I'm writing a realistic sci fi story involving interstellar conflict , where space warships (assume something like star destroyers from star wars) are the norm, These warships are capable of orbital bombardment and can essentially strike any place on a planet with immense force and rapid speed. In such a situation, would a traditional army and navy even be necessary? A single spaceship could wipe out an entire navy or army easily from orbit with no losses, which I feel makes them redundant due to the immense disadvantage. Would it be better to instead only have a group of elite marines to deploy from orbit to occupy objectives and perform delicate operations instead of keeping around a big army and navy to fight wars on the surface? I would like to have futuristic land and naval battles but I don't see how that is possible when a spaceship could just wipe everything out.
Space ships are like aircraft. They are very good at projecting power, but they are less good at things like gathering intelligence or holding indoor positions. A spaceship or aircraft can knock down a building, but it takes a person to take it more intact. It takes a person to protect people on the ground that you don't want to kill.
So, unless you're sterilizing a planet, if you want to subdue, not exterminate the population on the land surface, you'll need an army.
The same goes with a navy. If you're trying to control the sentient population of the planet on or under the water, unless you want to just kill everyone and everything, you're going to need tools capable of more finesse than an orbital bombardment. Sure, aircraft can prosecute attacks against ships and submarines - when they can find them. A ship or a submarine can be better at finding other ships or submarines than a spaceship.
You can bombard as much as you want, but until you are not able to set your boots on the ground and hold them there, you will not be controlling a piece of land.
Look at the most recent wars, like operation Desert Storm or the various operations in Afghanistan between 2001 and 2021: air supremacy and capability of destroying at will is one thing, having troops on the ground capable of actually controlling the territory is another.
Same will hold in your scenario. You can orbital bomb as much as you want, but with no troops on the ground to occupy it, you will not control anything. You still need army and navy.
I don't know if this should be a comment rather than an answer, because what I really wanted to say is "your question needs more details or clarity". Why is that? Because I see you start for the common, but unsubstantiated, mindset of "a spaceship on orbit will have the upper-hand over ground forces", which is something coming from Star Wars-like space operas. There is absolutely no reason why your story has to follow that convention, and if you want to be original, you maybe shouldn't.
Why a spaceship orbiting a planet has advantage over the surface forces? For the same reasons Obi-Wan Kenobi had advantage over Anakin Skywalker by being on higher ground: plot reasons. Or in other words, no reason at all. The supposed advantage of orbital control is assuming that the forces on the ground cannot hit you, but why that should be the case? Sure, the USA in Afghanistan or Israel over Gaza have a massive advantage through its air force... but this advantage is just the enemy having no anti-aircraft weapons. In a war where both sides have strong anti-aircraft capabilities, such as in the Russian-Ukranian war, the idea of air force as an infallible way to victory is contested.
Supposing that your enemy planet is not some backward shithole whose civilization is still at early bronze age, the idea of superiority of orbital spaceships is highly disputable. There are just two possibilities: your story takes place in a hard-scifi setting, where those ships have been probably been built in space because it's too hard sending all that weighty mass to space, and they move through the old Newtonian third-law of forces, expelling some kind of propellant to create a change of velocity, or it's a more soft-scifi approach where antigravity devices or similar methods of propulsion are in place.
In the first setting, the spaceships in orbit have the advantage that it is very difficult for the planetarian defense to hurl things up against the planet gravity well... but the thing is, they don't need to launch a lot of mass to destroy the spaceships. Any ICBM will do. And your spaceships are sitting ducks while in orbit. Their trajectories are predictable, and the maneuverability of those huge ships must be extremely cumbersome. Basically, you're an old WWI battlecruiser chased by modern fighter jets.
In the second setting your spaceships are allowed to have much more maneuverability and speed thanks to these non-newtonian technology... but so are your enemies. With artificial gravity, there's no reason why a planet cannot have armored spaceports where their defence fleet lies, and they can get to orbit to fight your invading fleet.
Every time that the trope of "orbital superiority" arises is because its advocats are assuming that they can bomb the planet at will, while they can't be hit from the planet. Unless there's a significant militar imbalance between attackers and defenders, that needn't be the case.
Why do we have armies and navies now?
Even without a space force, there are at least 2 powers on Earth that could strike any army or navy deployed anywhere on the globe and several others that could do so on a regional scale. Yet all of those maintain ground, air, and seaborne conventional forces. Why?
- MAD - if you can do it to them, they can do it to you.
- Genocide does not play well in the media. Even if you are totally impervious to the enemy's retaliation, there are likely to be elements of your own society as well as neutral third parties who are uncomfortable with total planetary destruction.
- Occupation - so you're OK with genocide and you don't fear retaliation, maybe you want to occupy the territory (with or without the native population) without having to rebuild flattened infrastructure.
Submarines are hard to find and hard to destroy
In addition to the need for infantry mentioned above. Do not underestimate the ability of water to keep things from being seen and absorb and dissipate most types of energy in large quantities. Submarines are very difficult to find without extensive sonar systems (which themselves kinda become a navy). And even if you've got a way to find them from space somehow, if they've dived it's going to take specialized weapons just to deliver fire on them. A laser or railgun is not going to penetrate and a missile will impact on the surface. Again meaning you need something a bit like a drone navy.
The navy is redundant/obsolete
In your scenario, there's no need for a navy. The point of boots on the ground is to handle the interpersonal stuff (which will definitely include skirmish-level firefights, but will definitely also include establishing control over the civilian government organisation). Transport between places will be by aircraft or intercontinental spacecraft.
Hell, even the navy today is no longer the navy of the days of battleships. The early 1940s established firmly that aircraft could kill any battleship with impunity, and sonar then let them kill any attack submarine with impunity too. From the 1940s then, the only role of a navy was to move aircraft around. The 1950s added the role of moving nuclear missiles around underwater, and the 1970s added the role of moving cruise missiles around. The navy has no other military raison d'etre.
If you've got guys in orbit prepared to deliver Rods From God, all those roles simply vanish.
This always bothered me in star trek and star wars. You have ships attacking planets. Planets that effectively have unlimited power, unlimited shield, and virtually unlimited weapons. If you had a navy, they are going to have whatever space shields your space ships have. And they will be able to provide tons more power towards it because they don't need life support systems. In a more realistic setting, attacking a planet should be suicide.
But let's take this a step further, why does a space ship need to shoot the planet at all? Fly out to the asteroid belt, and hurl one at Earth. With precise enough calculations you could hit the exact spot you want.
So a navy might actually be more important in the space warfare age. Simply to make the target not a city or a place with a lot of civilian damage. If I can put my giant space ship busting guns on a navel ship, then of course I am going to do that. Any counter fire will land in the ocean.
But let's now take this less sci-fi. You have a space ship, it fires a missile at the Earth. What do you do? Well the problem with intercepting missiles is we have to figure out where they are going. Usually we figure this out as they start their decent. Well your space ship can only shoot downwards. But does that affect anything? Not really, the missile is easily intercepted by a (much) cheaper ground or navel based missile. This dramatically diminishes the value of a space ship firing on the Earth.
When writing fiction I often try to look at reality, and it is a bonus if it is related to something actively happening now.
There is a weapon that can completely obliterate the enemy. There are also weaker weapons that can kill some of the enemy. Why would the less powerful weapon ever be used?
Ukraine is fighting a war with Russia. Russia has nukes and Ukraine doesn't, why does Ukraine bother with having an army?
I can think of a few reasons:
- Russia doesn't want to use nukes (for fear of angering other countries)
- Ukraine wants to be able to defend itself from other countries (eg. Belarus) that don't have nukes
- Ukraine wants to use its relatively weak (ie. without nukes) military to police certain regions under its control.
- Russia wants access to the Ukrainian controlled port. If Russia bombs it to ground zero, there won't be any port to use.
Israel is fighting Gaza. Israel militarily overpowers Gaza in every way. Why doesn't Israel simply bomb Gaza and end the war?
- Israel doesn't want to indiscriminately target and kill 2m Gazens.
- Israel wants to rescue its hostages and not have them die of collateral damage.
See if any of those reasons help you with your plot.
Welcome the United Spacey! We look back on a long history of shipping, transport, and warfare, going back almost as far as the Army! We were the big stick of Roosevelt! At one point we used to be called the Navy, but with the advance of spacecraft, we rebranded into the Space Navy, short Spacey. We are the only ones who went with the time - our carrier vessels ships swim and fly to the stars!
Join the Mobile Infantry! (Army)
Welcome to the Planetary Defence Forces!
You see those Spacy and MI people leave for war, never to return? Do your part here, right at home! Get your hands on the hottest and fastest interceptor fighters and blast those invaders to hell!
What are your war aims?
Do you just want to kill the entire population of a planet? Then you don't need ground forces. Also, you're a monster. (Hypothetical you. If you're writing a story with monsters in it, that's fine).
Do you want to kill specific enemies while harming as few of their innocent neighbors as possible? Do you want to seize infrastructure intact for your own use? Collect taxes? Enforce laws? Rescue hostages? Distribute humanitarian aid despite local despots who would prefer the population starve?
All those things require ground forces.
The question fails to consider that the enemy also has a vote.
Assuming a "Star Wars" type setting, a planetary system has the energy and resources to build a comparable fleet of spacecraft - indeed, by omitting the FTL drive it is likely that several warships could be built for the expense of one FTL warship. Gaining space superiority may not be anywhere as easy as you expect.
As well, fixed defenses can also be built on planets, moons and asteroids, with the potential to be much more powerful than anything capable of being carried by a ship. You might find yourself fighting through a system with powerful ship killing weapons hidden away in many different places, unexpectedly exposing themselves and firing when you least expect it. Gaining a superior position in orbit will require a powerful battlefleet capable of fighting it's way across the Solar System - enemy squadrons backed by fixed fortifications and weapons will be waiting to oppose you.
As well, while you may be capable of bombarding a planet into rubble, why won't the enemy try the same tactic against you? Your fleet might be better employed defending your planets than trying to attack another planet.
Finally, your scenario is actually self defeating - if you can just appear in orbit and bombard the planet, then large scale political organizations like "The Republic" or "The Empire" have no real use for most people - how can they protect you from what are essentially Space Vikings from plundering your planet? Once the technology arises to allow you to do this, then eventually polities will disintegrate into single planetary systems, and upwards mobility will be either becoming space pirates yourself or launching hyperspace bombardment missiles at any planet which has the ability to make spacecraft capable of threatening you.
Ground forces will still be needed regardless to man the defenses, control the population (or move them into shelters) and keep control of the territory. Some form of Commando or SoF forces will also exist to conduct raids against enemy systems, attempt to disable defenses and fortifications or act as forward observers for space-born weapons, which conventional armies will be needed to prevent landings or flush out landed units.
So your scenario is the trigger for disintegration of large scale political units, preemptive strikes against other planets and low level Commando warfare on the ground and in space in support of the larger weapons systems
You need more space assets to do what you want than you think
Planets are big on a scale we don't really grasp. And orbital shells are mathematically required to be even bigger.
Sure, if you only have a few brush fires a day you can divert your starship to act as overwatch and put munitions on target. But if you are fighting the population of the entire planet? Just monitoring is going to take a dense fleet of re-deployable satellites. Fighting?, that's going to take an enormous fleet to be able to reposition onto hotspots and hit them. And there's probably little reason to think that a spaceship isn't the least cost effective form of artillery out there.
Using a lot of surface forces is just cheaper, maybe cheap enough to be possible,
I assume you aren't using weapons of mass destruction because the only reason to bother with the planet is because you want it. If you just wanted to mine there are plenty of asteroids, so you probably want the biosphere.
"Why do we have soldiers when we have Battleships?"
"Why do we have soldiers when we have Tanks?"
"Why do we have soldiers when we have Aircraft?"
"Why do we have soldiers when we have Nukes?"
"Why do we have soldiers when we have Drones?"
"Why do we have soldiers when we have super-duper-future-death-rayweapons?"
When you read military history/argue in military history forums - this argument in some way/shape/form always rears it's head.
All throughout Wars - there has been some general or other senior person who, from a place of noble intention of not wanting to see young men endure the horrors of combat has suggested that insert superweapon could replace boots on the ground (and sometimes because they have significant shares in Superweapon manufacturer but we aren't covering that today)
And everytime this is discussed, someone - invariably from the USMC - stands up and says "The most dangerous Weapons system on the Battlefield is a Marine and his Rifle!" and then the commensurate 'OO RAHS!' from every Devildog in a 50 mile radius.
Whilst it may not seem that way, there is a lot more truth in that statement that we would care to admit.
Firstly - control of area - without boots on the ground, patrolling, projecting force, being a constant presence and deterrence - you don't control the area.
That actually has a really big deal - think of the Ho Chi Mingh trail in Vietnam, think of the insurgencies we've seen - even with massive technological superiority, the enemy were able to effectively move Men/Resources covertly in order to be a PiTA.
Secondly - Humans are really quite versatile and rather stealthy. Look at the Ukraine conflict with Tanks getting annihilated by small teams in well-prepared ambush locations. Even with top-tier surveillance systems, Humans can hide pretty effectively - buildings, tunnels - hell the USMC recently defeated an AI surveillance system with a bedsheet.
Thirdly - Threat, perceived threat and target prioritization. So, you're in orbit with your super-Death-Ship - you see a massed formation of big metal things with big shooty things and you decide to unleash a phase-shift-thermo-torp at them. That'll show them!
Then you see with your super-advanced surveillance systems that have crystal clear resolution a lone boy walking through the desert.
Obviously not a threat - what could they possibly do? What you don't know is that said boy has a radio and has just observed the trajectory of your torp that annihilated the decoys that you just blew up and has radioed in your position to the surface batteries that were hidden and the next thing you know - you are playing a non-consensual game of 'who can survive in the vacuum of space the longest'
In short - no Super-weapon ever replaces the need for maintaining control in a particular environment.
The thing with high science fiction is that if you can throw rocks from space, the planet can throw mountains back up.
Ignoring that, there’s only one reason to use massed orbital bombardment: genocide. But this has many problems, namely your neighbors.
In our current world we don’t nuke each other constantly because of reasons like “the people I didn’t nuke will likely not wait until its their turn to be nuked”. One thing people seem to forget is that the planet you orbital bombarded isn’t the only one, and everyone who knows you did it now has a good reason to do it to you until you aren’t able to do it again.
Another real world example: when nukes were attained, the USA assumed every war afterwards would be a nuclear war and that regular army would not play a major role. Since then regular armies played a bigger role than nukes (exception for the threat of nukes over firing it).
If what you want is resources, the buildings and infrastructure you destroy is some of the easiest to attain resources. Its a waste destroying it.
Some people argue “ah but in Galactic timespans just waiting for a planet to become livable again is a small thing”. But when you compare the effort and energy of making the planet livable again against the energy and effort it takes to gain control of the planet and subsequently start using the planet (even if you genocide everyone and repopulate) then orbital bombardment is always a bad strategy. In short term timescales its also a bad idea to bomb everything…
So the only reason to use orbital bombardment is either active genocide because you don’t care or limited scale orbital bombardments against specific targets so the planet remains useful.
But a thing anyone does is prepare for war. If orbital bombardment is possible, armies will prepare for them. Bunkers, underground infrastructure for long distance transport or holding territory, shields* (?), weapons to counter orbital bombardments, obfuscation of targets or decoys, defenses that prevent enemy space ships from easy targeting (and if fired from farther it gives a lot of time to intercept), placing military installations around things an attacker would want to keep intact etc.
We haven’t even discussed space-based habitats. Its likely you have access to space habitats and even likely that a significant portion of the population is inside these. Capturing these will be more useful than destroying them, especially with potential Kessler syndrome denying you the planet (and any of your ships being instantly engaged by other parties if you get close, no one wants to risk getting genocided by maniacs shooting space-cities out of the sky).
All this results in one thing: you will want an army. The type of army will depend on your strategic goals, current conflicts, threat of neighbors and available methods of arming yourself. But rest assured that armies will exist, and most armies will be used more than orbital bombardment.
*assuming you have shields, you don’t need vast powerful orbital shields. All you need is a shield with just enough power to create a shock to the orbital bombardment projectile and cause it to selfdestruct. Many small shields that can be placed in the path of projectiles would provide a whipple-shield effect and severely reduce the impact of orbital bombardments. The beauty of this is that things that move slower can easily penetrate the weak shields and still land.
Mobile Planetary Defenses
All of this answer assumes that both sides want to have control of the planet with minimal losses of civilian life and minimal destruction of civilian infrastructure. This also assumes a war between two multi-planetary political entities, both of which have FTL technology.
First, think from the perspective of the defenders. Attackers may show up with a large space fleet around any of their planets. A defensive space fleet won't be able to respond in time. Thus, each of their planets need anti-space defenses. There will likely be space-based defenses and planet-side defenses. The challenge with planet-side defenses is that planets are at the bottom of their own gravity well, so energy weapons will fare much better than kinetic or missile weapons despite the atmosphere you need to shoot through. The trope approach of planetary based guns (ex. https://starwars.fandom.com/wiki/Ion_cannon) are bad because they are stationary, and therefore are easy targets for orbital kinetic weapons. Mobile planet-side defenses are going to be more effective. The easiest way to make them mobile (assuming sufficient water coverage of a planet's surface) is to have them be boats. Hence, a water navy would be an essential part of planetary defense.
If the defenders have a water navy trying to shoot space ships out of orbit, the attacker isn't going to be able to stay in orbit very long. Thus, attacking a planet looks like the following:
- Space Battle: attack fleet shows up and fights the space-based defenses while being shot at by the planet-based defenses.
- Naval Drop: assuming the attackers win the space battle, attack fleet drops war ships to fight the defender's navy and attempts limited bombardment of key targets (such as any grounds-based anti-space defenses) before retreating.
- Naval Battle: attacker's navies fight across the planet's oceans to try and defeat defender's anti-space defenses.
- Ground Battle: assuming the attackers win the naval battle, the attackers now have space superiority. They now need to send in their army to capture enough key infrastructure points to get the planet to surrender. They also potentially need to do this before the defender's space fleet shows up to kick them out of orbit.
Summery of options for ground defenders.
Orbital defence systems: Sure your navy can blast/fight your way though them but its a race. The enemy will keep launching more more while your busy shooting them down. And planet 'big'/spaceships 'little' so their magazines will potentially be larger than yours. You might still win but it costs you time and effort.
Global ECM & EW networks. You can scan a planet optically from orbit but unless your EW systems etc are as good or better than the enemies a lot of the time your other sensors will at best be partially if not totally blinded.
Fixed Ground based heavy weapons: The fixed ground based systems can pack a punch as least as heavy if not heavier than your ship board capital weapons and they are well hidden (unless your spooks have done their job well). The enemy will hold them in reserve until your navy enters orbit and then fire them. They won't last long because once they fire they're almost certainly going to be detected but it doesn't matter. If one large ground based defense system destroys or badly damages one of your Star Destroyers it's a win net for the enemy because they can build what? 10/20/30/?) 'big guns' for the cost of your ship.
Mobile planetary defense systems: see submarines previously mentioned. Perhaps smaller and less powerful than land based systems but also much harder to detect and (relatively) more survivable.
Optical fiber networks: Your fleet 'networks' via photons traveling through a vacuum. These potentially be intercepted. Think of all those frustrating stealthed micro sats the enemy has deployed into Lagrange points and high orbits or perhaps even just automated listening posts on the local moon. The enemy however has a hard wired grid. Nothing to intercept or jam. Even mobile units can tap into the network when they need to.
Manpower: The planet already grossly outnumbers you. Whatever small % of its population it can in reality afford to employ in the military can leverage the high degree of automation the network allows into large manpower savings. It can automate it's fixed based defense systems so that they are 'manned' by very small cadre of human operators working from concealed locations. The entire planetary defense grid might only have a few hundred people operating it freeing many more soldiers for combat duty. Your fleet however still needs thousands of people just to say in orbit before it can even think about firing a shot or landing one pair of boots on the ground.
Mobile ground combatants: Soldiers, armored vehicles, supply chases and perhaps even robotic combatants all camouflaged and disbursed around strategical important locations just waiting for your marines to land. Again unless its a developing colony world they almost certainly outnumber you.
Conclusion. You have the all important high ground which is invaluable if or when you decide to launch a ground invasion but its not a magic bullet. You only really have two choices. A long a expensive (perhaps decades long?) siege as you slowly wear the enemy down from orbit or you go down and take the place. Good luck.
Because inhabitable planets aren't cheap
Repeat after me, kids: a reaction drive is a weapon effective in proportion to its efficiency. A drive powerful enough to enable somewhat-convenient interstellar travel is a really powerful weapon. So powerful, in fact, that it's trivially easy and cheap to destroy a planet.
Unfortunately for the Ministry of Defense's pocketbook, while the means to destroy a planet is cheap, the planets themselves are quite costly. Terraforming exists, but it's expensive, takes a while, and the result isn't ever as good as a naturally-inhabitable planet. In the end, it's usually cheaper to just send in the Mobile Infantry.
As a result, while space fleets can easily destroy planets, they mostly just use that capability against airless rocks with no civilian value. Limited orbital bombardments are used to support invasions of inhabitable planets, but the majority of the fighting is done by boots on the ground.
A military is as much about defence as offence.
Effectively, you're wondering why keep a standing military when you have the nuke.
The issue is nukes don't actually solve anything. If you start using one to vaporise the enemy, they'll do the same, and you'll both end up vaporise. If you want to actually takeover the enemy's land rather than leave it a radioactive crater, nukes are also subpar weapons. If you're defending your territory, you might prefer that it also doesn't become a radioactive crater.
We could of course entertain the idea of keeping elite commandos to do those jobs of taking someone's oilfields and other assorted stuff. But then that can be countered by keeping a slightly larger elite commando force. The logical conclusion there is a sizeable standing military that will deter others from trying anything stupid and/or allow you to do something stupid.
And I know this will anger military nerds, but a bunch of space marines is an army. Even if we operate with the assumption that you just have battleships and commandos, these commandos will need bases. By the time you account for all the infrastructure necessary to run these bases, you'll end up with a small army. You'll have commandos, yes, and training instructors, pencil pushers, MPs and JAGs, quartermasters, wrench monkeys, and so on, and so forth.
Simply put, if you entertain the idea of tickling others on their turf, you're gonna have an army. You can call it something else, but it'll be an army.
Where there's ocean, there's boats.
A similar rational applies to a naval fleet. You're going to have fishing boats and yachts or other recreational boats, and thus you're going to have a navy that can bring the law to the high seas when necessary, and operate search and rescue missions, and other naval shenanigans.
You could also use the opportunity to place a few submarines with ASAT weapons, and become part of your defence system.
If you're planning on invade Earth from space, you won't need to bring a navy. But at home, you're probably going to have one. Maybe not a big one, but you're going to have one.
And other benefits.
As it turns out, having a bunch of people on payroll sworn to obey your orders as a few other advantages.
You can order them to go risk their lives to save others. The US Coast Guards is a military branch. In a variety of countries, the military is called to help dealing with natural/industrial disasters. They can be trained to deal with such scenarios, however unlikely, and more importantly they don't have the option of saying "no way, I'm not risking my life for these shmucks who can afford a boat but not a life jacket".
You can call on them to maintain public order. The French Gendarmerie is a military branch, and acts as a substitute to police in a variety of circumstances. A number of armed police units are paramilitary if not strictly military. The advantage here is you can keep the big guns separate, because maybe it doesn't make sense for the police to have armored vehicles and snipers.
You can run a country with it. Juntas do work best with a sizeable military. Otherwise you might have to deal with elections and opponents, and who wants that?
Why send a starship in the first place?
Turning any unit in to a smoking crater is rather powerful. A number of the other answers have addressed scale of response, but one thing I have not seen addressed is doctrine.
A Weapon platform?The destructive power of a sci-fi spaceship can of course be immense, but if a civilization is capable of building one, they could as easily build a big missile, or drone, or interstellar satellite with FTL radio, or even a flying drone factory. If all that is necessary is to transport weapons somewhere, it gets a lot easier when you don't need it to carry and support humans. You can go faster, farther, be smaller and be way more power efficient. A number of probes have been sent to Mars, but an actual crewed mission is orders of magnitude more difficult. You also need to carry all your power or ammunition with you, whereas a planet with a civilization on it has vastly more resources. If the population of the planet is advanced enough to be a threat you're much better off tossing hypersonic asteroids at it if you want to knock it out. Being a solar system away and accelerating masses to near-c gives you much more advantage than being extremely visible and touchable in low orbit shooting lasers or what not. A starship is not a good weapons platform.
A mobile base?It may be that the weapons it can field don't have the impact to overcome a planetary defense, or are only to protect it from other starships, but how about using a starship as a marine base? A number of other answers/comments mention the value of ground forces, which is certainly true, but they were provided for in the question prompt. The question implies the landing forces are temporary and their function is to do things that big guns can't, but only as long as necessary. Presumably, they'd then return to ship, otherwise they would be an army/navy. Landing your forces is one thing, but supporting and withdrawing your forces may be extremely difficult. Your troops may have powerful weapons, but it is likely any culture that is a threat has weapons too. They will likely have heavy weapons like tanks, artillery and jets. While you can carry and deploy such weapons, they need maintenance and crews, which reduces the manpower you can put in to theater by replacing it with combat tonnage and support staff. Direct support from orbit may be possible with directed energy weapons or the like. Keep in mind though that if your starship needs to move elsewhere, for any reason, your ground forces are now at the bottom of gravity well with no support. If your enemy is smart, the moment you launch ground forces would be the exact time to blow up installations, attack occupation forces or kick off a coup on the other side of the planet. Reclaiming your forces, especially heavy equipment, from the surface will take a lot of energy and likely time. The best thing the enemy could do is get you to commit ground forces, because now they have a shot clock on the wall. They know you can't respond to anything else until you can pick your forces back up. If they can fend off pickup while containing your units, you're in the soup. A starship is a bad mobile base.
Why send a starship in the first place?What can a starship do? It can move people across space. I'd suggest that is the primary reason to construct one. The weapons it can field are secondary.
How to make starships useful, then? If I were to design a planetary invasion doctrine, first I'd want low orbit superiority with established intelligence, strike and suppression nets. Until we are talking about getting close enough to touch atmosphere, there's not much reason to move people there. Once low orbit is secure, land your marines to establish a drop zone. You'd want to set some type of quick supply route like a space elevator. Your ship is now a command post and is rooted. Get heavy weapons on the dirt and break out of the initial drop zone. In low orbit build/launch a waystation to host the ground umbilical. Widen the drop zone and install air and ground defense. Prepare landing areas, improved bivouac and drop vehicle crew and staff. Transfer theater command to the ground and drop regular forces to spell the landing elements. Space assets should set up transfer, fueling and monitoring locations at Lagrange points. Rinse and repeat the landing protocol, but with heavy weapon backup from the alpha site. Once safe zone is established, scale up power generation and set up automated industry to machine parts and ammunition dirtside while widening your zone of control. Next comes local food production and secondary services.
The other answers are great, but I want to point out that energy and material resources on the surface of a planet are much greater than could be stored in space. This means that a ground-based laser gun or heavy missile storage potentially could be more devastating than any space-based weapons, so an approaching fleet could be destroyed.
I expect that such ground-based guns to be operated by the army.
MY COMMENT on reddit reworked:
Inconsequential. The issue is that honestly by time civilization has capability traverse interstellar space and develop the weaponry for long distance fighting; Land warfare would be rendered a largely obsolete endeavor. SOldier or personal combat is really something usually only encountered in special circumstance where a limiting factor or necessity render big weapons overkill. Regardless; there are various instances when ground combat or "terrestrial" engagement is required. The issue is... if you have domain over the space above...orbital bombardment which can be pinpointed and accurate.? What exactly would necessitate deploying troops on the ground? Routing out insurgents, dedicated pacification of a group/faction.
Ground combat on the scale of Earth's historical wars Involving tanks and mechanized vehicles.... is likely obsolete except in instances where Enemy possesses a similar level of technology and historical parallel development.
The movie Star Trek: Insurrection (9th movie) did a reasonable job anticipating drone warfare. And Star trek II depicting stealth/subtrefuge. There are only a few types major circumstances where land combat rears its head.
Defending some kind of tangible asset that cannot be moved or relocated (Example: Star Trek: DS9 episode Siege AR558) or diplomat, VIP, protected status ethnic group) or location (Embasssy/base)
Repelling boarding parties from your vessel/station/installation. Or acting as a boarding party
Constabulary/peacekeeping function for a defeated or unstable population or mop up operations from a systemic problem BUG HUNT See movie "Aliens"
Specific Environment or technological encumberance that render specific strategic technologies unworkable. (Example: Avatar's unbreathable atmosphere, unique biology, Dune: shields are so effective, melee combat is only way kill assailants effectively)
Safeguarding your own populace from a terror/invasion attack (Example: Halo franchise, battles of Reach, Harvest, Earth, Star Wars: Battle of Hoth)
Can your ship differentiate between civilians, friends and foes? (Example: TNG: Who Watches the Watchers) consistently, reliably and accurately?
Do said rules of engagements require that a weapon be observed. Can the starship tell if a individual on the surface is carrying a sword, or a gun or a gun shaped toy? or if similar technologies.
Can the starship see if the person has a weapon under their clothes?
How much of a planet’s surface can this starship cover at any one time? Presumably most of the planet would not be under the starships line of sight, and therefore not immediately reachable. To support multiple teams on the surface would require a lot of starships or shuttles.
How does said weaponry of the ship from orbit protect you against hidden IEDs, teenagers with hidden knives walking up with a smile, a suicide bomber, a lone sniper or someone sneaking into your barracks at night?
If your military fleet needs to turn a planet into a desert to pacify it, is that acceptable? If the enemy is among a civilian populace; spread out in tactical formation across a continent, would take up a huge amount of real estate. At what point does vaporizing chunks of the planet’s surface become criminal, the real US military has sensory capabilities bordering on what we see in Trek, and the ability to project overwhelming and highly accurate lethal force on any square meter of the countries we invaded in the last 20 years… and yet, somehow that appears to be insufficient to defeat.