Little in the fragmentary records of that era remains, but there is no evidence to suggest that any of the worlds in what was then known as the Arkalas Cluster were anything more than a collection of backwater colonies adjacent to a well-travelled Warp route.
This changed in the latter years of the 8th century of the 41st Millennium, less than a solar decade after the conflict known as the Damocles Crusade.
Expanding outwards from their homeworld, and having reached the coreward edge of the Jericho Sector, the alien T'au began to establish a strong presence within the region, variously conquering, treating with or simply colonising the worlds they came across.
The trio of worlds were a distant goal at the time, many solar months travel for even their fastest ships, but all three were resource-bearing worlds, and thus a prize to be coveted.
Swift messenger ships carrying Water Caste dignitaries followed the intelligence-gathering drone-scouts, conveying greetings on behalf of the T'au Empire and expressing a desire for peaceful cooperation, endeavouring to appeal to "The Unity" as like-minded individuals in a hostile galaxy.
The T'au could attempt little more than this opening preamble before the Imperium arrived to reclaim their worlds.
Unwilling and unable to fully muster a defence against the crusaders, the T'au fell back from Argoth, Rheelas and Kaggeran, while gathering a force to contest the Imperium on Wrath, closer to the worlds the T'au had already claimed.
The Achilus Crusade
Kaggeran became the second Sept World to fall during Operation Hammerfall, with the first having been Rheelas. Within solar hours, the planet became garrisoned and was declared Compliant by the Achilus Crusade, having found only minimal signs of contact with the T'au.
This destabilised and demoralised the T'au forces enough that the Astra Militarum could round up and execute the leaderless and scattered rebellion.
Within solar days of his promotion to command of the Canis Salient, Lord Commander Sebiascor Ebongrave gave his first general order -- the quarantine of Argoth, Rheelas and Kaggeran, the three worlds that had suffered the worst of the T'au-sympathiser rebellions.
- Deathwatch: The Jericho Reach (RPG), pp. 57-58, 60, 63, 65
- Deathwatch: Rites of Battle (RPG), pp. 242