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.