Siege of Terra - Preparations: After the Drop Site Massacre conducted against the three Loyalist Space Marine Legions of the Salamanders, the Iron Hands and the Raven Guard by the Traitor Legions loyal to the Warmaster Horus on the world of Istvaan V, it became clear that eight of the eighteen Space Marine Legions had turned to Chaos and against the Emperor of Mankind.Horus openly and publicly declared that he would no longer follow the Emperor, believing him to be undeserving of the battles fought in His name, and the Warmaster took over the formal leadership of the Traitor Legions as their new overlord.The Heretic Astartes were supported by elements of the Imperialis Auxilia, a portion of the Mechanicum and the Titan Legions that had chosen to serve Chaos known as the Dark Mechanicum, a portion of the Imperial naval forces of the Armada Imperialis then still part of the Great Crusade's expeditionary fleets of the Legiones Astartes, and the daemons of Chaos.Over the course of the next seven Terran years of brutal interstellar civil war after the Drop Site Massacre, their aim soon became clear -- Terra, the heart of the Imperium of Man. The Sons of Horus, the Death Guard, the Emperor's Children, the World Eaters, and elements of the Word Bearers prepared to rendezvous at Mars. The rest of the Word Bearers Legion was tasked with destroying Roboute Guilliman and his Ultramarines on the world of Calth in the Eastern Fringes of the galaxy.The Imperial Fists Legion's Primarch Rogal Dorn and the Regent of the Imperium, Malcador the Sigillite, receiving the few survivors from the Drop Site Massacre of Istvaan V as they retreated back to Terra, became aware of the full implications of their position and the strength of Horus and the Traitor forces.Rogal Dorn immediately recalled all Loyalist Imperial forces back to Terra in preparation for Horus' invasion. Of the Space Marine Legions still loyal to the Emperor, the Space Wolves had just completed the Burning of Prospero, the Thousand Sons Traitor Legion's homeworld near the Chondax System, where the White Scars were stationed as part of the Chondax Campaign.Without warning, the Alpha Legion's fleet broke from the Warp and engaged the Space Wolves forces of the Primarch Leman Russ, hammering his smaller fleet and forcing Russ to resort to hit-and-run attacks. The Alpha Legion's Primarch Alpharius also attacked the nearby forces of the White Scars piecemeal in an attempt to draw that larger Astartes Legion into conflict.The White Scars' Primarch Jaghatai Khan wished desperately to aid Russ, yet as the Traitor Legions' starships attacked, he received the astropathic order from Rogal Dorn. Khan was to bring his Legion back to Terra immediately. Dorn also commanded him to relay the order to Russ and add that should he succeed in evading his attackers, only then should he attempt to head for Terra. Relaying the message and adding his apology for his inability to aid the Space Wolves, the White Scars made for Terra.Russ resolved to meet the Alpha Legion with renewed determination. With help from an unlikely quarter, the Space Wolves would eventually turn the tables on their attackers and make the Warp jump to Terra, well after the siege on the Imperial Palace had begun.Similarly at the world of Calth, the Ultramarines expeditionary force, battered by the relentless attacks of the Word Bearers, had dug in on the planet's surface, while Primarch Roboute Guilliman and the remnants of his Ultramarines' fleet began to organise hit-and-run attacks. Surveying the scene on the planet, Guilliman rapidly assessed his ground troops' positions, and began broadcasting orders to his men, coordinating each pocket of defence.One such pocket, under Brother-Captain Ventanus, organised a breakout, and retook Calth's defence cannons and laser silos, reaping a great tally of Traitor starships. Ventanus' victory evened the odds in space, buying time for the vast remainder of the Ultramarines Legion to arrive at Calth and drive the Traitors away. Reunited as a single force, the Ultramarines received Malcador's orders and set out for Terra, moving through the Warp as rapidly as they could even as they realised that they would probably arrive too late to make a difference.As this happened, the Night Lords arrived in the Eastern Fringe of the galaxy to engage the Dark Angels, and the Iron Warriors' armada broke from the Warp to engage the Imperial Fists' fleet marooned near the Chaos Space Marines' headquarters world of Istvaan V.Surviving the initial thrust of the Iron Warriors' attacks, the Imperial Fists' armada at Phall held fast and scattered the Primarch Perturabo's fleet, then made their own Warp jump to Terra in a desperate race against time.Meanwhile, in the Signus Cluster, the Blood Angels, granted new and terrible power by a mysterious mass rage (that would resurface again during the Siege of Terra) had triumphed, smashed the hordes of the Chaos daemons asunder at the Battle of Signus Prime and were able to make the Warp jump to Terra following the dissolution of the Ruinstorm in the wake of the Second Battle of Davin.As the Warmaster was heading towards Terra he received an unexpected communication from the Thousand Sons Primarch Magnus the Red. The Space Wolves had driven the Thousand Sons from their homeworld of Prospero after being manipulated by Horus into attacking the Thousand Sons homeworld instead of simply taking Magnus into custody to return him peacefully to Terra to face charges of the illegal use of sorcery by the Emperor.Devastated by the Space Wolves' destruction of his Legion's precious storehouses of knowledge about the Immaterium and sorcery in their lost Prosperine capital city of Tizca, and having turned to the Chaos God Tzeentch to save himself and his Astartes Legion, Magnus pledged his allegiance and the allegiance of the Thousand Sons Legion to Horus and Chaos in retaliation against the Emperor for this betrayal. The Thousand Sons were en route to Terra where they would link up with Horus' forces.Following the Battle of Beta-Garmon, which represented the last major Loyalist resistance beyond Terra, the door to the Sol System had been opened wide. Horus finally mustered his forces on Ullanor Prime after almost a solar decade of war. They included the Sons of Horus, World Eaters, Emperor's Children, Thousand Sons, Death Guard, and Iron Warriors, each commanded by their respective Primarchs.These forces were bolstered by elements of the Night Lords and 5,000 Word Bearers commanded by Zardu Layak, as the majority of the Word Bearers Legion remained under the command of the Primarch Lorgar, who had betrayed the Warmaster. The Alpha Legion had retreated from the Sol System warzone after delivering a in-depth map of the Sol System's defences to Horus.The great Traitor fleet left Ullanor behind and moved into the Sol System proper, gathering above Mars for resupply. The Red Planet had already fallen to the forces of the Dark Mechanicum during the Schism of Mars, though the Loyalist fleet in the system had successfully blockaded it to quarantine the Traitor Tech-priests' treachery.Of the nine remaining Loyalist Space Marine Legions, only the White Scars and the Blood Angels were able to join Rogal Dorn and his Imperial Fists in the defence of Terra before the arrival of the Traitor Legions in the Sol System.Three full Titan Legions of the Mechanicum and close to two million soldiers of the Imperialis Auxilia stood alongside them to face the Traitors in a battle that would determine the fate of Mankind for the next 10,000 years.But the Castellan of Terra knew that his forces were too few to do more than slow the ultimate Traitor victory. All they could do was buy enough time for Loyalist reinforcements to reach the Throneworld.