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They were offered a stark choice--bend their knee before the Emperor and swear service to the Great Crusade, or die by His hand.Though many set pride before what they regarded as slavery, others chose service and took up the Emperor's Warrant of Trade. The Rogue Traders Militant were expected to scout ahead of the leading edge of the Great Crusade, accompanied by their own armies as well as whatever assets had been ceded them by the Emperor.
During the Great Crusade to reunite all the scattered colony worlds of Mankind beneath the rule of the Imperium of Man, both the Imperial Guard and Imperial Navy were originally a single service: the Imperialis Auxilia (Imperial Army).
Those of the Legiones Astartes were often blunt of prow and slab-armoured, built to endure the withering storm of fire that accompanies a planetary invasion, their plasma furnace-hearts powering some of the most destructive weapons known to Mankind.
But beyond these practical needs, each fleet favoured the nature of its Legion, from the sable black marauders of the Raven Guard to the baroque crimson and gold battlecruisers of the Blood Angels to the brute functionality and unadorned steel of the Iron Warriors' siege-barques.
These armadas were designed to serve both as a source of deep-range patrols and rapid-reaction forces to respond to sudden threats, be they civil disturbance, rebellion or outside attack, both from within the Imperium and from beyond its borders.
In terms of the warships that physically made up these fleets -- and the Port Maw Armada was an exemplar of this -- they outnumbered and en masse likely out-gunned any single Expeditionary fleet of the Great Crusade, at least on paper.