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I've got an idea about boarding parties on spaceships but it's based on the continued existence of infantry, and infantry weapons that are designed to function in an open-system environment. The question is in a world where:

  1. orbital bombardment is a thing and in fact a common first resort weapon.

  2. environmental damage caused by bombardment can be mitigated to the point where it's just not a consideration.

and

  1. the vast majority of primary industry and manufacturing are in orbit so on-planet resources and infrastructure are not important targets.

Is there actually a reason to have large-scale armies that can invade worlds? There will always be reasons to have small units for assassinations, extractions, and espionage/sabotage missions on planet but that's a very different skill set and weapons philosophy to massed infantry which is what I'm interested in.

Edit: just so it's clear any group that is using orbital kinetic bombardment as a first strike weapon does Not care about civilians, neither casualties or survivors, at all. Sorry I thought that would be obvious.

Edit 2: MichaelK's quite right I haven't given enough context for really concise answers, there's a lot of good material here anyway but I like clarity too so background and hopefully a clarification of the question as well: In the universe I'm working on humanity started to spread to the stars with FTL starships centuries ago and has colonised a reasonably large number of star systems, with heavy colonisation out to 15 Light-years and rumours of colonies as far away as Canopus, FTL is limited and does not extend to direct communications. Conflict between/within human colonies is rare but tends to be catastrophic, there are rules of war and a "stellar navy" of sorts that can enforce some penalties but the reality of the war courts is that living victims get results and you're pissing into the wind trying to get convictions without them. This reality leads to really devastating bloodshed when a conflict "goes hot" because neither side really wants to leave anyone standing on the other side, it's in their interests not to. So orbital bombardment is a weapon that is actually useful because of it's indiscriminate nature.

Planets play less and less of a role in civilisation the further you go from Earth, orbital factories take advantage of orbital materials to create goods that largely spend their consumer lifetime in space. Planets make good living space for populations because artificial gravity is still expensive and/or awkward, and those who stay on-planet for work contribute to co-ordination and logistics within light-speed communications range. Planets can also be used to grow luxury real foods like meat and cereals that can't easily be grown in space. Almost all infrastructure is in orbit, including power for Arco-cities on planet.

Now on a planet using high-power kinetic weapons isn't much of an issue, you can get away with stray rounds etc... they'll come down somewhere, in space you don't have that option, a stray round powerful enough to punch through armour based on astro-engineering hardsuits that protect from micro-meteors can punch through civilian structural members potentially trashing the target and the boarding crew. From my point of view if the only troops who see regular action are in boarding actions where those risks are a constant consideration, and it's been like that for generations, then people being people they'd come up with projectile or energy weapons that function there but if planetary infantry are still a big thing then boarding parties wouldn't get the same attention and would have to go a different route. So is massed planetary infantry a working model when trashing everything at the bottom of a gravity well is not only feasible but also in some ways preferable?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Sep 1 '17 at 2:21
  • $\begingroup$ Using orbital materials is not realistic. It is one thing to mine an asteroid belt, but using random debris in planetary orbit would not work. There simply is not that much useful debris. I also find it is worth noting that rare, heavier elements are more likely to settle in considerable amounts in the larger gravity well of a planet. Also, why does simply killing everyone result in a lack of repercussions? With orbital bombardment there would be clear evidence that people had been murdered. I can't imagine a justice system being real where murder is nonpunishable if the victim is deceased. $\endgroup$ – Braydon Sep 1 '17 at 2:25
  • $\begingroup$ Are we talking about k2 civilizations here, or ftl civs. Because the reasoning changes based on that. $\endgroup$ – Garret Gang Sep 1 '17 at 23:32
  • $\begingroup$ One final question, how expensive is it to ascend/descend from/to a planet? Because unless you have a Handwavium drive, landing sufficient troops on a planet to take control of it is far more expensive then you can get out of the planet (in terms of resources). What is the maximum size of the space factories created? Cuts centrifugal force is a really inexpensive way to produce artifical gravity. $\endgroup$ – Garret Gang Sep 1 '17 at 23:37
  • $\begingroup$ @Braydon You know how difficult it is to get a conviction without eye witness testimony in modern courts? Multiply that by time delays, up to decades, in bringing a case to court and the fundamental inability of humans to give half a shit about things they can't see directly. No witnesses doesn't mean there isn't a crime it just makes it impossible to get any traction in court. $\endgroup$ – Ash Sep 3 '17 at 15:32

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I am going to assume you're wanting some plausible reasons for why a military would continue using ground forces after you have the equivalent space fire-power of, say, Imperial Star Destroyers.

This assumes your invaders aren't just mindless monsters hell-bent on destroying the galaxy. While such creatures make great monster-movie tropes, an intelligent species would have (we hope) some reason to attack beyond just destruction of the planet and all on it.

If that's not true; if their purpose is truly to wipe all life from the surface of the planet, then ground troops serve no purpose. Bomb the planet into a uniform-height dust bowl and move on to the next. The terraform crew can follow and do whatever they do.

Other Resources

You mention that environmental damage isn't a factor. Okay. But there are other resources that may not be merely environmental.

  • People (if life is precious)
  • Endangered species
  • Historical and cultural artifacts
  • Art (that isn't purely electronic)
  • Disruption to trade (raw materials go up to space station to be manufactured...)
  • Food production (is likely cheaper on the ground than in space)

Psychological

Maybe you don't want to instill terror in the hearts of all people, everywhere. Bombing from space might serve better as a a worst-case scenario to hold over your enemies' heads, rather than a daily fact of war. See also Nuclear Deterrence.

Collateral damage

Maybe there are citizens of your empire that you prefer to keep alive. Diplomats, spies, trade representatives, non-combatants, whatever. There's a cost for destroying them, and sometimes that cost isn't worth paying. Ground troops can evacuate key personnel from the surface rather than destroy them.

Hearts and minds

If you bombard the planet into rubble, you destroy any chance of the citizens ever becoming trustworthy, reliable, members of your empire. Any survivors out there in the galaxy will turn the destruction of their world into a reason to rise up in rebellion. That may mean guerrilla warfare. Or peaceful protests. Or open rebellion from other worlds. Remember the Alamo. Martyrs are powerful things; making an entire planet a martyr could have devastating consequences later.

So ground troops let you have more precision on where you strike and to what degree you destroy things.

Fear

A horror author once told me something like "fear isn't the threat of your own death, but the threat of not being able to prevent your friends' deaths." If the entire planet gets converted to rubble, that's an existential terror that, like global warming or the risk of eventual meteor impacts, doesn't register very high for most people.

But send in an army and suddenly the people not currently getting shot at have something to fear. Maybe they'll fear your army enough to surrender before shots are fired. Or at least before all life is laid waste.

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War is not about smashing things

If all you wanted to do was to blow thy enemies to tiny bits in thy mercy, then orbital bombardment will do the job without the need for infantry, that is correct.

But war is not about that. Smashing things to tiny bits is but a means to an end. The end is control. Control over resources; control over people; control over transport lanes; control over the enemy's military so they do not try the blowing-to-bits trick on you.

Blowing the enemy to bits can achieve some level of control: if the enemy's military is in tiny bits, then you have control over it... after a fashion. If the enemy themselves is in tiny bits then you have control over the enemy... sort of. If there are no enemies in your transport lane, then you most definitely control the transport lane.

However... that is not really a satisfactory win, now is it? You spent all that money, all that effort, all those weapons, all those lives of your soldiers... and all you ended up with was a big pile of tiny bits. That could be done better, now can it not?

Hence: infantry... the guys that actually go down there and establish the fact that you are now in control, and not just over tiny bits but of people, factories, mines, refineries, nuclear power plants (geez, aren't you glad that you did not put a relativistic kinetic railgun dart into that... what a mess that would have made... egads).

So in short: orbital bombardment is the big fist that smashes stuff to tiny bits. But the infantry are the ones that make it so that you have some kind of use of your win.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Sep 1 '17 at 2:23
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Is there actually a reason to have large-scale armies that can invade worlds?

Armies are require to police your populace, protect assets, and take things so why not train them in planetary invasion as well?

  1. The attackers don't value the lives of soldiers. Invade away. "Grunts die, its what they do"
  2. "Training exercise" to give your armies a blooding, test invasion before a mission critical world, test new equipment in a fight.
  3. Hardened defenses scattered miles deep under the crust of a hive world, requiring specialized equipment to drill to a hive comb, and troops to go down the rabbit hole.
  4. World hopping, moving armies from one world to another, this planet could be an island on the path to a final destination, and the cost in lives might not be that high verse the cost in time for your ships. Drop and go.
  5. Bureaucracy, army is going to do it cause the army is going to do it. Hoorah?
  6. Availability the army may have ships available while the the orbital bombardment armada is needed else where.
  7. Glory, propaganda. Boots on the ground and flag waving can make impressive pictures of success.
  8. Butchery over starvation, a ground invasion is preferable if you can't feed your people. A swarming species could use this as a practice to cope with overpopulation. A shrewd government could use this to force a people to invade another's for food. Its common accepted theory that normally people don't want war, this could be a good means.
  9. Auxiliaries are persons other than citizens that fight for your people in hopes of citizenship. A form of "required" progression might be to fight for the empire in a certain number of battles. Added bonus the soldiers you drop become the colonists after, they are literally fighting for their future home.
  10. Rogue army, a governor of a nearby world not in control of combat vessels could levy an army and civilian ships very quickly.
  11. Anti-ship weapons render large orbital bombardment ships neutralized. Does the world have an answer for drop ships in the millions though?
  12. Genetherapy of a society, send the most undesirable elements of your civilization on a suicide invasion/redemption by fire.
  13. Unique world, how do your bombardment weapons work on a water planet/water species? Not well I might think.

The way I look at it is Numbers is an advantage and a problem How do these advantages and problems compel a government of any type to send millions or billions to their deaths? The most common thread is that soldiers lives are cheap, and do no require much consideration. If soldiers lives are expensive, and there are other options then soldiers will never be used.

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    $\begingroup$ 8 and 9 are extremely germane points, 12 is nasty in a really useful sort of way too. $\endgroup$ – Ash Aug 29 '17 at 11:13
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    $\begingroup$ Read your most recent edit, I see what your talking about. Have you considered that kinetic based orbital weapons would kick up enough dust to render a planet uninhabitable for decades? dailymail.co.uk/sciencetech/article-2699854/… $\endgroup$ – CarbonDonuts Aug 29 '17 at 13:10
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    $\begingroup$ Yup that's part of the appeal. $\endgroup$ – Ash Aug 29 '17 at 19:10
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Force Fields

Planetary force fields. You think your tiny spaceship has shields? This is a planet without any need to optimise for mass or heat. The power of this shield would take 100 destroyers 100s of hours to pound through. And you know what else planets have defence batteries. Gun arrays larger than your ship connected to power grids that normally power cities. They will turn your destroyers parked to fire at full power to slag in seconds.

Good thing we got infantry. Piloted ships can dodge heavy fire and move through enemy shields using expensive energy intensive shield phase slip technology only viable for ships. Once we get our boys on the ground it's a different game. Secure a shield facility, pull the switch and the bad guys surrender or get to know what big mama has ready for them.

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Orbital bombardment would probably not be a first method or approach even if you didn't care about civilians. You stated that they wouldn't need to worry about blowing up factories because they're in space, but if this is trues then why bomb planets at all? There has to be something significant on the planet in order to give an incentive for orbital bombardment, and if there is something significant you might want to capture it instead of destroy it. If the planet didn't matter, no one would bother bomb it, and if it did matter, people would send soldiers to steal it.

If war crimes are no longer an issue in your world, then attacking forces may instead use other tactics such as deploying chemical weapons to kill most of their enemies and then sending in an army to capture valuable resources and infrastructure.

Even if there was no need to fight for planets, there would still be a need to fight for the alternative orbital infrastructure. This might not lead to massive armies, but boarding and capturing a sizable space stations or very large ships would probably require a lot of ground troops (or androids). In this scenario, soldiers may also opt for melee weapons for fear of stray projectiles causing holes/breeches, or in other ways damaging the space station.

It also may be better to not leave evidence of orbital bombardment. If the bombing could be traced back to a particular individual or group, then that person could suffer legal or political consequences. Also leaving evidence of a bombing could lead to retaliation from your enemies. For these reasons it might be better to send in an army to force relocation.

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  • $\begingroup$ The only thing on-planet that's worth anything is the population and they're only worth anything because they have a claim that you want to quit. The point about boarding parties is slightly on point but the question is really about whether there would be planetary infantry as well as boarding parties because without planetary infantry you only need to create weapons for boarding parties and the approach is then very different, energy weapons become more feasible and high-power projectiles are extremely undesirable. $\endgroup$ – Ash Aug 29 '17 at 11:11
  • $\begingroup$ I disagree on the planet mattering bit. Ina total war we would bomb construction yards and any other key military worlds to undermine opposition $\endgroup$ – Andrey Aug 29 '17 at 14:01
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    $\begingroup$ @Ash If I am understanding correctly this is in relation to a mining claim or other planetary claim being voided by the death of its population? This would likely not happen, as it would be to obvious that the planet had been hit with orbital bombardment. If claims matter I assume there is must be a government entity to enforce these claims. Sending in an army to forcibly relocate people would probably be less easy to trace back to you. If this is not what you mean I'm going to need more context. If nothing on planets is valuable why care about claims? Unless planets are just for mining. $\endgroup$ – Braydon Aug 29 '17 at 22:06
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What is the purpose of the war?

If your aim is just to burn everything and leave the planet a charred husk then troops aren't required, but for anything else, someone will have to get dirt on their boots.

One of the aims of the war is apparently total annihilation of the opposing population, that's a tactical consideration not a political one, why did the war start in the first place?

If there's anything you want to secure then you're going to need ground troops, whether a facility, technology, or otherwise, you'll have to put troops on the ground.

How much infrastructure do you want to maintain?

Is a breathable atmosphere one of your requirements? While a war of total annihilation is possible with orbital bombardment, the chances are that by the time everyone is dead, so is everything else, and the planet may not be habitable without appropriate PPE for several decades. Sufficient damage to eliminate resistance followed by ground based clean up crews would be a better option to leave you with a habitable planet.

Current experience

Current conflicts in the Near East are showing that air power alone does not win wars. Ultimately you will need to have boots on the ground. Are you going to declare victory and send down civilians after a bombardment? Unlikely, it's going to be troops.

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    $\begingroup$ Air power alone does not win wars if you care about minimizing civilian casualties. World War II was won in part by carpet-bombing civilians and thus crippling the enemy's economy. But fortunately that style of warfare went out of fashion. $\endgroup$ – Philipp Aug 29 '17 at 16:34
  • $\begingroup$ Also recent conflict, also less recent conflicts, etc. $\endgroup$ – fectin Aug 29 '17 at 22:52
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    $\begingroup$ @Philipp, Their economy was already up the Khyber, hitting civilians for their own sake brings no military benefit. Not caring whether you hit civilians for the sake of hitting a valid tactical target is a different calculation $\endgroup$ – Separatrix Aug 30 '17 at 6:59
  • $\begingroup$ @Philipp Nope, WW2 was not won because of bombardment. Whihtout groud troops there could never be a victory. You can still see the shotmarks on older houses, especially in Berlin. Of course, WW2 could also not be won without air superiority. The carpet-bombing was in fact counterproductive for winning the war, because the civilians support there country more if they are attacked; also the Cities where not very important for military. tl;dr: You cannot win a war without ground troops if you dont want to have tortal destruction (nuclear bombs) $\endgroup$ – Julian Egner Sep 1 '17 at 8:39
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One answer is already shown in The Empire Strikes Back. Although Vader may have some motive towards getting the Falcon, the main objective is to destroy the Rebel alliance (Skywalker really isn't that great of a Jedi at that time). They're there, all in one spot, and surrounded by a massive fleet. A single Star Destroyer, although its name most probably doesn't mean it's meant to destroy a star, might very well just blow the better half of Hoth to smitereens from the orbit, rebels included. There's a dozen of them in orbit.
You have to wonder why they still send infantry and care to fight rebels in snow speeders when they could just have it the easy way.

The question states that environmental damage from orbital bombardement can be sufficiently mitigated so it is not a problem. This suggests technology is well enough advanced to provide something like "shields" as per the common trope (and as shown in the above movie).

The second, and equally important reason is that you don't want to destroy either the planet or the enemy facilities. You want both the planet and the facilities intact and usable without much of a need to repair infrastructure. All you really want is to remove the superfluous humans that presently occupy your new possession. Infantry can provide that, orbital bombardement doesn't.

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The real issue - whether your infantry is a Greek Phalanx or a horde of Warhammer 40K grunts - lies in defining the strategic objective is, in conducting the war.

If your opponent holds a planet that is clearly vulnerable to orbital bombardment, he values it for some reason. General principles dictate that one possible objective is to deprive him of things he values, or to offer the threat of loss. If the population and/or certain assets based on the planet are valued, a planetary bombardment that destroys that value, makes no sense unless whatever utility you would gain by holding them yourself, is outweighed by the 'message' you send by destroying them. A planet reduced to its constituent raw materials is considerably less economically valuable than a functioning outpost - at least, your opponent thinks so.

Heinlein covered this in considerable detail. Warfare is controlled, constrained, and ideally surgically-applied violence, employed to achieve a political and strategic end. In a future where orbital bombardment constitutes 'normalcy' in warfare, infantry constitutes irregular warfare, whether it's a bunch of Mechanized Infantry a la Starship Troopers busting up an outpost of the "Skinnies" , or a bunch of punkass kids trying to obtain a very specific item via a reconnaissance-in-force a la Star Wars Episode VII.

If the aim is galaxy-wide xenocide of a species who is so alien that their calculus of 'value' is meaningless to you, sure, maybe a swathe of orbitally-bombarded planets is precisely the answer, and you don't see any reason in leaving room for negotiation, because you don't want to defeat the emeny, you want to exterminate them, but in many other scenarios, the strategic objective will always represent a cost-benefit analysis.

What do you want? How much will it cost you to get it? If deployment of whatever constitutes 'irregular forces' achieves it with less cost, effort, risk or whatever you want to minimize, then a prudent general would use them.

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There will always be infantry. Its shape will just change, as it always did.

Think about it, infantry has successively with society towards more training and specialisation. During the last century, a lot of contries have shifted from conscription to a professional army in which every soldier has a very specific role for which their have been heavily trained.

Given that, speculating about what infantry would look like in your future-tech settingh has a straight answer :

Robots.

With the development of AI and engineering, your setting's factions should be able to make robots that are precisely designed for precise tactics with AI developed for that exact purpose. That leads you to all kinds, sizes, equipments and behaviours aimed at given tasks or strategies.

Regarding the precise issue in your question, you can easily imagine modules that are thrown by warships at ennemy ships so that they nail through their hulls and infest them with dozens/hundreds/thousands of modular/self sufficient and self conscious/hive minded robots.

If successful, such a tactic would allow a faction to take control of another ship without destroying it, same thing with a planet.

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  • $\begingroup$ add self-reproducing robots and imagine a ball thrown from one spaceship to another, where the robots attack, take control, and use the enemies ship an equipment to reproduce themselves. $\endgroup$ – Julian Egner Sep 1 '17 at 8:42
  • $\begingroup$ @JulianEgner That's Mech-Zergs at that point :P $\endgroup$ – ksjohn Sep 1 '17 at 8:48
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Big Infantry is needed to control a lot of ground, and the planet is not going to be worthless.

The relationship between an Orbital station and the planet is likely going to end up an awful lot like the relationship between Major cities and rural areas in the current day.

When people think of the United States, it's the cities that come to mind. People think of New York, LA, or San Francisco. That's the first image. People don't often go to Kansas or Nebraska and the enormous farmlands there. They don't think of the mining operations in Colorado, or the fossil fuels in Texas and Oklahoma. However if Kansas and Nebraska go away, New York starves in a handful of days. Without Texas, transport in LA becomes a helluvalot more expensive.

Your planets will provide the raw material basis to feed the Orbital station. There is not going to be anything up in that station that doesn't get there from a long way away. Entropy says that your closed ecology may be able to last a while, but not forever. Your stuff will have to come from somewhere and planetside is the most logical place to get it.

So why an infantry that goes planetside? If you can send troops to control the raw material resources, you render the orbital station helpless. It's a just a siege that's shaped a little differently. Orbital station is not going to want to totally bombard the surface, because that could destroy the farms, mines and such along with the infrastructure that gets the needed stuff to the station. If they blast the spaceport that foodstuff lifts off from, they starve.

Finally, if you wipe the surface of a planet, you become the genocidal maniac. Nobody want's to be that guy (it's been brought up in other answers and it absolutely bears repeating).

Always remember, War is about control, not about destruction.

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  • $\begingroup$ Yes exactly war is about control, and if you can't have control if there are survivors, control becomes about destruction doesn't it? $\endgroup$ – Ash Aug 29 '17 at 14:45
  • $\begingroup$ "War is about control, not about destruction." Isn't completely true, society has breed you to think this, but there are societies that have existed through history which viewed warfare as total. Even modern day ISIS proves an example of this. Native Americans, Aztecs would destroy whole tribes in sacrifice to the sun. Genghis Khan killed so many people, and cities that the earth cooled for a time. $\endgroup$ – CarbonDonuts Aug 29 '17 at 14:47
  • $\begingroup$ @CarbonDonuts I would argue that the Aztecs, ISIS, and especially Genghis Khan support the argument that war is about control. Genghis Kahn's reason for obliterating cities was so that he could take the next city without a fight. He was frighteningly efficient about it. ISIS wants to control that particular area of the middle east in anticipation of the end of the world. Aztecs slaughtering entire tribes was so they they could maintain the favor of the gods, ie control over their own sphere of influence. Also keep in mind that ISIS and Aztecs might be thought of as primitives. $\endgroup$ – Paul TIKI Aug 29 '17 at 15:20
  • $\begingroup$ @Ash If you can destroy a thing, you do, in fact, control it. Think of destruction as a tool in warfare. It could also be thought of as an undesired side effect. "In order to control X, we may accidentally destroy Y." $\endgroup$ – Paul TIKI Aug 29 '17 at 15:25
  • $\begingroup$ @PaulTIKI Right I'm thinking of this as a situation where, to a great degree, to control Objective A, in the long term, total destruction is essential, are you saying that being able to destroy them is enough? $\endgroup$ – Ash Aug 29 '17 at 16:05
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the vast majority of primary industry and manufacturing are in orbit so on-planet resources and infrastructure are not important targets.

Manufacturing implies resources of some kind, and if they aren't being shipped from the planet, I assume they're being mined from asteroids or moons. With your bombardment technology, presumably you could blow up an asteroid; but then you lose its resources, and what's the point of that? Plus it would be really convenient to capture any mining equipment intact. So you need infantry to take control of the asteroids.

If you're fighting in the tunnels of an asteroid mine, most stray rifle shots will simply hit rock; no big deal, so you don't need extra-low-powered weapons. And if your asteroid has a 50 (or 500) mile diameter, you can't just send in your small specialized unit. You need the numbers that only infantry can provide.

It makes sense that the defensive infantry would be there to protect the high-valued target - they'd be useless on the planet if the attackers are just going to kill everything.

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Some of your premises are...unlikely.

This is about fiction so you can set it up however you, but if you are going for any sort of realism, remember that gravity is an advantage for a lot of things, as is having the foundation beneath you. In short, it is unlikely that all manufacturing would move to space regardless of technology level or even if the atmosphere was hostile.

Second, while less significant to your question, going to bombardment as a first strike does NOT immediately indicate a disregard for civilian casualties. Unless there is some affirmative reason not too, it is standard policy in most modern militaries to start with either artillery or air strikes and then follow up with infantry. Of course, I am talking about strikes on legitimate military targets rather than civilian cities, but for the most part the military targets will be at least somewhat separated from the civilian ones for a variety of practical reasons.

Your main forces feed into your special forces.

AS others have discussed, especially MichaelK's excellent answer, infantry achieves things that bombardment cannot. I won't go into detail since they already did other than to second that point.

But even if you assume that large armies are largely useless in your world, they will still exist. You said, "There will always be reasons to have small units for assassinations, extractions, and espionage/sabotage missions on planet..." But you derive those units from your main army. If you really needed those, you would still maintain a reasonably large army just so you had somewhere to vet and select your special troops from. Major League Baseball needs the Minor Leagues to exist for instance and the NFL needs college football to exist which in turn needs high school football. If you really need those special small units, you will subsidize a relatively sizable main army to select them from.

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  • $\begingroup$ Okay a single round kinetic bombardment does about the same damage as a small nuke, or much more, targets that aren't purely "oh look a population centre" for that kind of hit are limited, in fact humans have never used military nukes for any target that wasn't a civilian population centre. I'm also going to have to disagree with you about SpecOps teams the very term assassin comes from a group without any previous military experience, training, or affiliation who trained for specific mission profiles and were hideously successful in their self-chosen role. $\endgroup$ – Ash Aug 29 '17 at 17:19
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    $\begingroup$ Its your fiction, but there is no reason a "single round kinetic bombardment" would need to be as powerful as a nuke. We have the technology today to perform them in a variety of sizes, they are just ineffecient. As for assassins, the word comes from the hashashin. The hasashin were a true military with fortresses and units. Only a small portion practiced assassination as we use the term today and their effectiveness, other than spreading fear, is debatable. $\endgroup$ – TimothyAWiseman Aug 29 '17 at 18:10
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Defences can be hardened or hidden vs orbital bombardment

At the onset of war, the defending planet will get devastated from orbital bombardment. Because this is a common occurrence, it means that defenders would likely have people in bunkers or hidden locations for a counter attack. The devastation and rubble of the aftermath makes it hard to search for their locations, or the location of any hostages or POWs who might still be alive.

Humans with flexible thinking are good resources to ferret out human resistance

For the purpose of going thru complex terrain that cannot be scanned from orbit, the attackers have to land a moderately sized force in a land invasion. These have to be people and not drones (except in supporting roles) because machines aren't good at searching out people. In addition, the soldiers used get to loot any leftover baubles so they get many volunteers for this position, especially from the poorer populace.

Undersea or underground bases cannot be easily attacked remotely

Armies can follow human transportation links such as tunnels, elevators etc. to chase after retreating defenders and wipe them out before mediators can stop the war. Trying to solve the problem with more bombs might not work, and would attract attention from the stellar navy incoming to mediate the conflict.

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  • $\begingroup$ You're assuming far too low an upper limit to the weapons people are willing to use, if you're attacker drops something this size as a starting proposition and they just get bigger, it really doesn't where you are or what you're under. $\endgroup$ – Ash Aug 30 '17 at 19:55
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If you want to destroy things, you don't need infantry - you use artillery, tanks, air strikes - and their far future equivalents. However, even today artillery use can be denied by counter-battery, air strikes can be interdicted by AA or air cover, tanks can be held in check by missiles, minefields, other tanks, rivers without fords and bridges, ...

Infantry is for when you don't want to destroy everything. Do you want to use the oil fields and refineries? Artillery and air strikes won't help. Do you want to fill your ships with fresh foods? An already existing population might be willing to provide, or be convinced by means other than orbital strikes on their farm. Do you want to have a safe place for recreation for the crews of your spaceships? A security force is needed if the population is less than friendly.

Planets might have value out of proportion to their "worth" if their location in galaxy is crucial, and working with a less than totally hostile local population might be much better than destroying everything and rebuilding (with things brought by spaceship from large distances)

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  1. the vast majority of primary industry and manufacturing are in orbit so on-planet resources and infrastructure are not important targets.

Your third point could be incredibly wrong, unless your raw materials for manufacturing come from outer space. The larger you consumption is, the larger your infrastructure to obtain your raw materials will be. You still have to have the infrastructure to get some of your raw materials, in particular, mining. Many minerals come in abundance from mining of one form or another.

To support the mining you also have to have the infrastructure:

  1. Mining infrastructure
  2. Refining infrastructure
  3. Refined transportation
  4. Processing infrastructure
  5. Distribution

Oh, and don't forget about surveying and so on, to locate new resources.

Even if you can get your raw materials from outer space, you may find that not all processes are capable of working in space, due to gravitational requirements (unless your space stations, etc, have some form of false gravity). Also, the quantities that you could mine or obtain in space would be far less and most likely more expensive to obtain than traditional mining methods.

Still, I do believe that refining or smelting iron in space could be ... interesting!

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If your universe has truly trashed everything at the bottom of the gravity well of planets, then there will not be any reason to use planetary armies. Heck, you won't even need orbital bombardment either, because there is nothing worth bombarding down there anyway.

But I don't think your description of your universe is consistent with "trashing everything at the bottom of the gravity well".

What are the people doing down there? How is the planetary economy related to the space infrastructure? How are they affording all the energy and other imports? Do they truly generate nothing of value to be exported (because, if they do generate things of value and export them to space, then the planets are practically a part of the space infrastructure, which is inconsistent with "trashing everything at the bottom of the gravity well")?

If you can make things consistent, then you might get better answers.

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