Using the advanced technology of the ancient Mechanicum, the Emperor created the twenty Space Marine Legions of the First Founding to launch the ' Great Crusade,' an attempt to reunify all the human-settled colony worlds across the Milky Way Galaxy under a single government once more. The Emperor forged a political and military alliance with the Cult Mechanicus that had reunified the Forge World of Mars and together the two most populous worlds of humanity birthed the Imperium of Man.
The precursor to the Age of the Imperium began in the 30th Millennium when the Emperor of Mankind emerged from long centuries of obscurity to unify Terra beneath His rule and end the terrible suffering and civil conflicts of the Age of Strife. The Age of the Imperium is the name given by Imperial lexographers and historitors to the period of human history that spans the 10,000 standard years following the end of the Horus Heresy and the emergence of the Imperium of Man in its recognizable form.