To whet appetites while I feverishly write this day, here’s a comment I left elsewhere a while ago on the general topics of planetary invasions, and the ongoing argument of the “just slag the planet, no need for troops” vs. “massive world-spanning groundpounder warfare” schools of thought.
My conclusion, in a nutshell: on any post-colonial planet that isn’t easy to nexus-interdict, it’s less what you might prefer, and more what you’re going to get anyway:
For myself and my setting, I concluded that at least some aspects of the “kill it with nuclear fire” school are going to be more or less inescapable because of, as you point out, how good the planetary defenders have it.
What the Ley Accords – the Eldraeverse equivalent of the Geneva Convention, essentially – actually says is that you can’t use planetary bombardment indiscriminately on civilian populations or to make terror strikes, and once you’ve disabled the orbital defenses and “own the high orbitals”, you’re supposed to ask for their surrender before you start firing on the legitimate military targets…
…because once you’ve started dropping heavy enough hellflowers (air-burst antimatter sterilization/EMP weapons), stoneburners (sub-ground-burst anti-bunker burrowing antimatter shaped-charges), and plain old k-rods (Rods From God) to take out deep-running submarines, crust-embedded fortresses, giant planetary lasers, etc., etc., there’s no way not to do major damage to the planet even if you’re not trying to, or indeed if you’re trying not to. If you’re lucky, you’ll get away with a few dozen simultaneous earthquakes/tsunamis/wildfires/hurricanes/massive radiation events/etc. worth of damage. If you’re unlucky, you throw enough debris into the air to give you a particle winter and a major extinction event. And either way, depending on how careful the planetary government is to keep its military facilities in the middle of nowhere, you’ve got megadeaths to gigadeaths.
The polities that are both (a) established galactic citizens, and (b) halfway civilized, all understand this, and that you’re supposed to surrender the planet when you lose the orbital defenses, because while you might not be able to take it back, you definitely can’t un-wreck it.
(Even if you intend to fight a guerilla war groundside afterwards and are willing to absorb the damage from that, you may still find it worthwhile to surrender any formal planetary defenses you invested in. At least that way they’re only going to be dropping tactical k-rods on you…
…but there’s no upside to engaging in a pissing contest with starship-class weapons and their planet-mounted equivalents when the planet is going to take all the collateral damage, and the fleet in orbit doesn’t have to worry about that.)
Thus, Imperial admirals hate having to fight galactic newbies (who might be under the impression that you can fight and win an orbit/ground battle without taking horrific collateral damage), or worse, the kind of fanatics who don’t mind taking their population and ecology along with them when they go. (Although, in practice, there’s usually someone in the latter’s command structure willing to introduce their leader to a bullet rather than let him initiate Ragnarok.) Even Caliéne “the Worldburner” Sargas-ith-Sargas, the IN’s mostly-tame sociopath, thinks it’s a little messy and inelegant.