Where do you grow food on Mars and Jupiter? Farm habitats. If you can have pressurized housing, you can have pressurized farms, and the asteroid belt provides a tremendous resource base for industry, as many asteroids are made almost entirely of iron and nickel. Although I'd personally like to see actual modern nations appear in the unification war. Who says China or the United States will be gone by 2310?
Earth has several thousand years of head start..not to mention huge areas for farming various crops, seas for fishing and farms for animals...and no pressure domes.
Any colony would be severely limited in what food it can produce on it's own (I don't really see a big meat industry) and it's doubtfull that it can produce enough to sustain BILLIONS of people, not to mantion that domed cities and farms are a hell of a lot more vulnerable than anything on earth..
A single stray shot breaks the dome - *BAM* - there goes the city..or all your crops. Not a side I would choose to be on in case of a war...
Orbital bombardment seems to be rare or nonexistent in FreeSpace, except for the Lucifer's genocide campaign. Also, many colonies might be backed by Earth nations who might use the colonies in a proxy war. Earth itself is fragmented into many nations. It's not like a few tiny dome cities against an entire planet.
And who even really needs farming? In the 24th century they could probably synthesize mass quantities of food.
FreeSpace wars are largely won by naval power. The asteroid belt has enough metal in it to build a virtually limitless number of ships. If one side has control of space, the other side really can't do anything but hunker down on their planet, because they sure as hell won't be able to fight off the enemy fleet.
