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For the past few days I’ve been thinking a lot about the idea of a medieval-style siege occurring at a planetary level—that is, an aggressor attempting to “starve out” a planet by cutting off its supplies rather than attacking it directly. Here we’d assume that, for whatever reason, the aggressor does not want to cause any geological or significant ecological damage to the planet it is attacking.

Given that, in most cases, you’d imagine that an entire planet would be self-sustaining and could survive indefinitely with its outside trade being blocked, how might this scenario play out? Would the attacker try to disable the major infrastructure of the planet with tactical strikes? What technology might achieve this?

Any thoughts on this (or, even better, examples from Sci-Fi) would be greatly appreciated. Thanks!

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    $\begingroup$ How could a defender fortify a planet? Would they have to armor the whole surface and move all housing and industry underground. To paraphrase Bean from Ender's Shadow, a normal fortress must guard the whole perimeter while the attacker can strike any point, but in space the planet defender must guard the whole surface area and the attacker can still strike any point. He concluded that planet defence was untenable in space and thus attack must be the only option, along with other other concutions. $\endgroup$ – sdrawkcabdear Feb 29 '16 at 22:47
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    $\begingroup$ Check out this answer for some other thoughts on the subject. $\endgroup$ – Avernium Feb 29 '16 at 22:49
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    $\begingroup$ Could the attackers perhaps block the Sun? Or at least limit it's power to cripple the defender's ability to remain self-sustaining, with a threatening population. Perhaps tow a large sail to L1, and eclipse enough of the star's light to severely impact the defender's planet's agriculture and economy. $\endgroup$ – BM- Mar 1 '16 at 3:38
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    $\begingroup$ There is one thing you entirely forget to tell us about: the... tech-level of both sides. If you laying siege at a pre space race your options are completely different than when you doing the same with someone who can put a whole plant below some kind of energy shield or - even worse - shoot back from armed satellites or PW-Lasers from the surface. $\endgroup$ – Confused Merlin Mar 1 '16 at 9:41
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    $\begingroup$ @ConfusedMerlin I believe tech levels of both sides are deducible from the question. Multiple inhabited planets exist. Attacker has efficient enough space travel to send an invading fleet. Defender doesn't have efficient enough space travel to become dependent on large-scale interplanetary supply chains (see Trantor example). Defender is either on par, or slightly lower on tech and much higher on resources. No planet killers there (uninvented or banned). Looks detailed enough for me. $\endgroup$ – Dallaylaen Mar 2 '16 at 13:58
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I very highly suggest reading these two resources:

Much of the details of the siege will depend upon the exact objective of the aggressors. For instance, what do they need to keep intact?

  • anything?
  • biosphere?
  • industrial infrastructure?
  • population?

The more of the planet you need to keep intact, the more effort and time it's going to require to conquer.

If you're just trying to eliminate a threat, then bombard the surface with enough asteroids and/or nuclear weapons to destroy all civilization, it doesn't matter very much whether you kill the population or not since the survivors will be too busy just trying to survive to think about getting even.

Presumably your attackers want to do more than that and they want something on the surface of the planet captured relatively intact. Since the attackers don't have the troop strength to capture it directly, they're resorting to breaking the will of the population to make them give up that item.

The Attackers

Let's assume the attackers are coming from another solar system.

Advantages

Then the advantages that the attacker has are:

  • They get to choose targets
  • They can concentrate their forces
  • They can run away if/when needed/wanted
  • They can change their strategy as needed
  • They know the defender's objective (to make the attacker leave)
  • They can see large concentrations of the defenders (either military, industrial, or population on the planet surface).
  • They can hold valuable features on the surface of the planet hostage (e.g. "if you don't surrender, we'll nuke LA")

Disadvantages

  • A very long supply-line, so long that, in effect, anything lost during the conquest you can assume can't be replaced.
  • They will be completely visible to the defenders at all times (you can't hide in space).
  • Unlike normal siege operations, the attacker doesn't get resupplied. Presumably the defenders planet is the only inhabitable one which means the attacker might run out of food before the defender does.

Defenders

Assuming the planet is inhabitable to begin with (siege of an airless body of the Moon would be totally different), the defenders have many advantages.

Advantages

In a planetary siege the defenders have many advantages:

  • You have access to your industrial base
  • You have access to your population base
  • You have more troops
  • You have more resources
  • You can replace your losses
  • You have access to terrain in which you can hide (e.g. oceans, mountains, etc.)
  • You have a tremendous ability to dump waste heat (for those super powerful laser beams to sweep the enemy from the sky)
  • You mostly should be able to feed yourself (this might actually lead to mass starvation of the population but a suitably determined defender might allow an excess population to starve while he kept his military fed)
  • If the defenders know the attacker's objective (e.g. capture the biosphere relatively intact for colonization), then they can hold that objective hostage (i.e. "if we lose we're going to bombard the surface with our remaining nuclear weapons so you don't get it either!")
  • If the attacker doesn't have enough vehicles/satellites to watch the planet from all sides, the defenders can hide activity (ground movement, launches, etc.) from the attacker using the bulk of the planet.

If necessary, a war of attrition strongly favors the defenders. Exchanging almost any number of defenders to get a single attacker is probably a winning strategy for the defenders.

The ground and especially the ocean provides an awesome opportunity for mobile forces to remain hidden from view. The defenders need only use passive sensors to watch the attackers, move nuclear powered submarine nuclear missile boats into the path of the attacker's orbits and then launch missiles when they're out of view. Once the rockets finish the boost phase they become much harder to detect.

Disadvantages

Due to the nature of the engagement, the attackers can see any concentrations of effort (industrial, population, military, etc.) and bombard them to keep the population properly subdued.

In space combat and under normal circumstances any object that isn't thrusting is at risk. If your enemy can predict where you'll be at a certain point in time, they can ensure an asteroid or similar unwelcome event happens at that point in space and time. If you are on a planet's surface, this means you.

Any fixed defensive installation that you have will get at most one shot before the attackers figure out where it is. Then they'll stay out of your firing envelope and ensure you get to see the closeup view of the underside of an asteroid.

Planetary Sieges are not Medieval Sieges writ large

Based upon the above, there will be almost no similarity between the two. The attacker is the one who will worry about the duration of the siege.

But really, read those resources. There's a bunch of information there and many people put a lot of thought into them.

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  • $\begingroup$ Thanks for the resources. I agree about planetary sieges not being equivalent to medieval sieges—this is something I considered myself. I think the medieval siege was very much a product of medieval technology and what that meant for attacking and defending fortified positions. Still, as per some other responses, I think it's not unfeasible given the right scenario. $\endgroup$ – Mark Mar 2 '16 at 8:23
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It depends on how much damage to the planet is acceptable.

If you're okay with banging it up a little bit (collateral damage is a thing, after all), but don't want to cause too much long-term damage, it would be pretty easy to bombard the major food production sources from orbit. Just imagine what dropping a few tungsten rods on Nebraska and Iowa would do to America's grain production.

If you don't want to harm the planet at all, it's a bit trickier, but still doable. Remember that the key to a siege is making the besieged want to surrender. It's majorly an issue of morale, and food just tends to be one of the easiest ways to hurt an enemy's morale. It's easy to say that you'll fight to the death when the table is full, but it's much harder when you're trying to extend your last loaf of bread another day, when you're giving up your portion so that your child might survive.

The battle of morale is a battle that you can fight constantly, even without trying. Never let the people on the planet forget that you're there; plan your ships' orbits so that there is always a ship in view of every major population center, and they can't look up without seeing you. Maybe you could drop a rock into an ocean, causing a tidal wave to crash into a city. Communications on such a short distance are easy, so send video of your crew eating, laughing, generally having a good time (with the implication being "Surrender and join us, and you can be happy too!"), and generally not living under the siege of an armada of spaceships.

The entire time, though, you'll be wanting to watch out for the planet fighting back. You have spaceships, they have missiles that can reach space. If they're capable of space flight, you may need to destroy their shipyards so that they can't fight back as easily. That's really the big issue with a siege; you only besiege a place when you can't conquer it in an outright assault, so the planet must be about as advanced as you are. They will fight back, so at some point you'll have to ask yourself how much you're short-term damage you're willing to do to the planet.

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  • $\begingroup$ Love this! Surprised there aren't more examples of something like this from fiction. $\endgroup$ – Mark Mar 2 '16 at 8:34
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This kind of sci-fi setting treats planets as nations or islands. A planet may be all industrial or have some resource it sells, and imports food. Treat it the same as islands and ignore most scaling issues; e.g. Star Trek was an allegory of south pacific islands and some have noted the correspondence between Kirk and Hornblower.

For a specific example in well-received fiction, look at Trantor in Asimov's Foundation series, and the Puppeteer homeworld in Niven's Known Space.

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  • $\begingroup$ Or Corsicant, for that matter. $\endgroup$ – Xandar The Zenon Mar 1 '16 at 2:31
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Assuming the following:

  • the planet is self-sustained;
  • both sides are at comparable/same tech level;
  • the defenders are capable of destroying the invading fleet if it comes close (as in @Jim2B's answer);
  • the invading fleet is capable of keeping the defenders grounded for a long time (*);
  • efficient interstellar signaling requires going to space;

-- a siege looks possible and feasible.

The shortage of food/resources/goods is not a big threat. Yet the defending planet would be deprived of technical progress, as reinventing all the wheels on one's own is much less efficient than knowledge sharing.

Based on the speed of progress in the given universe, being suspended for several years may become a huge and long lasting setback. Many would consider surrendering a lesser evil in such case. This also depends on attackers' claims and reputation, of course.

Defenders' internal politics could also tip the balance, for instance a government might want to keep themselves in power at all cost for as long as possible, and screw the progress.

(*) As for the "keep defenders grounded" claim, I think that such balance is possible. An ascending ship should suffer from the same issues as ones in space, plus it must struggle against gravity.

Attackers could organize rotation of forces (not to be confused with orbiting :) ) and maybe even build a temporary base on a nearby uninhabited planet. This would decrease the wear of their fleet.

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    $\begingroup$ Some of earth's greatest tech developments have come when there was an aggressor to fight off. The attacker presents a unified enemy for the population to unite around. $\endgroup$ – boatcoder Jul 12 '16 at 16:10

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