system
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human
[40K] Could the Great Crusade have succeeded without Astartes?

Yes, the Legiones Astartes played a vital role in the Great Crusade, but they didn't do everything. Mortal men and women fought in the crusade. If the Emperor of Mankind decided that making an armies of post-human killing machines with demigods heading them was too risky, would mortal forces have been able to conquer the galaxy?
gpt
>Yes, the Legiones Astartes played a vital role in the Great Crusade, but they didn't do everything. Mortal men and women fought in the crusade. If the Emperor of Mankind decided that making an armies of post-human killing machines with demigods heading them was too risky, would mortal forces have been able to conquer the galaxy? 

Possibly?  But at greater cost. They would have been facing equivalent or stronger enemies, and the gains would be less.

Contrary to popular belief, Space Marines (High Gothic "Adeptus Astartes") are not meant for battle line formations.  Even in the great crusade that was rare.  Space Marines are the "tip of the spear" assault units.  Their primary function was to shock and awe human worlds into surrender by orbital drop assault against their leadership.

Imagine if today, right now, 1000 drop pods landed outside Washington DC.  Out of these drop pods came 8 foot tall giants in shining metal armor.  They would stride through our defenses, eliminate our leadership in a single blow.   And then would come the message from orbiting space ships, "Surrender and join us, or face this in battle."

Most human worlds just surrendered.  At which point the Emperor essentially added them and their resources, largely intact, to his Imperium.  This is what permitted the exponential growth of the crusade.

The other benefit of the Astartes, is that when focused they present far greater firepower in a narrow front than regular troops can provide.  They're also more capable of facing the alien terrors of the galaxy than regular humans.

Without Astartes, the much slower pace of successful conquest would have greatly increased the cost of expanding the Imperium.  Eventually it would have just ground down his resources.