The earliest days of mankind's expansion to the stars is a time of dark myth and legend, unremembered by all save the Emperor of Mankind. Terrible wars and anarchy engulfed in the galaxy and as human civilization fragmented, alien races and vile creatures of the Warp seized their chance to plunder unprotected worlds to enslave their populations.

The future survival of the universe and human kind stood the test of time and following the treachery of Horus the threat of extinction lifted Guilliman spread the word of the Codex Astartes. This would be the corner stone of the future Imperium. The surviving Space Marine Legions were broken up into Chapters of 1000 space marines, and trained to be the best fighting soldiers in the Universe.

(quoted:Games Work Shop Codex: Space Marines ISBN: 1--84154-526-0)