Post-Heresy separation of land and space forces

The Horus Heresy provides the institutional explanation for the later fleet structure. Guilliman breaks up both the Space Marine Legions and the formerly combined Imperial forces, separating the Astra Militarum from the Navis Imperialis. The arrangement accepts variation among Mechanicum, invasion-oriented, and general fleets while deliberately limiting any one leader's ability to command an overwhelming combined force. It sacrifices some unity of command to make another empire-wide military rebellion harder to organize.