Lynata said:
To conquer includes to hold, else it would just be one attack after the other - or not even that, as the enemy simply goes into hiding once you arrive and plops back out when you're gone. This is why the Marines were followed by millions of human troops. Which were also used massively during the Horus Rebellion, by the way, for contrary to popular belief that battle was not fought by Space Marines alone either.
Yes, there are Chapters that specialize in siege warfare (Iron Hands iirc), but they are specialists who are called upon when there is a stronghold that is too tough even for the Guard, which is a situation that, though this is mere conjecture, does not come up very often.
There's also a GW fluff blurb about Canoness Aspira leading the Order of the Bloody Rose - about 6.000 Sisters during peak times - through the liberation of a hundred worlds from the tyrant Denescura, and you don't see me using that as a justification to circumvent studio canon.
Not really, no. That's why garrison troops (whose job it is to hold) are generally of poorer quality than front line elite infantry air assault brigades. Capture and hold are two distinct roles on the battlefield, as perhaps best illustrated by the role of armoured units to capture, which then cannot physically hold terrain and are replaced with infantry. The Heresy era indeed employed a lot of Guard troops, but the fiercest of fighting went to the Legions, and not solely in the role of small strike teams.
The fact remains that the Marines WERE a massed forced, and that's what they were intended to be. 'Legion' is a fairly indicative name, after all. The Great Crusade employed Astartes a thousand at a time, not piecemeal as kill-teams. Claiming that the marines were never intended as a battlefield force and have always operated merely to decapitate command structures and hit vital targets is incorrect.
[For a start, there's those black-clad field police marine units...]



