First off, I'm talking about a true, professionally trained army. Humans also learned, very early on, that they are not alone and the universe is very Darwinian. Also, human nature doesn't change, even after going into space. We are still an argumentative bunch that doesn't really turn the other cheek. EDIT: There are multiple threats; other humans (piracy, etc.) and unfriendly aliens on the fringes. The human threat would normally be handled by the marines, but responding to attacks from the aliens who may want a planet back, would be the army's responsibility.

In my universe, the marines are meant as shock troops that ship out with the naval units. In case of need, a fleet would ship out with a transport with the heavy equipment needed for an assault and the ship board marines would be combined into a fighting force. Much like the US Marine Corps, these troops are meant to take a beachhead and allow the army troops to come in and continue the fight. Travel in the universe between stars and in Einsteinian space still takes time, so deploying from a single planet could take months or even years.

How then should the army be structured to maximize training efficiency and minimize deployment times in case of trouble?

I'm looking at the US Army as model for my army. There will be troops trained in combined arms tactics and in specialized fighting techniques. By that I mean that a typical regular division would be mechanized infantry with armor mixed in at the regiment level and the requisite support units to allow each to deploy on its own. It doesn't make sense to me to keep enough of the heavy lift ships to deploy all mech. infantry divisions at once. I would only want to deploy 25% of the divisions at once.

There would also be specialized divisions such as full armor, cavalry scouts or light infantry as well. These would be scattered throughout the occupied space on a regiment level. The regiments would also have the specialized lift ships on hand to deploy.

Since the typical division is mech. infantry, deploying one would require sufficient heavy lift capacity. Should I have pre-deployed equipment that the trained soldiers could fall in on much like the POMCUS sites planned for use in the REFORGER operation. This equipment would be on heavy lift ships in orbit near likely trouble areas and maintained by civilians. This would minimize the need for heavy lift capacity but the response time would not be quick. However, these troops would be highly trained and motivated.

The other option I'm thinking of would be like the National Guard model. A division would be on each planets in the federation (or whatever) and the units would be a mix of professional cadre and reservists. Many of both would be recently discharged marines and soldiers. This would maximize the response time, but the heavy lift capacity needed would be extreme. Additionally, like the National Guard, training to standards would be an issues as well.

Post Answer Edit: I read abot 50 pages of Starship Toppers and Rolling Hot by David Drake last night. I think I'm going to go with a hybrid strategy. I'm going to have several main bases scattered through human space. Additionally, I'll have several forward deployed regiments near the frontiers. For deployment, I'll have a fleet of heavy lift ships that can ground and carry an armor company each. During "peace time" these ships will be psuedo-civilian conducting trade between the bases and the frontiers.

  • $\begingroup$ Have you read Heinlein's Starship Troopers? He presents a solution for human "mobile infantry" deploying from starships against alien planets, with training modelled after the U.S. Army. $\endgroup$ – Jasper Jul 9 '19 at 21:08
  • $\begingroup$ Also, have you read any of Tom Kratman's Terra Nova books? A Desert Called Peace explains that only 25% of the military's units are deployed at a time. The training assumptions are based on the U.S. Army. Indeed, one of the books (The Amazon Legion) is a rewrite of Starship Troopers. $\endgroup$ – Jasper Jul 9 '19 at 21:18
  • $\begingroup$ @Jasper, if you mean how Heinlein uses capsules to get to the planet surface, that doesn't make sense for armored personnel carriers, tanks and artillery. I'm actually more interested in strategic deployment, not the actual assault. $\endgroup$ – Madere Jul 9 '19 at 21:20
  • $\begingroup$ This seems to me like a question partly about space travel, so I added that tag. Feel free to roll back if I'm mistaken. $\endgroup$ – a CVn Jul 9 '19 at 21:26
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    $\begingroup$ Hi Madere, when you get a bit more reputation you'll be able to upvote answers you like. We ask that you wait a full 1-2 days before accepting an answer as best. This encourages other people to answer, which is of course what you want: lots of great answers to choose from. There's no time limit and you'll always get your 2 points. $\endgroup$ – Cyn says make Monica whole Jul 9 '19 at 22:20

A strategy depends on the analysis of potential threats. Since nothing is specified in your question, I'll outline a few situations.

If all the worlds in your spacefaring nation have no threats and are at no risk, then its immaterial where the Army is at any given time. A central location that permits travel to any given place in the nation's system systems works as well as any other.

After that, its a matter of response time, presumably dominated by interstellar transit. You need your soldiers, their weapons, and ammunition able to reach the conflict within whatever the maximum time local units are expected to hold out in case of an attack by another spacefaring nation.

If the region of space is highly contested then its reasonable to build permanents bases that can support ongoing operations out of their reserve forces. These would likely provide training, R&R, medical, and repair services as well as act as a depot for munitions and spare equipment. These will need to be well defended since they would be really important to any defensive plans for that region of space.

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  • $\begingroup$ Thanks. I'm still world building, but hope to get the nasty stuff out of my brain and onto paper before too long. $\endgroup$ – Madere Jul 9 '19 at 21:53

The question is flawed. Space wars should be fought by space navies, not by space armies.

I think there is a scene in Heinlein's Starship Troopers where a recruit asks what is the use of ground troops in an age of atomic weapons and push button warfare, and the drill sergeant throws a knife and pins the recruit's hand against a wall and asks him how he is going to push the button now.

But the drill sergeant's answer is flawed. The ground troops are not going to march or ride in trucks to another planet light years away. They are going to be transported by faster than light starships to the enemy planets. And enemy planets will have space radar and other systems to detect approaching enemy ships at a great distances.

So when a convoy of transport ships with soldiers and escorting space battleships from a hostile power are detected approaching a target planet - or more likely approaching the entire star system the target planet is in - the target planet's planetary defense system will be alerted and do everything it can to destroy the approaching ships.

Maybe it will turn on a force field so the approaching ships will smash into the force field and explode, or at best be stopped far from the target planet. Maybe it will fire giant ray guns many times larger than the approaching space battleships, so that they have many times the range of the ray guns on the approaching space battleships and can destroy all the approaching space battleships before the approaching space battleships get within range to fire their own ray guns. Etc., etc, etc.

So the fleet of space battleships escorting a convoy of army ships approaching a target planet will have to fight and deactivate the target planet's planetary defense system, and also any fleet of space battleships that might be assigned to guard the target planet, before the transport ships can land the army troops to take over the target planet.

But once the invading fleet of space battleships neutralizes the target planet's planetary defense system and/or defending fleet of space battleships, the invading fleet of space battleships should have enough firepower to exterminate all life on the target planet. So the invading war fleet could send a message to the planet giving two choices: Total extermination or total surrender.

Furthermore, in real life advanced space traveling civilizations are likely to have a lot of their assets in artificial space habitats. And an army would have even less of a role in conquering space habitats. Space habitats might also have defense systems and/or defending space fleets that an invading space fleet would have to defeat before it would be safe for transport ships to land troops.

And since space habitats would be enclosed and airtight, landing troops on the surface of a space habitat would be less useful than lading them on the surface of a planet. Getting through the surface of a space habitat and into it without the cooperation of the inhabitants and without destroying might be impossible. So the soldiers of the space army would be restricted to occupying and garrisoning the space habitat once the occupants of it had surrendered.

It is possible that all artificial space habitats will have faster than light drives installed and whenever and when ever an invading forces is winning they will active those drives and evacuate the star system, leaving it to the invaders. That would give a space army less to do occupying an conquered star system.

So the role of the invading army in the transport ships would probably be to land unopposed by the natives and form an occupation garrison. So it seems to me that a space army would mainly be used as garrison troops instead of invasion and conquest troops.

And that means that a space army garrisoning a planet would have little power to defend against invaders, since the invaders wouldn't send in invading soldiers once they neutralized the planetary defense system, but would instead demand total surrender, first. So once the planet surrendered, the garrison soldiers would be required by the laws of space war not to resist but to peacefully surrender to the incoming enemy soldiers, with severe penalties - possibly up to exterminating all soldiers and civilians on the planet - for violating that rule.

Of course in your future civilization the space navy may be officially called the space army of the future Human government. The organization can be called army instead of navy, and the members can be called soldiers instead of sailors, and they can have army ranks and organization instead of navy ranks and organization. But the main fighting and defense roles will be done by what is effectively a space navy, even if it is called an army, and the roles for a proper army in space warfare will be comparatively minor compared to those of a proper space navy.

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  • $\begingroup$ Exterminating a whole planetary population by means of space bombardment by space battleships is good if you just want to deny the planet to your enemy. But if you want to occupy and use its assets you might not want that planet to be a sorched Earth or undergoing a nuclear winter. In order to take and hold it so you can tax it's civilians, grow crops there whatever your planetary empire needs, you may need to engage ground forces. It also depends on the tech that an author decides exists in their verse. $\endgroup$ – Sarriesfan Jul 10 '19 at 20:40
  • $\begingroup$ With weapons available now, and theoretically possible in the reasonably foreseeable future, ships in orbit have a range of responses running from "take out the vehicle" to "nuke the continent". They aren't limited. The point of the matter is that they can make that threat. $\endgroup$ – Keith Morrison Jul 10 '19 at 21:11
  • $\begingroup$ Whether navy or army is the name of your space forces is irrelevant as no terrestrial term fits well with space. $\endgroup$ – Daniel Jul 11 '19 at 3:56
  • $\begingroup$ @KeithMorrison yes they can make the threat, and the planets system of government can decide their response. Depending on the scenario a writer chooses those threats may be empty and the planets government may know it, "Nuke us? Then how will you feed your expanding techno-hive planet?". The point is that it is not automatic that planetary bombardment trumps ground forces, it's possible to come up with ways in which that is negated. $\endgroup$ – Sarriesfan Jul 11 '19 at 18:38
  • $\begingroup$ I'm sorry, but navies, even space navies, cannot seize lands. A space navy can isolate a planet or star system. It cannot, however, seize the planets in usable condition. Marines or an army is needed for that. As for using kinetic or nuclear warheads against a planetary surface, that will be banned in my universe. $\endgroup$ – Madere Jul 11 '19 at 20:31

A planet with billions of people on it will need millions of ground troops to control. With such travel times and volumes of troops needed, there are some important factors to consider. First, you need ships to be BIG, like Independence Day big.

The risk disparity here is that it does not take a huge ship to nuke a species out of existence. Nor would it seem that you would need a huge ship to nuke a giant troop carrier. As M.A.Golding points out, this creates a massive disparity between the Navy and the Army.

To fix this problem; you need your universe to include very powerful force fields that scale up in effectiveness in proportion to your shield generator size. This way, big ships can't be hurt unless you have equally big weapons. Bombers that can fire a 50 megaton nuke become useless if it takes a 50 petaton explosion to break a transport's shields. So, not only are your troop transports gargantuan, but so are all your warships designed to stop them. This keeps the investment in waging a ground war from out-spanning the cost of preventing one with a cheap, small navy.

As for deployment, I would suggest each planet have its own standing army designed to resist a massive incursion. Transporting armies across space in advanced space ships is likely to be way more expensive than building up the army itself. To put things in perspective, the cumulative armed forces on Earth today are about 60 million; so, even a less populated border world could still muster a truly massive army.

The way I would handled deployment is to keep my whole navy near my inner worlds. A single ship is simply too expensive to risk leaving alone on the border to be overwhelmed by a superior force. If a border world is invaded by 10 alien ships carrying 1 million ground troops a peice, then I would have the defending world's army send this info to the home world, and use their local army to resist the invasion. Wars between forces that size may rage on for months or years until your homeworld shows up with enough forces to drive them off, or decides the cost of defending the planet it too great.

The last factor is that conquest needs to be way more valuable than the rock you are conquering. If you just wanted the materials, then it would be easier to genocide a species with nukes as already discussed, then wait a few decades to move in and colonize. To control for this, I would make most wars about assimilation instead of destruction. Sure you could colonize a new world and wait a few hundred years for them to build up enough infrastructure to contribute to your civilization in a meaningful way, or you could subdue who is there and use their existing infrastructure and labor to add to your empire.

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