So, in my setting, for plot reasons, I want to have both a space-based Army and space-based Marine Corps. Here’s the thing though: I don’t know how to make them different enough without them seeming like functionally the same branch. Using irl military forces, how can I make my space equivalent branches more separate? What duties would fit both?

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    $\begingroup$ Do you have other branches, like Space Navy and Space Air Force? $\endgroup$
    – Alexander
    Sep 29, 2021 at 16:16
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    $\begingroup$ @Alexander: Yeah. Air Force is more like the Aerospace Force now though (they do in atmosphere, orbital and occasional deep space missions) $\endgroup$
    – DT Cooper
    Sep 29, 2021 at 16:21
  • $\begingroup$ @Alexander Now that the Space Force is a real thing, would the Space Navy be what you mean by Space Force, or do you mean a branch that uses spaceships to deploy naval assets onto planets? I know it sounds ridiculous, but water worlds could be common enough that you need branch specifically for fighting on these worlds. $\endgroup$
    – Nosajimiki
    Sep 29, 2021 at 17:50
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    $\begingroup$ Weber's Honorverse has dealt with this question; reading a few of those books might be informative. $\endgroup$
    – Matthew
    Sep 29, 2021 at 20:06
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    $\begingroup$ One branch gets rid of little green men, and the other gets rid of little green amphibians? $\endgroup$
    – alephzero
    Sep 30, 2021 at 1:57

14 Answers 14


Space marines do ship to ship action.

Space marines, much like real life marines, mostly operate with other ships. They have ships which are specialized to fight other ships, and lots of equipment to allow them to board ships and space stations and mining colonies and other things floating around in the sea of space.

When they go in a system they'll handle raiding mining colonies, checking merchant ships, and fighting large space battles. They have a massive advantage in ship to ship fights, and the marines are well trained for quick snatches and grabs.

Space armies do planetary invasions.

Much like real life armies, space army handles land, aka entire planets. They have way more troops and drones to handle the huge land area, and much longer ranged ships.

Their ships are not built generally to handle ship to ship fights. They are built to stay out of range of the truely massive defences a planet can muster and bombard the fixed defences from afar with heavy weapons, and then put heavily shielded ships into orbit to disgorge huge numbers of drones and troops to take control of planets.

They're also built to hide behind the truely massive defences a planet can muster and shoot at marine ships from rival navies.

The soldiers are built to hold a planet for an extended period of time after it's been pacified by artillery fire.

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    $\begingroup$ My thoughts exactly. Rather than focusing on the modern definition of a Marine, this harks back more to then Ancient definition of Marines: soldiers who specialize in ship boarding. While other answers are correct that Marines and Army have become more or less synonymous in modern military tactics; in space, there would be a huge difference in the training & hardware requirements of a force designed to deploy IN space vs FROM space. And when you introduce a combination of space habitats and planets, you have enough demand to need both. $\endgroup$
    – Nosajimiki
    Sep 29, 2021 at 17:47
  • $\begingroup$ Not meant to detract from this answer, as I gave it a +1, but I'm not so sure I agree with using "massive defenses" ... get big enough rocks, or even "tungsten thunderbolts" and the political/sociological will and the planetary defenses are no longer "massive" enough. $\endgroup$
    – CGCampbell
    Sep 30, 2021 at 9:22
  • $\begingroup$ That's why I noted that the space army would have lots of artillery. Planets can draw on the entire electrical supply of a civilization to make some really powerful railguns and the vast resources in a planet, and so could get pretty good at deflecting artillery. Enough big rocks or tungsten thunderbolts would break that though, and ships at a great distance could easily dodge any return fire from the planet. $\endgroup$
    – Nepene Nep
    Sep 30, 2021 at 9:25
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    $\begingroup$ @CGCampbell The idea that an orbiting fleet will always have advantage over planetary defenses is a sci-fi trope that obviously has never been put to a test. It is based on the idea that mobility grants an advantage to the attacking fleet, and that you need much less energy to hurl rocks down rather than up a gravity well... but that depends a lot on the technology available. How do they put these massive ships in orbit? If they have antigravitational devices, hurling rocks to orbit is not a problem. On same tech level, a planet has much more resources than a ship or a fleet. $\endgroup$
    – Rekesoft
    Sep 30, 2021 at 10:04
  • $\begingroup$ This more or less matches usage in existing military-SF such as David Weber's Honor Harrington series: honorverse.fandom.com/wiki/Royal_Manticoran_Army. (Although in that series the Army doesn't have any warships, just atmospheric small craft and maybe transport ships. Defending the planet from external threat is the Navy's job, the main branch of the military. If enemy ships get into position for kinetic strikes on your capital, you've already lost. The Army only comes into play when you need to hold territory after they surrender, against resistance movements, and stuff like that.) $\endgroup$ Sep 30, 2021 at 12:25

For the US, the theoretical difference is mission:

  • Army = territory control
  • Marines = amphibious assault (i.e., landing under fire)

But that's only theory. The USMC hasn't conducted an amphibious assault in decades and the US Army has (over the course of the last 100 years) conducted more amphibious assaults than the Marines. There are also differences are chain of command (USMC is part of the US Navy) and traditions. Even the chain of command difference is mitigated by the US's regional combatant command structure.

More importantly they hold different institutional mindsets.

WARNING: Gross generalizations follow.

The Marine Corps tends to see itself as an assault force. This creates an emphasis on movement and punching into an enemy force to establish a foothold while downplaying staying power. All teeth and no tail.

The Army tends to see itself as a more traditional land army charged with methodically taking and holding territory (yours or theirs). This creates an emphasis on staying power over assault. The teeth might be smaller, but they can keep biting a lot longer.

IMPORTANT Before the comments go crazy, let me clarify that the above is overgeneralized and talking about institutional mindsets, not capability. The USMC is perfectly capable of holding territory and the US Army is perfectly capable of assaulting an enemy position.

In practice, there's very little difference in how they operate on the ground or how they are used. I would recommend taking a similar approach in your own world.

  • Your marine corps sees itself as the republic's (or empire's or whatever's) fist. They bloody the enemy and step back. They'll think of the army as little more than a police force.
  • Your army sees itself as the republic's arms. They grab and hold and never let go. They'll see the marines as hyperactive children who can't hang in a fight when it matters.

Depending on the scale of war you have, you can even make theoretical missions literal. But as you get bigger in scale, you'll find that everybody does assault and everybody holds.

  • $\begingroup$ more importantly, at least traditionally, the army tend to have the more massive equipment as the marines have a much lighter load. The army operate the massive battle tanks, landing docks (constructed by the Army Corpse of Engineers) etc as the marines tend to operate lighter assault vehicles keeping them quick and agile to carry the fight to the enemy. Now, the modern marine assault vehicles are becoming more akin to tanks now a days $\endgroup$
    – Sonvar
    Sep 29, 2021 at 17:52
  • $\begingroup$ @Sonvar that's part of what I had in mind when I differentiated staying power $\endgroup$
    – legio1
    Sep 29, 2021 at 18:06
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    $\begingroup$ @Sonvar I know military engineers can do amazing things under fire, but less so once they're dead - though it would be a good use of zombies (typo: you've got corpse of engineers) $\endgroup$
    – Chris H
    Sep 30, 2021 at 11:05
  • $\begingroup$ blah.... the one time I needed autocorrect to take action, it doesnt $\endgroup$
    – Sonvar
    Sep 30, 2021 at 13:30
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    $\begingroup$ good thing you can't fix that typo, is awesome. Worthy of a Pratchett-style narrative $\endgroup$
    – lurscher
    Sep 30, 2021 at 19:43

They overlap a lot, because it is a new merger.

The Space Marines are actually the army of a different smaller nation or group that was amalgamated into the larger group. In their old role they were functionally much the same as the Army is for your larger group. The Marines keep their old command structure and colors and tech. Persons in the Army often refer to them by the name of the country they came from, not "Marines" which is a new name given to them.

It is a recent merger. The Space Marines are loyal but they routinely get tangled up with the Army and vice versa because they are similar groups. For political reasons the two groups are not getting a lot of direction and management from the top. A lot of how these two divide up jobs (or fail to do so) is being figured out on the fly by officers on the ground.

That will be fun to write and give energy to your story.



Your Marines are tasked with operating as an infantry force in nonstandard environments. Variable-G outside a given range, 0 G hazardous/no atmo, ship-to-ship/ship-to-station, and similar combat situations. The Army, is tasked with planetary combat, to include Planetary Assault from orbit.

The delineation here is fairly straigtforward. The marines need special weapons, tactics, and training to fight in their assigned battlespace. Guns that work in a vacuum or are "safe" to use shipboard or have no recoil for low-gravity work are unlikely to be the best weapons for fighting a battle on earth. Battle drills and equipment meant for places where any hole in your suit means you're combat-ineffective are unlikely to be the same for combat troops on a normal planet. (for example, marine grenades might rely less on concussive force, but put more emphasis on creating punctures a spacesuit can't survive but a ship's hull can.) Your marines are likely to have little or no heavy equipment outside your spaceships/fighters, and adapt their strategy and tactics accordingly. Obviously tanks/arty/etc are no good ship-to-ship fighting in cramped corridors. But they're just as likely to be essentially useless in a 0G environment or a high-G environment unless your talking very short-ranged stuff. (check out WWI artillery tactics for insight on just how much the gravity/spin/atmo of a planet being known matters for artillery.)

Meanwhile the Army, because it's designed to fight in more "open" areas, would have larger amounts of traditional support in the form of artillery and tanks that need a more "normal" environment to operate effectively. Plus it's HARD to train good infantry. So it makes sense to silo soldiers into "fights in 0G" and "fights on normal battlefields" as quickly as possible. In this case when they pick their branch of service! Your army would also have different needs and goals. Your army is wildly more likely to need long-term plans to conquer a planet or even a city. Meanwhile a ship or station fight is going to be over fairly rapidly. So even things like "what sort of logistics do I need?" are going to be wildly different.

I should note this doesn't necessarily make your marines "better" than your army soldiers. (though they'll probably disagree with you!) A marine squad specializing in 0-G station fights is going to be annihilated by an army squad in the woods. And vice-versa. It just means they both have different jobs to do. heck, depending on your background you might even have MORE marines than army troops, because you need more station-clearers and ship boarding troops than soldiers to invade planets!


Space Army:

  1. Large force.
  2. Participates in official conflicts.
  3. Requires an act of parliament (congress) to use.
  4. Slow due to their size.
  5. Primarily focuses on Space combat and land operations.

Space Marines:

  1. Small force.
  2. Can carry out "unofficial" operations
  3. Serves at the pleasure of the President / Dictator / Prime Minister
  4. Focuses on speed.
  5. Can engage in any field of battle: Space, Land, Water, Air.
  • $\begingroup$ Your space marines sound more like SEALs (or, uh, SLAWs?) and should be some form of special forces within another military branch. $\endgroup$
    – jamesdlin
    Oct 2, 2021 at 22:20

The US armed forces have different missions/primary functions:

  • Army is the main land force: taking and controlling areas of dry, dry land.
  • Marines have the specialized mission of amphibious assault: going from sea to land.

This doesn't mean there are no overlaps, but it does mean they have different priorities.

So your space army is all about dominating regions of space, whereas the space Marines are about establishing footholds to/from space from/to planets. Ultimately, these are just names, so you can define these as your wish, but this would be the analogous functions.


Gripples are (not) Doopleknops.

Exactly what it says on the tin. The members of the space army are all dually-trained in Gripple tactics. Now every Gripple is by extension a Doopleknop, so can be called in reserve for any Doople-Knopping operations that might arise.

However, interplanetary operations require a significant amount of Doopleknopping (in some solar systems up to 1.8 units per capita) and as we all learnt in school (at least where I am from) not every Doopleknop is a Gripple. Hence it is economically efficient to have a specialised force (navy) that specialises in Doople-Knopping operations, and leaves Grippling to the army.


The Planetary Army knows where it will fight, the Space Marines don't.

  • The Space Marines are trained for multiple environments, including zero-G
    Even in basic training, they get the free-fall assault course. If there is a moon, they learn to operate there, too. If the setting allows it, they regularly deploy to other worlds for training.
  • The Planetary Army may possibly deploy to another system ...
    Most of them serve their entire term on a single planet, training to defend it. When they are deployed, they get specific training for that destination. A career sergeant might have served on one or two other worlds. Probably not three.
  • The Space Marines have orbit-to-surface weapons
    Every Marine might be trained as an infantryperson, but they also have aerospace assets for ground attack. They operate from Space Navy carriers, troop transports, or surface bases.
  • The Planetary Army has surface-to-orbit weapons
    This includes both aerospace assets for space attacks and ground-based beam and missile batteries. They operate from permanent and expeditionary surface bases.
  • The Planetary Army includes water and underwater units
    When there are suitable bodies of liquid, submarines can evade orbital attack and maintain the defense for months or years, firing SAMs from just below the surface. Hunter-killer subs and surface craft hunt those planetary defense subs. The Space Marines don't have that. (I realize that goes against the multi-environment ethos of the Marines. It is about having big ships.)
  • The Space Marines deploy a Regimental Combat Team, or less
    There may be divisions in the TO&E, but they are administrative headquarters. Above them is the Space Marine Corps, which is not a deployable corps at all.
  • The Planetary Army deploys divisions, corps, or armies.
    Small brushfire wars are the job of the Marines. When the Army comes, it is the sledge hammer. For instance, the corps-level or depot-level Army maintenance is much better than anything the Marines have, yet the brigade-level maintenance companies are much worse than what a Marine regiment has. Same for medics, intelligence, logistics, ...
  • The Space Marines have battlesuits/powered armor
    It firs with the idea of dropping on different worlds, etc. It also fits with the idea of relatively small units -- more suits in a battlesuit platoon than tanks in an armor company.
  • The Planetary Army has Main Battle Tanks
    Those things are too heavy for highly maneuverable dropships, and not suitable for every environment. But where they work, they devastate their enemies.
  • $\begingroup$ Although I like some of the divisions, you are basically saying "space marines do everything, army does only planets". You are training marine units for fighting on a variety of planets but also space combat. That is like having an air-force that is also trained for urban combat, mountaineering and desert warfare. Better specialize it. Just like navy/marine units ferry army units to their destination the space marines would ferry theirs. The distinction being the army does the actual planetary assault as planetside fighting is their area. $\endgroup$
    – Demigan
    Sep 29, 2021 at 18:51
  • $\begingroup$ @Demigan, it is more like "Marines do little contingencies on short notice, Army does big contingencies with long preparation." If the Army deploys to a vacuum world, they will be issued space suits and get the training; there is plenty of time while their Corps logistics troops put everything else into shipping containers. $\endgroup$
    – o.m.
    Sep 30, 2021 at 6:09
  • $\begingroup$ Looking again, that does sound plausible. I would expect army and marine units to basically be deployed for the same situations but with only that distinction. If that is enough distinction for OP is his problem to deal with. $\endgroup$
    – Demigan
    Sep 30, 2021 at 8:06

For clarity I'm calling Space Marines by a more realistic name: Espatier.

This has all to do with training and what you can expect from your men. It would be easy to assign Espatiers to planetary assaults but that is a tall task. It would be like asking a pilot to do urban combat training. Sure you could, but it would not be efficient.

Espatiers would be trained for 360 space combat, "any direction, any gravity" woukd be an easy motto. Habited/industrialized asteroids, space stations, orbital rings, sattelites, space ships and more space-based things would be their primary targets. Assaulting planets would be the task of the army, since training the Espatiers for ground combat in many biomes and terrain types would be too much.

Since someone might bring it up: planetary invasions are often portrayed extremely simplistic. "Blow up forces in space, bomb/invade the planet". However the energies of most sci-fi would change things. An antimatter reactor on a planet can easily be hundreds of times more powerful than that of any FTL-capable ship. This lets you negate much of the advantage of being above the planet gravity well. You can make any assault a multi-stage battle that has to happen simultaneously. You have to fight the defenses in orbit simultaneously with the high-powered defenses on the planet, guided by sensors that are larger than the ships in orbit. Additionally defensive space ships would wait and attack any planetary-assault ships. This makes fights about handling orbit and planet at the same time: Espatier groups try to protect army elements as they push through. The army starts engaging on the surface while Espatiers try to clear orbital elements so a reasonably safe area can be made to keep landing army and supplies.

  • $\begingroup$ "An antimatter reactor on a planet can easily be hundreds of times more powerful than that of any FTL-capable ship." <watson> Heh. Heh. Um... do you know why we put those reactors on ships in the first place? Because having them on a planet is a disaster waiting to happen. A ship-sized reactor failing too close to a planet is an extinction event. What sort of LUNATIC would want an even bigger reactor on a planet? </watson> In many settings, you have to be really, really desperate to be willing to take the obscene risk of using shipboard power sources on a planet. $\endgroup$
    – Matthew
    Sep 29, 2021 at 19:57
  • $\begingroup$ (...and in case it isn't clear, <watson> is used to indicate a Watsonian, i.e. "in universe", comment. Your setting may differ.) $\endgroup$
    – Matthew
    Sep 29, 2021 at 20:02
  • $\begingroup$ @Matthew <watson> my fellow namesake you have to lay off those deathsticks. If every ship reactor is an extinction event there would be no "safe" combat possible the moment you get a single ship in orbit. Because in our world and alternate worlds we glimpsed there are dampening fields, shields and alternate physics this is of no concern. By the time any army is in a position to truly blow a ground-based reactor they'd better capture it.</watson> any sci-fi already takes liberties. Sentients would have no place as commanders of ships with computer technology they would have. $\endgroup$
    – Demigan
    Sep 30, 2021 at 4:10
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    $\begingroup$ "For clarity I'm calling Space Marines by a more realistic name: Espatier." Maybe explain why you think this is a more realistic name. I've never encountered it before, so it just comes across as cryptic. $\endgroup$
    – NPSF3000
    Sep 30, 2021 at 4:16
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    $\begingroup$ @NPSF3000 since I got the name from Atomic Rockets and found their explanation logical I'll let them explain: "Espatier" is a twofer. Its formation exactly parallels "Marine" (also French-derived, as are nearly all basic military terms), and it also parallels the English word "spacer," but with a nice shade of meaning — a spacer is anyone who lives/works in space; an espatier is a space soldier. (projectrho.com/public_html/rocket/astromilitary.php) $\endgroup$
    – Demigan
    Sep 30, 2021 at 5:27

Most answers focused on the different specializations of the two branches. But there could be a lot of other reason to maintain two functionally identical but organizationally separated armies.

  • The two armies have very different leadership concepts. For example, one army consists of drafted soldiers, while the other is a professional army. The difference in mentality between conscripts and career soldiers require completely different leadership paradigms. Conscripts are kept in line by propaganda, intimidation and discipline; professional soldiers by career opportunities, rewards and personal agency. To prevent the methodologies which work on one kind of soldier from affecting the other, both branches are kept completely separated.

  • They are under different political leadership. For example:

    • One is controlled by the executive branch and the other by the legislative branch.
    • One is maintained by the central space empire and the other by the individual planets.
    • One is paid for by the government while the other is privatized.
    • One is secular while the other is controlled by the space-church.

    Neither political power-block wants to give away their military power to the other, and neither can afford to rock the boat by demanding it. So they keep maintaining two redundant armies.

  • They are under the same political leadership, but that leadership has reasons to be afraid of a military putsch. So they used a divide-and-conquer strategy and separated their military into independent but functionally similar branches. If one stages a coup, they can use the other to strike it down.

One fictional example to study in this regard could be the Warhammer 40k universe, where the Imperium of Man maintains not just one or two but at least six independent ground armies, each under a different department of the government (Imperial Guard, Space Marines, Sisters of Batttle, the military forces of the Mechanicus, the military forces of the Inquisition and the "Custodes", the personal guard of the Emperor). Many of those can be divided further into sub-armies which enjoy considerable autonomy from each other.


I'm assuming this is science fiction set in our own universe - extrapolating from where we are now is difficult, as the nature of combat in space is not well understood. One way to go about this is to research realistic space combat, and try to see from there how the specific challenges of space combat would facilitate multiple branches.

Currently, we have the Space Force because the need to protect satellites was viewed large enough that the Air Force Space Command could be spun off - this is all similar to how the Air Force came to exist from the army.

A personal thought of mine - it probably would not be called the "space army" or "space marines." If the needs of a future Space Force facilitated future sub branches, I'd imagine they'd be named something else. Current Space Force troops are referred to as guardians - you might invent a new type of troop that follows in the spirit of the marines, but for the Space Force.

In short, it all depends on the military activity of your world, the geopolitical (solpolitical?) situation. How you divide it will depend on the environments being fought in. Are you fighting only in space? Then you might have that branch be focused on pinpoint navigation stuff. Are you fighting in the pitch black craters of the Moon? Maybe you might have a special ops branch.

There's many potential avenues you could take it.


Frame challenge

An army is designed functionally. They dont just say "lets have some pretty names for some random different units and organisations".

Yes historical stuff may persist due to tradition, but also, if the Marines were not functionally useful as such, on Earth, they wouldn't really matter so much either and probably wouldn't persist at some or other reorganisation.

So heres the thing. You have to start by asking what a space army needs to do. What kinds of tasks are involved. Then, within that, what specialisms may be required. Not by just positing an earthbound army and trying to find some use for its historical units.

So in your universe, what exactly does the space military have to do?


The Space Army Travels in Space and Fights on the Surface

It’s intended to garrison planets, asteroids, space stations and so on. It would typically land on a friendly celestial body and provide security, If there’s a mission that involves taking control of a contested planet, the Fleet would attempt to land it on a part of the planet where they won't disembark under fire, and they can set up a forward operating base.

They usually wear light power armor and operate defensive weapons such as anti-space cannons, and might patrol in lightly-armored vehicles. Some of their units are meant to be capable of rapidly taking control of a continent (ground, atmosphere, seas, low orbit and underground if needed), or surviving an orbital bombardment to deliver a return strike, and operate weapons such as hovertanks, bombers, blaster sats and submarines.

When they deploy over interstellar distances, it’s typically in large troopships.

The Space Marines Assault from Spaceships

These are the special forces you would call on if you want to do an orbital insertion into a city on another planet, a boarding action against a spaceship or station, or a ground mission on a hostile planet. They normally use heavy power armor and small drones. They deploy from small, stealthy ships big enough to carry a platoon, which might be transported in a larger cargo ship and released close to their target.


They aren't different so much because they're split from the same body. For political reasons.

Because of politics, Homeplanet is not comfortable in having one and only one body (headed by only one General) that manages the entire fleet of their space capability. So, recently, the Government of Homeplanet splits sizable chunk (~25%-ish) of the Space Army into the newly formed Space Marines.

They say they want this new force to specialize in agile and precise operations, such as:

  1. Fast orbit-to-land invasion and land-to-orbit extraction.
  2. Ship-to-ship infiltration and subversion.
  3. Higher capability in faster-than-light scouting operation.
  4. Headquartered (or has primary base of operations) on some specific colony Kepler 221-B that requires frequent special operations.

To achieve such goal, the Marines is assigned mostly the standard-class and lighter-class ships and gears from the bulk of Army equipment. They still get the heavier-class stuffs, just not so much. When compared per capita, Marines has slightly more agility, while Army has slightly more firepower.

This sounds just like overrated Space Army's Special Operations Force to me.

Yes, it basically is, indeed. Space Army may even already have a Spec-Ops division. But, the thing is, the Government of Homeplanet wants something not to be on the same General's chain-of-command. And they're not telling publicly why.

To this day, Space Army and Marines look like two redundant space forces, one with about a third of the other's size. They're still trying to specialize and differentiate from one another as to justify their superiority over another. The only tangible difference between them is that Marines is very experienced in operating around Kepler 221-B because it's their home turf. The Army, well, they're more experienced everywhere else on the star cluster.


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