Initially a battle against Orks led by Warboss Urlakk Urruk, the campaign took several years. When the White Scars finally defeated the greenskin hordes, they found themselves blockaded by a fleet of the Alpha Legion under orders from Horus and effectively trapped in the system before they even knew the Horus Heresy had begun.
After a tense standoff with the Alpha Legion, Jaghatai Khan took control of his fleet from the Swordstorm and skillfully executed a formation known as The Chisel which smashed a wedge in the Alpha Legion fleet and allowed their escape. Multiple Alpha Legion ships were destroyed during the brief but intense battle in space. After receiving conflicting orders from both Horus and Leman Russ over who the real traitors were, the White Scars fleet then made course for Prospero. The Khan was determined to discover the truth of this mayhem for himself.
Later Mortarion remarks that that something foul had taken place at Chondax, as the original plan by Horus was for the Alpha Legion to recruit the White Scars. Instead, either under orders from Alpharius or Omegon, they took overtly hostile actions, avoided any attempt at communication, and kept the White Scars occupied until they could recieve a message from Rogal Dorn warning them of the Heresy. Indeed, the only reason this message could be delivered was because Omegon had dispatched his Effrit Stealth Squad to destroy a Pylon blocking White Scars communications. It seems that the Alpha Legion, or at least elements of it, wished for the Scars to remain true to the Emperor.