Planetary invasions

I won't rehash the discussion up to now, I'll just give my premise for
the campaign I am building.
Colonies will be basicly 2 types, those with or without a breathabnle
atmosphere.
Invading a colony without a breathable atmosphere will be much like 17th
century siege warfare.	Once you defeat the colony defenses, the colony
surrenders without an invasion.  Pressurized domes or caves are too
vulnerable to defend directly and too valuable to be blown up without a
good reason.  That isn't to say that you won't get fanatics who just
want you gone who will blow them up anyway.  But conflict between major
powers will by necessity become somewhat stylized.  There is also the
possiblity to sabatoge, resistence to occupation, which could make for
many intersting senarios.
The situation is a little different for colonies.  Most colonies will
have reletively small populations.  When they get big enough the UN
takes over and they become part of the core.  6 million people may seem
like a lot when they are scrunched together in a place the size of
Israel but spread them over an entire planet and they are less
concentrated than Alaska.  More like NW Territories.  These worlds will
probably have a few cities, industrial centers etc. and the rest of the
population will be spread in small communities.
There may be a militia in the cities but not in the hinterland (they
wouldn't really need one).
When invading a world to just take it over, it isn't neccessary to take
the whole thing; just the key parts.  spaceport, major cities,
industrial centers etc.  The invaders will not usually have huge forces
available, just too expensive.	But neither will the defenders.  Their
militias will be small because there is little surplus population; they
might have a garrison of regular troops.
These invasions will consist of a few pitched battles up to about
brigade size to take key objectives.  If successful, the invaders then
may be faced with a low intesity guerrila war to pacify the planet.
They will be thinly streched to do so if the population is restive.
there are a lot of places to hide on a whole planet.  This will make the
high tech guys vulnerable to bands of frontiersmen with more primative
weapons.
Anyway, that's how I'm going to do it.

Tim