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I am writing a fan story for a well known universe with typical Sci Fi armies. (For those who want to knows which, in this universe people seems to love Pauldrons and gothic architecture)

They basically fight like WWII in spaaAAAAaace, and their best writers often include real life concepts and tactics from this era. Theirs space ships are a WWII navy, in space. Their air force are a WWII airforce with space jets, their tanks are space tigers. They have space marines, humongous mecha but don't like to bombard planets full of humans because it lower the number of sla ... loyal workers.

But in the canon of this universe, we often see woefully undermanned force attacking planets. Sometimes with as few as a dozen thousand men, and not super-men, to attack/control a billion+ population.

So here is my question, how strong should be one of these army to take a pretty centralised, typical Sci Fi colony with a population of 500 millions, slightly lower tech than the invader.

Let's assume that the invader can have a max of a few hundred space-marines/spartans equivalent and less than a dozen humongous mecha, and none of that for the defender.

We will also guess that this is a conquest war, not a extermination, the locals rebelled for, say, taxes and freedom of something (dirty traitors). They will seriously fight for what they want but are still within the scope of negotiations.

Edits after comments:

  • The planet is WWII England ... in space ! Not everyone is in the military, but most spent 1-2 years in training. Military have a lot of initial support that will go down if the war start to linger on damage their lives too much.
  • The task force carry enough supply for the first few month of combat and will be resupplied by out of system sources if the planet start producing&trading with the empire of the task force.
  • The task force can receive reinforcement.
  • The task force will need a few month and up to a year to reach the rebel planet after the war declaration.
  • Annihilation of the planet population and production center is the last resort. As much as possible the world should keep its production capacity. So, actually, a good part of the infrastructures and population can be targeted, we only need to keep the key industries up and running, even if we have to import other resources afterward.
  • The planetary government knows this and will first try to make the planet too hard to conquer. If they can hold long enough they will have more negotiation tools. (The task force will be needed elsewhere and they can ask for a better status)
  • The defender have very limited access to offensive weaponry space wise. Let's say that, due to their static position, it's trivial for the attacker to dispose of them in the first few days of the assault.
  • The defender have very limited access to low orbit transports. But the attacker have dropship in high number, can unload its whole capacity in a week and can move its space marines around the planet in a matter of hours.

Edit for bonus : Invasion Objective

Bloggs answer is very satisfying, partly due to sheer logic and partly because it justify the existence and usefulness of small sized space marines task forces in many universe.

So, just as a bonus, for this very universe, the core goal for the big empire is to get resources, manpower or industrial production from each of its worlds. So the goal of the taskforce would be to keep the factories running, but to manage to get their production offworld in less than 6 months. If this is accomplished, the taskforce or a part of it can stay longer to get compliance or further negotiate a reintegration of the planet in the empire.

(PS: My first guess was around a dozen million men, factoring in superior tech and a potential higher fighting ability for the invader. With all support scaled up to the task, so some scores of thousands of tanks and planes.)

NB: I now realise how wide this question is due to political, military and scifi questions, still it's something that seems really interesting to me because official writers get it wrong very, VERY regularly.

NB2 : I am editing this as an anonymous user, the StackExchange did not link this question to my account after its creation so It's still an anonymous question, sorry guys :/

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  • $\begingroup$ I've posted an answer because I like the premise, but this question is really, really broad. How large is the enemy armed forces? What's the general level of armament? If the government capitulates, will the army? will the citizens? Exactly how against collateral damage are you? What are your resupply routes like? Is every member of the opposing planet behind the cause to the same degree, or are there different factions? All of these factors (and more) can play into a successful conquest/occupation of a state. $\endgroup$ – Joe Bloggs Dec 16 '15 at 10:05
  • $\begingroup$ Look up "Gunboat diplomacy". $\endgroup$ – Euphoric Dec 16 '15 at 10:47
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    $\begingroup$ Did you forget to log in before updating your answer, or did some other anonymous person try changing your question? $\endgroup$ – JDługosz Dec 16 '15 at 10:48
  • $\begingroup$ I see you tried, but can you be more specific? You need to add more specifics on technology advancement, level of automation / AI, distance between enemy planets, how many other planets are there.... and so on and so forth. $\endgroup$ – Abhinav Dec 16 '15 at 14:48
  • $\begingroup$ One little correction: If I get the setting right, the tanks are more like WWI ;) $\endgroup$ – Bounce Dec 17 '15 at 10:11
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Two things:

How military is the planet? If you're invading space Switzerland, where everyone has a rifle and is trained to use it, then you'll need a pretty huge army. If you're invading Space England, where the vast majority of the population are unarmed except for a specialised armed forces, it's a different question. Both: however: run into the following problem:

Conquest and occupation are two very different things

Your space dudes control the high orbitals (presumably) and can drop rocks on the centralised government. Well done. The planet is conquered as the local authority has no choice but to be supplanted or die. If they choose to die: drop in the space marines. The enemy military can't intervene particularly because they've just had their head chopped off and can be demolished piecemeal by your marines and a few thousand soldiers. Also: You own the skies.

However: Once you've 'conquered' the planet, what's the chance that the 500 million strong population is going to do as you say? Pretty much nil. For that you need further occupation. If everyone on the planet is armed, then you effectively need one soldier per-head just to disarm them, or you're going to have to kill a lot of people. Even if not: You run into the 'resistance', which will effectively lock down your military into a bone grindingly painful war of attrition for every second they're on the planet. Then the thing that matters is the ratio of military might/the number of resisting citizens. Two examples to think about:

1: Iraq: Relatively low military presence compared to the number of people that needed policing. Result: Long lasting, generalised pain.

2: France (WWII): High military presence compared to the number of people that needed policing. Result: More targeted pain designed to disrupt and inconvenience the occupying army.

So really: If they're willing to negotiate, you should take along one ship, your space marines, mechs, and whoever else can fit in the transport just in case. Then the enemy surrenders. If they're not willing to negotiate: You can't win this fight. Glass the planet and start again. If it's somewhere inbetween then it depends upon their will to fight, their ability to engage in guerrilla warfare, and your unwillingness to drop rocks on them.

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  • $\begingroup$ "your unwillingness to drop rocks on them" - I actually laughed out loud. +1 $\endgroup$ – AndreiROM Dec 16 '15 at 15:18
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I agree with Joe Bloggs, invading a planet and actually holding a successful (and profitable!) occupation is nigh impossible, given even just the logistical problems.

I guess the question is why the planet is to be invaded in the first place.

  1. Rebel uprising: I'd go for shock and awe. Conquer the capital, then propose a cease-fire under your terms. If they don't comply, nuke one of their cities from orbit and turn it into a sea of molten glass. Repeat until compliance is achieved.
    1. Making an example for others: Yeah, just glass them. Or go all Alderaan on their ass, that will work as well.
  2. Access to resources: In that case it is enough to conquer the area surrounding the resources you're planning to harvest/mine/... Make it clear that any disturbance to your operations will be punished severely, but that the planet will be more or less independent apart from it. If there are any incidents escalate to #1.
    1. Resources already produced: Impose a tax/tithe/toll and let the locals do all the mining/harvesting.for you. Then just visit periodically to pick up your goods. If the planet fails their quota... See #1.
  3. Goods/Tech/Knowledge: Drop in a strike force, take what you want and leave. Let the rest of the planet carry on.
  4. Rule the Galaxy: If your aim is to control as much of the galaxy as possible, I'd try to establish vassal worlds/systems. Force them into compliance by force (see #1 :P). Vassals will generally be expected to pay a tithe to their liege. Maybe the liege (i.e. you) will also have some input on how they should govern, i.e. replace the existing governments/command structure with either locals loyal to you/your cause, or your own governors/officers.

These are all ways to avoid having to conquer and occupy the entire planet, so don't quite answer your question. Still I think they provide some ideas on how such invasions/conquests could be tailored depending on the needs/reasons leading up to the conflict.

Essentially it really all leads to 1) Tell them what you want; 2) if they don't to it, nuke them; 3) repeat. Saves you the cost of invasions, and prevents your military forces to be spread thin over too many occupied planets.

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  • $\begingroup$ The reason for the invasion is simple because it is their god-emperor-damn-right to rule the whole galaxy, as they are the superior race. $\endgroup$ – Bounce Dec 17 '15 at 10:20
  • $\begingroup$ Hehe, obviously. ;) I addressed this point in an edit. $\endgroup$ – fgysin reinstate Monica Dec 17 '15 at 12:26
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As Joe Bloggs says, it's a very broad question, and difficult to answer directly.

Here is an blog post which talks about a 3:1 ratio of attackers to defenders as a general rule. I've heard other ratios discussed as well.

But in almost every situation where an attacking force has come upon an entrenched and aware enemy, the attacking force is only successful when it has overwhelming numbers.

From Thermopylae (where the 300 Spartans held out) to Khe Sanh, an attacking force requires a higher ratio of attackers to defenders.

The alternative is a long, protracted siege. Cut off supply lines, deny reinforcement and wait until the enemy starve. Much harder when you are blockading an entire planet.

An example: The American Vietnam war. There was a disparity between the two sides in terms of training, experience and equipment.

You can argue that the North Vietnamese Army/Viet Cong (for simplicity I'll refer to them as NVA) had less in the way of hard training infrastructure (a lot of their training was undertaken in the field) and had a generally lower tech level of weaponry. What they did have were numbers, and the experience of thirty years of fighting against European armies (Japan, then France, then America). The NVA had possibly an overall higher level of discipline due to the hard nature of life in a communist country, and immediate repercussions for not following an order (AKA a bullet in the head). Also, they had the homeground advantage, and a well embedded intelligence network that went as high as the upper levels of government.

The U.S. and allied forces on the other hand, had a training infrastructure the NVA could only dream about, weapons and support systems that could dominate a battlefield. Their command and control system was much more elaborate and generally more immediate than the NVA's.

When the U.S. and allied forces attacked a dug in NVA force, the attrition was often horrendous despite the areas being prepped with napalm and bombs from B-52s and other aircraft, and artillery fire before the troops went in.

The NVA faced a similar problem when attacking a fortified position. Their approach would be to use surprise as much as possible, and throw much higher numbers at the position until their enemy just ran out of combat effective troops. A hole would open up, and NVA would pour massive numbers through it and overwhelm the defenders.

During the Tet Offensive of early 1968, the NVA sent anything up to 100,000 troops against dug in positions all over Vietnam. Roughly half of all those troops were killed. It more or less ended the Viet Cong as an effective fighting force, and in theory was a victory for the U.S. led forces (despite the footage of the U.S. embassy being overrun in Saigon being a major catalyst for the U.S. withdrawal).

At the end of the day, if you send a few hundred space marines against a population of 500 million, even if they are only armed with rocks and spears, your marines are going to have a very, very hard time.

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I really like this question but as others have pointed it, it really is "it depends".

How brutal are you willing to be?

Let's say you decide to make an example of a highly populated but not very industrially productive are of the globe - and exterminate everyone in that region from space? You'd probably scare everyone into behaving.

Larry Niven's Footfall discusses this scenario.

What level of cooperation do you really require?

Subdue - crush their ability to fight back?
Submission - give them the alternatives of death or cooperation but essentially leave them in control of their fates other than that?
Conquer - sweep away their old system of self-rule and impose your own system of rule?

The farther down you go on the list, the greater military investment that it will require.

The price of cooperation

You could probably "buy" their support with technology for less investment than it would take to subjugate them. Given the interstellar supply lines, your resources are literally irreplaceable without their industrial capacity. Their industrial capacity does you know good without you upgrading their technology to your standards.

So you're going to have to give the technology over anyway. Why not use it to buy what you need? Vernor Vinge's Zone's of Thought series (e.g. A Deepness in the Sky discuss how this might work).

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  • $\begingroup$ You can go the Foundation way of things. Give them technology, make them dependent on the technology to the point that disruption of your exports makes their society collapse. $\endgroup$ – Eleshar Dec 13 '18 at 17:57

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