Great Rift they had suffered significant losses to the Traitor Legions' forces.

It is a rather confusing term since no-one is quite clear who is and isn't part of the conflict, what battles are covered under it and various other details about it. For example: Do the Space Marines that have turned to Chaos since the heresy count as being part of the Long War? Do the Lost and the Damned and the Daemons of Chaos count as taking part as well? Does every conflict fought between Chaos and the Imperium automatically count as part of the Long War? It tends to be one of those terms that people toss around casually without referencing it further. This could be GW's stubborn refusal to let too much fluff out of its forbidden depths. Then again, it's very much possible that, in universe, they aren't really sure what they mean when they use the term either. It's...well, the Chaos Marines buzzword for how much they hate the Imperium.

Actually there is likely a very good reason the Long War gone on as long as it has, due to the leadership of a certain armless failure....

Another point is that Chaos Gods are more interested in war itself as a process rather than it's final result. Especially given according to the Cabal the ultimate Chaos victory would be their undoing. I mean eternal war spreads eternal bloodshed and carnage, massive death tolls and epidemics, struggle for power and epic level mind games between the leadership of opposing factions and leads warriors involved into new peaks of sensual experience. So they subtly undermine their servants' plans if they come too close to the ultimate victory. Just as Planned.

Timeline of Warhammer 40,000
The Times of Old Wars of Secession - War in Heaven (60.000.000 BC) - Fall of the Eldar (M30)
Pre-Heresy Age of Terra (M1-M15) - Dark Age of Technology (M15-M25) - Age of Strife (M25-M30)
Great Crusade (M31) The Last Church - Rangdan Xenocides - Interex - Faash - Council of Nikaea
Horus Heresy (M31) Battle of Isstvan III - The Burning of Prospero - Drop Site Massacre - Thramas Crusade
The Battle of Phall - Battle of Calth - Signus Campaign - Imperium Secundus - Siege of Terra
Time of Rebirth (M31-M32) The Great Scouring (M31) - Start of The Long War (M31) - The Legion Wars (M31)
The Battle of Skalathrax (M31) - Creation of the Codex Astartes (M31) - Second Founding (021.M31)
The Forging (M32-M34) The War of The Beast (544.M32-546.M32) - The Beheading (546.M32)
Nova Terra Interregnum (M34-M36) 21st Founding (M36)
Age of Apostasy (M36-37) Plague of Unbelief (310.M36)
Age of Redemption (M37-M38) Abyssal Crusade (321.M37-121.M38)
The Waning (M40-M41) Gothic War (143-151.M41) - The Macharian Crusade (392-399.M41) - The Macharian Heresy (400-470.M41)
Wars for Armageddon (444.M41, 941.M41 and 991.M41) - Damocles Crusade (742.M41)
Time of Ending (M41) The Vaxi Atrocity (731.M41) - First Tyrannic War (745-746.M41) - Sabbat Worlds Crusade (755.M41-780.M41)
Siege of Vraks (813.M41-830.M41) - Massacre at Sanctuary 101 (897.M41) - Badab War (901-912.M41)
The Vaxhallian Genocide (926.M41) - Second Tyrannic War (990.M41-993.M41) - Third Tyrannic War (997.M41-999.M41)
Octarius War (999.M41) - 13th Black Crusade (999.M41-M42)
Age of the Dark Imperium (M42-ongoing) Indomitus Crusade (999.M41-111.M42) - Ultima Founding (M42) - War of Beasts (001.M42-025.M42) - Plague Wars (~111.M42) - Psychic Awakening (M42)