Garth 2 wrote:The think is, we don't know really how many survirors there were and what state they where in after the visit.
We know there was enough to rebuild, and then inturn improve on there technology, we know that they explored and knew enough (or figured it out) to place sensor platforms on the traditional approach route.
But combine with the lenght of time between visits, the population, as stated by Horus, start to forget and focus on domestic concerns, which have a habit of drawing down your availalbe armed forces especially if you then slip into civil war.
After all, its clearly stated that Colin was the first Leader who managed to stop a visit (and therefore keep/maintain a population centre) and connected to that capture enough information on the enemy to learn something about them (after all Horus nor anyone else, mention any real information on the enemy just the left over Human battlefleets so presumably the Achuultani destroed or reclaimed any of their own vessels after the battle before moving on).
Not entirely accurate.
It is known that the immediately previous incursion culled humanity back to only the population of Birhat being survivors.
It is known that one of the predecessor Imperiums seeded life on a large number of planets, including Terra and Birhat.
It is also known that there have been more predecessor Imperiums than the "official" number, but they're even older, with even less evidence left behind of their existence.