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Comments welcome.
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The thing about combat is that it's very much constrained by logistics. What can I say, armies are
ravenous when it comes to supplies, and you do need to be able to supply that. And that means you either fly it in, you rail it in, you sail it in or you drive it in. This inevitably means that no matter what, even fairly newly settled worlds are likely to see a lot of combat in and around the places they live. Even fully automated mining colonies are likely to see most combat happen around major mining complexes and any surface/orbit infrastructure. It's where the goods are, it's where the money and the people are, and it's where you can move supplies around the easiest.
That doesn't mean that there won't be combat outside those inhabited regions, many battles for sections of the planet will start with maneuvers to cut off or establish corridors of supply, and remote sections of roads, railways or bridges are really good choices for that because long supply lines are really hard to defend continuously. There's a reason WW1 and 2 saw extensive convoying, and it's this exact problem.
And the issue of logistical path constraints gets even worse the higher the colony cost, but interestingly enough there's reason here to have a cut off below which the risk collateral damage (well, to non-infrastructure installations) drops considerably. A heavily populated planet with a colony cost requirement is likely to have an expansive network of transit systems that move goods and people between major population areas, which may be covered, but below that population count there simply has neither been time nor the desire nor the economical development to fund such a convenience, and all travel will be in fully environment rated independent vehicles. Which means that every battle will effectively be an assault on a fortified city simply because of the way infrastructure constraints work in real life.
Sure, you are less likely to damage the facilities manned by the people in a sprawling environmental hazard protection complex (or a bunker in other words), but you are extremely likely to compromise their infrastructure and thus their lives, likely causing increased casualties through losses of infrastructure alone, while combat in the city itself will inevitably involve the civilian populace who have no option of escape.
It also makes sense in this case for defenders to prefer a defense further afield, because otherwise the risk to habitation zones becomes much greater.