The Imperial Navy is one of the armed forces of the Imperium of Man.While the Imperial Guard represents the Imperium's ground forces, the Imperial Navy is responsible for the fleets of starships that maintain order between the stars and planets in the Imperium, for all space and air support provided to the infantry of the Imperial Guard, and for transporting those Guardsmen across the galaxy to the Imperium's myriad warzones.
Collectively, these massive war fleets would come to be referred to as the Armada Imperialis.
Under this form of organisation, each Imperial Cruiser would have a single Imperial Army regiment assigned to it.
Imperial Army regimental commanding officers held command over both their regiment and the warship assigned to them, making a single warship a tactically flexible combined arms unit and minimizing the damage to the Imperium in the event of the loss of a starship, its crew and its assigned troops in the Warp.
During the Horus Heresy, however, it appeared that some Traitor Army regiments used the power of the starships at their disposal in order to forge interstellar empires for themselves in the fires of anarchy that swept the galaxy during the seven bloody standard years of the Heresy.
This tendency to make use of the power of an Imperial starship combined with that of an Imperial Army regiment to establish tyrannies on many worlds led to the eventual split of the Imperial Army into the Imperial Guard and the Imperial Navy to deliberately foster a bureaucratic and inter-service rivalry between the two.