Horus Heresy - Fortifying Terra: As the years of conflict across the galaxy passed, the ultimate aim of the Traitor Legions' destructive interstellar advance soon became clear: their goal was the conquest of Terra, the heart of the Imperium. With this conquest, Horus could fulfill his promise to the Chaos Gods and throw down the Emperor of Mankind, their greatest enemy. 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 Fringe 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 Isstvan 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. 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. Without warning, the Alpha Legion's fleet broke from the Warp and engaged the Space Wolves, hammering their smaller fleet and forcing Leman 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 their Primarch Roboute Guilliman and the remnants of his 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 planetary 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 Traitor Legions' headquarters world of Isstvan V. Surviving the initial thrust of the Iron Warriors' attacks, the Imperial Fists' armada held fast and scattered Perturabo's fleet, then made their own Warp jump to Terra in a desperate race against time.Meanwhile, in the Signis 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 they had faced asunder and were able to make the Warp jump to Terra.As the Warmaster was heading towards Terra he received an unexpected communication from the recently betrayed Thousand Sons Primarch Magnus the Red. Magnus pledged his allegiance and the allegiance of the Thousand Sons Legion to Horus and Chaos in retaliation against the Emperor for his betrayal of the Thousand Sons and the destruction of their homeworld of Prospero. The Thousand Sons informed the Warmaster that they were en route to Terra where they would link up with Horus' forces.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 Solar System. Three entire Titan Legions of the Adeptus Mechanicus and close to two million soldiers of the Imperial Army stood alongside them to face the Traitors in a battle that would determine the fate of Mankind for 10,000 years.