I see the subtleties of my argument confound you - my point was, had Japan called the US bluff and gone for a third or fourth attack, they'd have realised the US had no more bombs! And then what? Anyway, we're arguing the toss over an example used to prove a point over something I didn't actually say.
Actually, they'd have found the US had established an assembly line and was cranking out a couple bombs a month indefinitely. Not to mention the conventional bombardment, which meant that they would essentially run out of targets big enough to hit within a year. And the starvation coming from the combination of the blockade and the inability to transport what food they did grow due to the complete destruction of their transportation grid...
In reality, I doubt any colony would have a massive population or vast urban sprawls anyway, so it would actually not be the bloodbath you seem to think it would be. Take Star Trek - most colonies are very small and when invaded and occupied it was done with relatively small forces. It would take centuries for a colony to get up to the size of Earth, at which point an invasion does become a problem
If the colony isn't heavily populated, it also probably hasn't been heavily fortified either (since spending on defenses would take away from the investment in actual useful infrastructure that the colony will need while it is growing). So stick a cruiser in orbit above each of the major cities, and selectively target any resistance (and I do mean selective; with complete orbital control and decent tech, you could drop a baseball on someone's head from orbit).
But the main problems remain:
-Any fleet capable of sending in ground troops is capable of sending in missiles (since each troop ship could have just been loaded with explosives instead).
-It's very unclear that any invasion would be less destructive than a bombardment anyway. Urban warfare is obviously monstrously destructive, but it's not even the worst case. Consider the example of invading an unterraformed colony, where people are still living in Outpost-like enclosed spaces (e.g. a moon base). To actually have troops invade, you have to penetrate the facility, and since they probably won't open their airlock voluntarily for your troops, that means you have to bust a hole in the wall and depressurize that section (presumably the colony will have auto-sealing sections to prevent one accident from killing everybody). You then will have to bust a hole in the automatic seal to get to the next section (because again, they won't open it for you unless they are surrendering). And repeat, until the place surrenders or the entire place has been depressurized (incidentally killing everyone, and probably causing massive damage to the infrastructure).
-If the populace is so fanatical they won't surrender even when they literally have no way of damaging your fleet, the last thing I want to do is expose my ground-troops to them. The fanatics won't be useful as a conquest (because they will continue to resist until the ground troops kill them anyway), and they will just get a bunch of my people killed. If I absolutely need something on the planet, I can selectively bombard defenses in that area until it is safe (and make it clear I'll bombard anyone building new defenses or approaching the area I am interested in). Yes, that will tie up a couple ships, but that's still far cheaper than supporting a full-fledged invasion/occupation. If I just want to deny the planet to my enemy, a couple ships in orbit to play watchman are perfectly adequate for that too. If I want the planet as a base, I am much better served building one in orbit or on an uninhabited planet in the system, since that way I don't have to worry about my important military institution being attacked by suicidal fanatics on the ground.
