Because of the haphazard manner in which they tumbled to Urutsk from the great heights of the Space Beyond the Sky <Aya 'Eye-Ah'>, the variety of troops and armed units became so dispersed that individuals or detachment-sized remnants of squads and larger groups simply became associated with enclaves of other survivors.
From this jumble of civil servants, constabulary, home guard from dozens of environments, to elite and specialised warfighters, isolated societies cobbled together their individual tables of organisation, training, and deployment as Black Winter set in with its scouring celestial wind of space debris and plasma storms.
In some locales, due to the distribution of more favourable geographic features, limited surface activity was possible over the time-forgotten span of the Scourge. However, the vast majority of the scattered Imperial cantons were forced literally underground and expanded their holdings over the centuries.
It was not long before the tunnelling unearthed and awakened long dormant creatures and monstrous civilisations as recent as their own, also escaped to Urutsk. These wars, The Onyx Battle
While certain kingdoms and other cultures did meet during that long period, few had the resources to maintain larger communities and had often kept the contact secret from their constituents. Now upon the surface, the enclaves and empires of thirty or more generations removed cousins gradually were reunited.
Naturally, the grand traditions of the Imperium made the re-integration of the disparate colonies more easily possible. Of course, there were those who desired no part of the resurrected behemoth that had created the Cataclysm, and from these 'deserters' came the first intra-Human wars.
But Fate was kind to renew the enmities with the non-Human, and in some cases, also the Allied Peoples (human-like aliens). More often than not, Human enemies were brought past their bickering and resource grabs to unite briefly against their common foes.
Through these various organic mechanisms of transmission and re-implementation, there became a great commonality to Human military endeavours that was retained as various groups spread out to seed the barren planet with animal and plant life from their long-destroyed homeworlds and others once held within the Imperium's grasp.
From the hereditary melange that had defined the Core of the Imperium ('The Sphere of Suns'), as had been recorded in myths of the long vanished past, Humanity became a species of multiple peoples. As the Human pool of bloods split and refined, specialised, and re-introduced other groups, the ethnicities of Urutsk came into being in only a few hundred years. With these divisions, martial procedures changed and were lost to time and replaced with more ideal methodologies and practical measures as suited each group's particular needs.
As the planet grew lush and green, as the rains fed the multitudes with blue and white-tipped marsh grains and fattened game animals, in many cases, the non-human races began to find more ideal conditions above ground, and expanded rapidly -- often out-breeding humans by several generations.
These groups usually fell upon each other as readily as upon Humans, but in some cases, great bands or more subtle confederacies were formed, and these both were a thorn in Humanity's side for much of the Mad Spring.