The Solar War refers to a series of battles fought in the Sol System from 010-015.M31 during the closing stages of the Horus Heresy. The war saw Horus begin his drive on Terra, capital of the Imperium.
With the Emperor preoccupied with the War Within the Webway and Malcador the Sigillite waging the Shadow Wars, it was left with Imperial Fists Primarch Rogal Dorn to oversee the defenses of both Terra and the greater Sol System. The defenses of the Imperial Palace were significantly strengthened, and much of its aesthetic was destroyed in favor of strategic defensibly. Following the Schism of Mars, Kelbor-Hal's Dark Mechanicus was victorious in claiming the Red Planet, but an Imperial blockade trapped the traitors on their own world.
As Horus' forces inched closer to Terra, Dorn organized the defenses of the Sol System into a series of Spheres. Each consisted of not only Imperial Fists forces, but also loyalist Imperial Army and Mechanicum.
- The First Sphere was the outermost defensive zone from Neptune to Pluto.[2a] It was commanded by Sigismund.
- The Second Sphere oversaw the Jupiter region.[2a] It was commanded by Halbrecht.
- The Third Sphere oversaw the blockade of traitor-occupied Mars. It was commanded by Efried.
- The Fourth Sphere was commanded by Camba Diaz and oversaw the region between Terra and Mars[2a]
- The Fifth Sphere was Terra and Luna, overseen by Rogal Dorn himself.[2a]
For the next several years, the loyalists would counter traitor-instigated uprisings and attempted breakouts by the Dark Mechanicus on Mars.
The Alpha Legion announced its entry into the Solar War by exploding the freighter Primigenia over Terra, killing millions by raining debris. This was accompanied by Alpha Legion organized uprisings at Terra's Damocles Starport and Triton. All of this culminated in the first major battle of the Solar War, the Battle of Pluto. The battle saw the Alpha Legion drive on Pluto, linchpin of the loyalist monitoring of the Sol System. During the battle, a fleet of over 200 Alpha Legion warships assaulted the planet, but after Alpharius himself was seemingly killed by Rogal Dorn the traitors retreated.
- 1: The Horus Heresy Book Six pg. 30
- 2: Praetorian of Dorn (Novel) Part 4, Chapters 1–6
- 2a: Part 1, Chapter 4
- 3: Secrets of the Solar War — Day 2
- 4: The Lightning Tower (Short Story)
- 5: Weregeld (Novella)