The universe is populated by a decent number of different spacefaring civilisations and races, the largest spanning over hundreds of systems. They have formed relations with each other: Diplomacy, trade, and of course war.

Most nations can field large military fleets and the largest nations have constructed huge dreadnoughts up to a kilometer long (though nothing planet destroying). Space battles often have epic proportions, with hundreds of vessels exchanging fire.

Yet, encounters of ground forces are far smaller in scale, with usually less than a hundred of combatants. Tanks and large mechs exist, but are a rare sight. Instead, most forces are small elite teams.

Why would a nation have so much bigger space than ground forces, when it has the economical power to field millions of soldiers or tanks, or even build huge titans?

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    Why invest resources in ground troops when space superiority determines who actually makes it to the ground alive? – Muuski Feb 1 at 19:38
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    That's the same reason navel warfare is at a larger scale then ground warfare. And if you think the scale of navel warfare has shrunk, just look at a carrier battle group. Just because we don't fight big navel battles anymore doesn't mean it's shrunk only that most countries cant afford the scale of it. There is also the age old "high ground" deal. – ArtisticPhoenix Feb 1 at 21:02
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    @ArtisticPhoenix I'm being overly pedantic, but I just want to point out that it's naval warfare. Navel actually means bellybutton, and is not a particularly large war front in most cases. – Ethan Feb 1 at 21:39
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    @Ethan - ugg, spelling and grammar strikes again, I'm very Dyslexic. It's a constant chore for me with things like there and their, and even d and b's when I hand write ( which is almost never ) – ArtisticPhoenix Feb 1 at 21:44
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    @DonaldHobson: Pounding a planet's surface into dust and steam and lava is sufficient for most purposes. Blowing a planet up isn't necessary unless you're showing off or fighting staircase gods. – Beta Feb 3 at 19:13

14 Answers 14

up vote 101 down vote accepted

The reason no space-faring nation would invest in large, expensive ground forces is the fact that they have no defense against your kilometer-long dreadnoughts. However, a small special forces team could still be used because their defense is the risk of collateral damage.

Got a 100,000-strong soldier-and-mech army on your planet? The fleet clears that problem up with a quick and cheap barrage from space.

Got a tiny special forces team causing a ruckus on your planet? Unless the fleet wants to take out an entire city to kill 10 enemy troops, they can't do anything except send in their own small teams.

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    What if the enemy forces are dug in in the sci-fi equivalent of bunkers to resist the bombardment? Is that a problem? – Sinthorion Feb 2 at 10:44
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    @Sinthorion: Try housing 100,000 soldiers in bunkers. Keep in mind that for structural stability reasons, the rooms must be small and sturdy. Are the soldiers all inside your massive bunker complex? Good. The enemy is landing in 25 minutes. Everyone must leave the bunkers ASAP in order to prepare for their arrival. Good luck organizing that. And that's not even factoring in other tactics such as e.g. sieging them and keeping them trapped and starving, or bombarding them with biological weapons that linger in the environment, thus trapping them in their bunkers. – Flater Feb 2 at 11:27
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    "Got a 100,000-strong soldier-and-mech army on your planet? The fleet clears that problem up with a quick and cheap barrage from space." That's it in a nutshell. – Len Feb 2 at 15:35
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    You don't need to kill. You just destroy their supplies. How do you feed troops that size in a bunker? One week's worth of water and food will take up a ton of space. – Nelson Feb 3 at 5:32
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    there is also the issue that rocks are fairly cheep in space once you have a galactic civilization, and you can just keep dropping them on the bunker until it is crater. orbital drops make modern munitions look like fire crackers. All you really need to do is collapse the entrances, then the bunker is just a really big coffin. – John Feb 5 at 7:25

If you have orbital superiority, small teams are the only ground forces worth deploying.

You can drop a nuke from orbit and incinerate a city. You can drop 'rods from God' to precisely annihilate bunkers with the raw power of kinetic energy. You can drop precision-guided munitions, turn sands to glass with lasers, or wipe out an army with a railgun shell.

What you can't do is extract a VIP, or secure a hostage, or fight an insurgency door-to-door, or rendezvous with an asset, or take a command post, or seize a broadcast station. Your policy options go from 'sternly worded letter' straight to 'obliteration from orbit', with crater size being your only variable.

Your ground forces do not exist to subdue cities or destroy armies as they did in the past- that function has now been taken over by your fleets. The ground forces exist to go where a spacecraft in orbit cannot see, do things a spacecraft cannot do, and surgically apply force in ways that spacecraft cannot.

This is not to say that you shouldn't invest resources in ground troops: On the contrary, they should be significant investments, the best-of-the-best, as much diplomats and strategists as they are door-kickers and trigger-pullers. Because wars aren't always won by inflicting as much damage as possible, and accomplishing the primary objective of any war ('making the enemy stop being disagreeable to your policy') requires applying exactly the right level of violence in exactly the right way. For some cases, the right level is nuclear armageddon and the right way is rained down from orbit. For others, the right level is a single bullet and the right way is delivered from point-blank range.

The side that does this well will accomplish its objectives faster, more completely, and with less collateral damage than the one that applies the sledgehammer approach of orbital bombardment to any and all obstacles.

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    Assuming you're an occupying force, you're going to need boots on the ground for nothing else than to be a reminder of the situation and act as a police force in critical instances. Ideally, once you have control of the orbits, whoever is in charge on the planet surrenders. You park some troops in their center of government to remind them of that, but then using the existing power structure to maintain law and order and only bring in the big guns when necessary. – Keith Morrison Feb 1 at 21:37
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    @KeithMorrison I hadn't touched on occupation after the fact, but you're absolutely correct. Boots on the ground serve as a visual reminder of who's in charge, can deal with civil resistance (eg riots) with minimum necessary force, and can carry out hearts and minds operations. Forces like warships in orbit (or, in the present time, missile-armed drones) don't give you that. – Catgut Feb 1 at 21:57
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    You don't have to remind the average person who's in charge. As far as they're concerned, it's still the "existing power structure" (EPS). You only have to remind the EPS that they take orders from you. So the main job for your "troops" is to escort EPS leaders to meetings at your Governor's throne room on your Imperial Star Destroyer, then when the meetings are over to escort the compliant among them back home, and the non-compliant to the transports headed to the prison planet. – Monty Harder Feb 1 at 22:18
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    Worth noting this reasoning already applies writ small to the modern world: you can't take over a city with just jets and bombs, no matter how powerful or precise they are. You still need ground soldiers to actually do the boot work of securing the city – Pingcode Feb 1 at 22:22
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    I'll be honest: Your first three paragraphs sound like a pretty dope pitch for a P&P where you play as precisely these elite infiltration teams. I'd play it. – Suthek Feb 2 at 13:00

Planets are slums.

You are stuck at the bottom of a gravity well, you are exposed to uncontrolled and polluted biosystems. You are immobile and vulnerable to planet killers. Plus, who wants to spend their lives in full-G, halving your lifespan, just so you can walk around on a planet?

About all planets do is cheaply (in terms of resources of value to the space-born) incubate poor people. And people really aren't that expensive to make even in space, and the space born are properly adapted to where people are useful (space).

There is nothing much of value on planets, so space born civilizations don't spend resources on planet based armies. Rarely there is a reason to do planet-based operations. These are mostly non-military; trading in primitive gravity-bound art, biologists looking at how life adapts to harsh planetary conditions, mineral formations that are useful and produced by chaotic planetary environments easier than in a factory in space, etc.

To that end, there are some specialists who are equipped to visit planets: down-trippers. With reinforced bones and circulation systems (either a throwback, bioengineered, augmented, or an uplifted primitive) they can operate in a full gravity environment.

They don't have huge budgets (compared to space navies), but the cost of modest personal security tech is cheap enough (for people in space) and planets hostile and dangerous enough that they look a bit like special operations troops.

In the event that there are military or intelligence objectives on planets, independent down-trippers are contracted to do the work. Maintaining a real military force for down-tripping would be like the military maintaining camel cavalry in this day and age; way too niche to bother with.

Large armies invite aerial bombardment. Any time more than a few hundred soldiers are in one place, asteroids, nuclear warheads, and chemical bombs are dumped on the area.
The only way to be secure against this bombardment is to totally control the space around the planet with your large fleet. If your fleet is in control already, then you simply bombard the enemy ground forces to oblivion. Even your ground forces may only require a few men in any given area. Armed with hand-held nuclear-rocket launchers, a hundred troops easily have the firepower to oppress local inhabitants.

Or, if you want to be more creative, perhaps the post-singularity nanite swarms that wander the planet like locust disapprove of large groups of humans and dis-assemble them.

A third option is you have star-trek teleportation devices that can only beam limited amounts of matter. In the fast-paced warfare of the age, only instantaneous teleportation is swift enough to avoid death. A large army takes an inhibitive amount of energy to teleport.

It's mostly logistics. It's unbelievably difficult to maintain supply lines for hundreds of thousands of people in hostile territory. Sun Tzu gave a baseline efficiency of 20-1 for this (as in, it takes 20kg of food to get 1kg of food to troops in enemy territory.) The US army in the Middle East gets even worse numbers for fuel - we're talking about dozens or more of gallons of fuel required for each gallon delivered to some remote outposts. This is only going to get worse in space, where distances are even greater (although it's probably more a question of fuel vs. food. Still, resources are resources.) For smaller numbers, say thousands, it becomes easier to work into overall fleet logistics. 10,000 men in a fleet of 300 ships and 30,000 crew is not that hard to maintain. 100,000 men in a fleet with 30,000 crew is going to really stretch things.

It's also versatility and training efficiency. I'd much rather have a company of elite soldiers who I can use to board enemy ships, defend against boarders, and make pinpoint strikes than three companies of good soldiers who are trained for one of these things. Now, I may want to groom specialists among the elite company, but I think most of the training would be universal. Fewer troops to train means I can train them more intensely and more frequently with the same overall set of resources, and these are all high-value missions where I really don't want to screw things up.

It's about the value of the targets In an era of space warfare, what is most important? The enemy shipyards, docks, and fuel depots. Guess where those will be - that's right, in space. There's not really much worth taking on the ground initially, and the things which are worth taking are small installations and production facilities, not huge swaths of land. Small, elite forces are much better at taking those kinds of targets anyway. Of course, you want the whole thing eventually, but when you're in the thick of the fight, what are you going to focus on?

Speaking of taking the whole thing, there would need to be large armies at some point, but these are only brought in once the skies are clear and the enemy military is neutralized. These won't see much straight-up combat; their experience will be more like resistance and partisan fighting. Therefore, no mechs, no tanks, and no large-scale engagements.

Basically, if you want to invade or attack and degrade an opponent, you are not doing it properly, if you have a huge army and no way to transport it to the target in any reasonable amount of time, if at all. You also are not doing it properly if you have no way to prevent the army from being destroyed in transit. You also are not doing it properly if you do not have the ability to first attack the landing site before actually putting your army on the ground.

If you have a huge amount of coastline to defend or an entire planet, and have no naval assets and no standoff weaponry that can be deployed from your location, then you are in serious trouble even if you can see the approaching invasion. The whole of your attacker's army is grouped up on individual vessels, and you cannot destroy it in those vessels until they are very close to your borders. If they get a foothold, they now have to face your land army, but also have the benefit of the terrain to use for cover from your own land army and supply lines coming via methods of transportation which you cannot attack while it is enroute to supply the army now on your land.

The Soviet Union had this problem, in a way. They had no way to counter attack the Germans in Western Europe or North Africa, because they had no navy and had little port access to that part of the world's oceans anyway. Recently, it is argued, this was the reason for the conflict in Crimea - some fools allegedly decided that they could tamper with Russian trade and military access to the Mediterranean, by manipulating policies regarding Russian port access at particular locations in Ukraine. With the reported "warming" of relations between Russia and Turkey, the former NATO threat from trying to sail through the straights of whatever at Istanbul is diminished. (It probably should have never been there, to begin with, but hindsight and current levels of education are drastically increased compared to Cold War decision making).

Basically, science fiction is using real-world examples of naval power when making imaginary space fleets.

This is all stuff I have read from historical accounts or supposedly authentic digital copies of declassified U.S. papers. The U.S. Navy continues to be as vulnerable as anything else to nuclear attack, among other things. Publicly, it got to the point where it was described as having the capability of simultaneously engaging and defeating every other navy on the planet. This, in implied fashion, indicates that you are not going to have much luck launching a naval assault on the Americas (or on anything else the U.S. Navy does not want you to attack).

It omits the fact that the U.S. Navy is entirely vulnerable to anti-ship missiles launched from aircraft or land installations. It is particularly vulnerable to swarm attacks from missiles carrying nuclear payloads or large conventional warheads. A science fiction example of this is the Death Star opening fire on Rebel cruisers (which were basically heavily armed aircraft carriers) in "Return of the Jedi". The only way to prevent the things from getting "nuked" was to pull alongside their Imperial counterparts so that any more Death Star participation would result in "friendly fire" incidents involving Imperial Star Destroyers.

I am trying to mix history/facts into this answer to state that it is likely that Science Fiction writers or movie directors probably get their inspiration, for many stories, from real-world examples.

A final thing to consider is that, like science fiction spaceships orbiting a planet, modern navies of some nations can attack anything located anywhere on the planet via ballistic missiles launched from submarines. The U.S. Trident II missile, if I remember correctly, can deliver several nuclear warheads per missile at ranges of something like 5,000 miles. The missiles can be launched from beneath the surface of the ocean. U.S. Tomahawk Cruise missiles, in their original design, were supposed to carry nuclear warheads and could be launched from the "new" B-1 bomber and B-2 Stealth Bomber. This was a serious problem, in terms of balance of power, because the B-1 was capable of very high airspeeds compared to other heavy bombers and the missiles themselves could not be intercepted when they were initially deployed. The missiles could also be set to literally "loiter" in the airspace of an opponent, until they were told what actual target to strike.

The U.S. ballistic missile arsenal is really a bunch of pilotless aircraft with particularly large effective attack ranges. It is described as being a different "thing" than the navy, but it really is not any different. Some of the missiles can launch from underground silos in the U.S. (if they haven't been secretly decommissioned haha), others can be launched from submarines. I do not believe that any nuclear variants of the Tomahawk missile remain in service, but I could be wrong. That missile could be launched from everything from submarines to surface vessels to aircraft, if I remember correctly, and had an effective range of over 1000 miles.

If you want to effectively fight anybody at any location in the space in which you are aware that there may be reason to fight, you want a large navy first and a large air force/missile fleet first. Even if you are only maintaining a defensive position, you do not want an approaching navy to get close before you can start attacking it to stop the invaders it is transporting.

Prior to the intercontinental ballistic missile, U.S. policy was that the best way to prevent an attack on the U.S. was to destroy it when it mobilized overseas (which probably explains why we were messing around in Korea and why we were quick to establish basing in as many locations near Soviet borders as the host nations would allow). This policy appears to have been the reason that we deployed nuclear missiles in Turkey, when those missiles still only had intermediate attack ranges, in the late 1950s or very beginning of the 1960s. They were already there when the Cuban Missile Crisis began (and may have been the actual reason that the crisis took place?)

Lastly, your invading army is not worth a damn if it doesn't have fuel, food, and ammunition. Neither is your defensive army. There has been a time when the U.S. economy was at the mercy of oil imports from places very near to Soviet borders. The U.S. nuclear arsenal also needed quality uranium, which is described as being in much greater abundance in places in the Eastern Hemisphere. Without a navy, the U.S. had no way to project influence on the decision making about those oil shipments or to maintain control of the situation that kept those shipments moving. Europe also relied on those shipments.

Science Fiction writers who are artistically representing warfare in space, and using real-world parallels, may be drawing from history classes or military experience or even government input on their work.

  • Welcome to WorldBuilding Matt! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus Feb 2 at 22:39

Why bother? While elite units to take out and defend planetary targets are very useful for espionage, assassinations, and sabotage, what can a tank do against a space ship?

As soon as the space fleet is defeated the attacker has a choice, they can try to carefully pick off any planet bound defenses or destroy the main areas of resistance and much of the surface.

Removing surface defenses, when they are in place, is a simple matter of time and careful targeting from a safe distance, at very low risk for the attackers. The area around the defenses will be rendered useless magma, but the rest of the planet will be largely intact.

Or if they're in a hurry, the attackers can send some meteorites at any city or military base that doesn't surrender immediately. Defenses might take out a few of these, but not enough to worry about. While the widespread destruction is frowned upon, if a planet is foolish enough to resist it is their fault for not surrendering.

In this situation an army is about as effective as a child throwing rocks at a tank.

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    I'd call it as effective as a child throwing rocks at a jet bomber -- throwing a rock at a tank there's always the microscopic possibility that it lodges itself in the treads or the turret's gearworks and jams something up, causing some damage that raises the gesture from 'pointlessly futile' to 'symbolically defiant'. In this case, the child's thrown rock can't even reach the ships. – Shadur Feb 2 at 11:35

I think the premise is wrong.

A basic fact of warfare is that if you want to hold ground, you need troops on the ground. Or a castle wall and a mote.

So, if you're simply holding a small area then you only need a small number of troops. But as that area increases, the garrisons increase. Patrols, reaction teams, assault forces, etc. Troops are also more "surgical" than airpower.

We have ample demonstrations that while air (orbital) superiority certain have an impact on enemy forces, they are but a component of the combined arms approach. We have several examples right now of complete air superiority, yet they alone can not do the job. The large numbers of indigenous forces continue to fill, and refill, in the holes that the aircraft make. Only by occupying the ground is it possible to keep the enemy forces from coming back.

Obviously this is different from a scorched earth policy of "simply" obliterating the enemy forces from above, reducing their number to below an effective fighting strength. If you genocide the inhabitants, then obviously you don't need anyone on the ground.

  • But in this case by controlling space there is nowhere else for troops to come from. this isn't like air superiority this is like naval and air superiority on an island. – John Feb 5 at 7:14
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    Traditionally, you wanted to hold ground - however, that's not really true in this scenario. Traditionally, holding ground is necessary to secure yourself from attack, to run your supply lines through that ground, to gain resources of that ground, deny the enemy use of those resources, and to have a position that's good for defense. In space all that doesn't require going down to the planet if you control the space around it - all your supply lines go around the planet, the offensive/defensive positions for spaceships don't require holding the planed, and you can get resources by blackmail. – Peteris Feb 5 at 9:36

There are some good answers above that touch on some of the most important points (such as orbital superiority) but I'm going to add a few additional important things to consider.

Offense vs. Defense

A ground army in a space-faring civilization is inherently a defensive military tool. It can't be used to siege another planet without the addition of space transports, and space dreadnoughts to protect the space transports. When given a choice, Civilizations will act against their own best interest and endanger their own citizens to generate offensive capability.

Due to the distances involved, sustaining a large number of troops like that in a hostile environment becomes exceedingly difficult. Even with FTL travel times you're look at potentially hours before you're able to reinforce or evacuate units. The US Military has better turn around then that in most war situations.

Vulnerability to Interdiction

[Transitioning your forces always presents a danger].4 If you have four hundred men manning a space dreadnoughts weapons, it's more effective then 400 troops sitting in a cargo hold. The best bang for your buck (literally, in this case). The US military recognizes this even against the advice of nearly everyone.

Large numbers of ground troops in transit are vulnerable to interdiction - the damage done to a single transport of 400 man can be caused by a small one man attack ship, providing a greatly enhanced force multiplier for the enemy.

Shifting Strategies

Military techniques will have evolved to keep up with the times. Sieging a planet is inherently different then seiging a city. You need to not only be able to interdict incoming shipments (which a dreadnought can do) but also to destroy enemy assets being developed on the ground.

Ancient warfare would have siege engines for the later and large roving patrols of troops for the former, but now our single dreadnought can do both, and we all know "planet-hopping" is a viable strategy.

If you're looking for other literary examples, the Tour of the USS Merimack series has battalions of Marines deployed on board it's space Battleships. These marines are not merely there to dispel boarders (or provide boarding action against hostile ships), but they are also responsible for manning the ships manually operated cannons and operating it's flight wing. There's a historical precedence for US Marines to have this kind of training, and it allows the world to have both a ground military and a space military that operate hand-in-hand.

DISCLAIMERS:

  • I don't want to debate US Nuclear policy this was just topical.
  • Ipicked the US military because it's large and internationally known.
  • The dubious effectiveness of Island-Hopping is not the subject of
    this post.
  • I have no affiliation with the book series in question, I just thought it was relevent to mention here.
  • I'm only human, don't sacrifice me to the internet gods because of something I said here.

There are great answers already, but I think there's one more reason to consider: the gravity well.

Every piece of military material you send to a planet's surface will need to be launched back to orbit (unless you want it trapped there forever, which usually isn't a great option). Getting things into orbit is extremely expensive for even moderately sized planets like earth. Exponentially expensive.

It's not unreasonable to have a sci-fi SSTO capable of carrying a half-dozen special ops to their mission and back to orbit. But once you start dropping mechanized infantry, it will be really hard to get them back to the mothership if things turn south (until you've captured/built the planet's space elevator, mass cannon, whatever). You might have a lot of resources in space, but you still probably can't afford to just drop large amounts of military tech and never see it again.

So mechs/large forces will only be dropped onto a planet as a shock and awe campaign that is the final prelude to total planet domination. And since most planets would probably negotiate some peace treaty once the threat of orbital bombardment arrives on the horizon, this would only be used rarely.

  • In the real world, a lot of military tech does get left behind in war zones - even if it didn’t get shot up, it’s probably either worn out from heavy use or becoming obsolete. Even on earth it’s better to just build a replacement and the single use nature of stuff used in combat is a normal part of any military budget. – Quentin Clarkson Feb 4 at 22:49

Quite apart from the bombardment issue, mechanized forces are no longer practical. It comes down to the balance between offense and defense--and for ground vehicles offense is solidly ahead.

When a small infantry-launched missile has a high kill probability against a distant vehicle nobody will send vehicles into a combat situation. Infantry, however, does not have a useful defense against artillery.

Combine these and you get a situation where the only survivable offensive ground forces are small units that move on their own feet and rely on stealth to reach their targets.

Defenders don't have the same limitations but any large base will draw kinetic attacks from space. Thus they are also limited to fairly small units.

Furthermore, all fights must be short because otherwise the defenders call fire from space and take cover in foxholes just before the kinetic weapons hit--near but not on the battlefield. The shock wave is much nastier to those in the open rather than those with a bunch of dirt between them and the shockwave. The attacker must hit and run before there can be a response from space. (Yeah, they can occupy a high value target--but they'll get nailed when they leave.)

why don't modern militaries train troops on shield walls and melee, because modern technology has made melee largely irrelevant. Ground troops have to get to the ground, if you have a decent space defense force that will never happen. So most of your ground troops are only for dealing with local rebellions or as a last ditch defense force. You might have a few other elite specialists for stealth missions but that is going to add up to a very small proportion of your troops.

It is much the same reason an island nation might put a lot more effort into naval/air defense than ground troops. Ground forces don't matter if you can keep them from ever reaching the ground, so defenders will put as much as they can into space defense and the attackers will do likewise. You also have the issue that you need a much larger force to cover space space since it is a 3 dimensional battlefield, there are no defensive lines, only defensive spheres that adds up to a lot more assets.

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    Police forces still train in melee. Anti-riot teams look very similar to Roman soldiers in tactics and types of equipment. – Muuski Feb 1 at 20:47
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    police are very different than the military. – John Feb 2 at 16:21
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    Unless your goals are to kill all life intelligent life on the target planet, you'll need a police force on the ground enforcing your rule. See all occupations ever. – Muuski Feb 2 at 18:04
  • Why wouldn't that be your goal?the chances of a planet optimized for one species not needed some alteration to make it habitable for a different intelligent species is pretty small. But even if you are right an occupation force will still only be a small fraction of your overall military. – John Feb 5 at 7:09
  • @Muuski why would you need to enforce the details of what happens on that planet? You control their interstellar trade and have absolute power over their lives without setting a foot on their planet - an ultimatum of "we'll nuke one city for every day where the quota of shipments is not met" can give you most benefits of ruling that planet. – Peteris Feb 5 at 10:35
  1. Ambient force field.

When on the planet's surface everything is under influence of high gravitational forces. On the other hand, in the "free space" you have much weaker forces to bare with.

  1. Life sustain systems

When on habitable planet the planet itself supports individual's vital needs. On the other hand, in free space the vessel must have its own life sustaining technology. Bigger vessels therefore have higher power-to-supply ratio.

  1. Different battlegrounds

The difference between space combat and ground combat is similar to difference between battling in a forrest and battling on a plain. Heavy cavalry can be easily defeated by agile archer units in forrest, but they can wipe them out easily on a plain in a single raid attack.

Large ground forces can be easily targetted and wiped out by a single blast from the orbit, on the other hand many small units spreaded all over the surface force firing at one unit separately.

Heavier spacecrafts can sustain heavier fire, because their surface is square-proprtional to their size byt their volume is cubic-proportional. Twice as big craft hase four times bigger area to defend but eight times bigger space to carry troops, ammunition and technology.

It is the manevruability versus strength decision. In forrest you have to have ultimate manevruability (Individual soldiers), on plain you can go bigger (Tanks), aeroplanes are even bigger and spacecrafts follow such patterns. You can see how the warfare changed by introducing tanks (WW I) and heavy bombers (WW II) to the battlefield.

The rapid mobility of space forces means that the enemy can bring everything that can float to any place in space. Your home-world can go from all-clear to a sky full of battle-wagons in a minute.

If your own space forces cannot meet the enemy with enough force to make the engagement too expensive for the enemy, then you are best off not having any military assets on your planets at all. No factories, no supply depots, and certainly no C3 assets; when the hammer goes down, it will go down on planet-bound military assets first.

The only exceptions are concerned with accession (recruiters and the guys who process new recruits, and maybe your boot camp), and then only if the planet has a substantial population from which to recruit. Everything else will be in orbit around otherwise uninhabited systems, and in deep space.

This all assumes that your enemy prefers to avoid civilian casualties; if that isn't near the top of their priorities, then your planetary presence has to consist of people who are prepared to bug out at a moment's notice, and people who don't mind a Tsar Bomba landing on their heads.

So the reason you won't have extensive ground forces is because you will have arranged your affairs so that you don't need them.

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