I am trying to make a space based strategy game, in which different factions try to take over all the planets in a solar system. The first step anyone should take to take over a planet would be to destroy any spaceships (and surface-to-space defense installations) defending it, after this, an attacker may choose to destroy the planet from space (making it uninhabitable), negotiate a surrender, lay siege or invade the planet. While I do have some idea as to what the first options would involve, I am not sure what exactly an invasion would look like.

My question is therefor: What would a full-scale invasion of a planet, by a technologically equal civilization, involve?

Based on these assumptions:

1. Invading a planet involves putting your troops on the ground, (which is the difference between invading a planet, and negotiating a surrender or destroying the planet from space).

2. The invaders want the planet as intact as possible, which includes both its infrastructure, ecosystems (if any) and the civilian population. The player may be allowed to choose how great length their troops shall go to to protect the planet, but certainly invading a planet should leave it better off than the Drop-a-few-thousand-antimatter-bombs-from-space-option

3. Even though the societies depicted in my game will be (significantly) more advanced than our society, their infrastructure would still look mostly like ours: They may have fusion reactors, quantum computers, hyperloops, space elevators, mass drivers and superconductors, but they will still have a physical energy, communication, and transportation grid.

4. In my game magnetic shields may be used on some planets to protect the inhabitants from solar radiation, but these shields are not powerful enough to provide any protection against the weapons of this time.

5. There is no teleportation technology, therefore the invaders can't just beam their troops down, and the defenders can't use a stargate to evacuate their planet or get help.

6. The weapons depicted in my game will mostly be more advanced versions of modern weapons: There may be antimatter bombs, antimatter propelled surface-to-space missiles, super-advanced stealth technology and surface-to-space artillery (possibly mass drivers), but there are no FTL torpedoes (in fact no FTL at all), no lightsabers, no hand phaser, no hand lasers, no death star, no gravitational-wave weapons, no photon torpedoes, no invisible elite soldiers and absolutely no magic.

7. (in response to comment) I assume that the attacking ships are very heavy ships build in orbit, which properly can't survive entering the atmosphere of the planet they are attacking, and won't try to land on it (since it will be unlikely that they will get off the planet again) The attackers will, however, have carriers which can send smaller lander vehicles which can take troops to the surface (and back).

8. (In response to comment) One of the main reasons for asking this question was that I wasn't sure when an invasion should be considered a success, but at this point I think the invasion would be successful as soon as all centrally organised resistance either surrendered or was destroyed, though when this happens on an entire planet can be extremely hard to tell, and judging by modern invasions of countries, fighting hardly ever ends when the capital is captured – just look at Afghanistan.

9. (In response to multiple answers and comments) While life-bearing planets probably will be self-sufficient in food, I think that all planets to some extent will depend on imported goods. At least because some resources aren't available on all planets and moons – of which Helium-3 probably is the most important resource.

Even though only one answer could be accepted, i have tried to take inspiration from all suggestions, thanks.

• Can the attacking ships land on the surface of the planet? When is an invasion a success, when government centers are captured? – user37130 Apr 22 '17 at 18:28
• Have you read Footfall? – JDługosz Apr 22 '17 at 21:37
• offtopic, but I recommend you the grammarly browser plugin, very effective on errors you had. Easy to use, helps me a lot.(not perfect, but very useful) – MolbOrg Apr 22 '17 at 22:18
• The problem with your question is the fact that the answer highly depends on the technologies used. Example - if the planet defense is done mostly by orbital forces, and they do not have some tricky survival technologies on the planet itself(we today) and the attacker is capable of producing(or deploying) a large tinfoil sheet which blocks the sunlight, on the planes they will suffer to survive. But if they use the thermonuclear power to produce all their food and goods etc - they will be immune to such type of low-tech attack. And that is just one many small things which makes the difference. – MolbOrg Apr 22 '17 at 22:31
• When you say lay siege to the planet, what does that entail? All planets are self sufficient in terms of food and water, because dilivery of those resources would not be economical at all. So kinds of supplies would be deprived of the planet? – ntchapin Apr 22 '17 at 22:37

The invaders wants the planet as intact as possiple, which includes both its infrastructure, ecosystems (if any) and civilian population.

Once you have control of orbit I can think of a few ways this could go down.

1. Negotiate surrender. After fighting, point out that the planetary government has lost and that being conquered by you would be preferable to being conquered by your adversaries. I think of the Germans at the wend of WW2 who fled west to surrender to the Americans rather than the Soviets. This sort of interaction might happen with no-one firing a shot, which means you also capture the surface to space defenses intact.

2. Negotiate fealty. Maybe you could control this planet without having them surrender. They come under your control as an independent allied state in your confederacy or allied to you like a feudal baron. Again the situation needs to be such that they see an advantage for themselves in the situation.

I can imagine this working in a situation where planetary government was divided. One governmental entity allies itself with the outworlders (you) and allows them to come down and set up a presence in that territory. That entity then gets help from the outworlders against its interplanetary rivals. Maybe too subtle for a game but inside help would be good in this scenario: agents who influence government to favor this sort of alliance in advance of the spaceships showing up.

1. Show of force. I am thinking here of Hiroshima. The alternative to the nukes was an invasion of Japan, whom the Allies thought would probably fight to the last person. The nuclear bombs demonstrated immense force superiority such that even the Japanese realized that they had to capitulate. From orbit, with planetary defenses down, a show of force should be pretty easy. Once they realize you are able and willing to make any given place a crater they will get in line.

2. Work around.. Find a lightly populated and defensible base of operations and just take that. Leave the rest of the world to do as it will. Especially if they are politically divided, no one entity might see an interest in taking on the formidable aliens solo.

3. Rescue.. In world war 2 the peoples along the western USSR initially hailed the Germans as rescuers, saving them from their Soviet overlords. For reasons unclear to me the Germans did not run with that, but crushed these people worse than the Soviets had. But you don't need to. Rescue a planet from your rival who has conquered them. They will ally with you in gratitude.

As opposed to totally crushing the populace back to the Stone Age, these "take it intact" options also offer the in game possibility of an uprising - for example if you are too draconian in taxes / food levy / drafting soldiers etc.

Just because there is resistance does not mean you cannot use the planet. An active insurgency does not mean a successful insurgency. Unfriendly polities elsewhere on planet may or may not hamper your use of territory you control.

More destructive / total war options seem less interesting: e.g. neutron bombs / plague or bioweapons etc.

• Hiroshima and Nagasaki were a show of force, but not for Japan it was aimed at the Soviet Union. When the Emperor announced Japan's surrender on the radio nuclear weapons weren't mentioned. The Soviet Union had declared war on Japan and that's why they surrendered. It wasn't an alternative to an Allied invasion. Agree with you about not using brute force to subdue planets. Destruction only hardens resistance. Why the Germans behaved as they did: they were dealing with Slavs and Slavs were untermenschen. Not so smart. – a4android Apr 23 '17 at 4:17
• Actually the nazis believed it was the destiny of the German people to expand eastward. That made the people living in the area surplus of the requirements and their "rights" something to be ignored as a matter practical value. You can probably best understand this by comparing to how native americans were treated under the manifest destiny of the US to expand westward, but amplified by much higher population density. This is actually relevant as the invaders might also want to replace the current inhabitants with colonists. – Ville Niemi Apr 23 '17 at 12:46

The scenario you describe is not that far off what you get in the Warhammer 40 000 universe and is covered in quite some depth in the Horus Heresy novels.

Here there are a number of degrees of planetary attack.

• Total destruction : rendering the planet permanently uninhabitable destroying it entirely by saturation bombardment and/or chemical and biological weapons.
• Extermination of the populace : typically a heavy but targeted orbital bombardment followed up by troop landings and an all out ground/air campaign involving a large range of unit types.
• Targeted Assault seizing key strategic locations eg military command centres, seats of government, infrastructure hubs and landing zones/ports for bringing additional reinforcements and equipment.

So if we generalise a bit there are a few key elements.

• Insertion vehicles capable of delivering troops direct from orbit ideally capable of operating in combat zones.
• Air superiority craft capable of protecting the troop carriers from air and ground fire.
• Close air support
• Elite infantry capable of assaulting directly from drop craft or from a landing zone a moderate distance away. These troops should be capable of operating for a moderate length of time without external support and have their own organic transport and support weapons. They should be adaptable and capable of aggressive high mobility assaults and enough tactical flexibility to adapt to unexpected or changing circumstances.
• Regular infantry and heavy armour to hold ground, consolidate gains, follow up initial assaults and provide heavy support, flank protection and security.
• Logistics capable of resupplying troops and equipment from orbit efficiently and in bulk.

This is broadly the way that modern combined arms forces work. You have specialist self contained airborne and marine units which are able to take key strategic locations rapidly and at a distance from establish bases which are followed up by heavier and more numerous units which can hold the ground which is taken initially and has the logistic capability to then apply sustained pressure on the enemy

Actually this model is not a million miles away from the allied invasion of Normandy in 1944 (as long as your technological hand-waving allows the English Channel to be reasonable analogy to 'from orbit').

Here you had initial operations by irregular and special forces to take key positions like gun emplacements and bridges and disrupt the enemy logistics and command and control, then followed up by direct infantry (and a limited amount of specialised armour) assault of the beaches in force supported by air superiority fighters, close air support, and strategic bombing. Then once you have secured an initial beach head a rapid build up of heavy armour, equipment, troops and supplies required for a rapid advance and build of of pressure to retain the initiative.

Orbital Bombardment

Nuking the planet is unacceptable, but with this level of tech the attackers would be able to use conventional, highly accurate weapons to bombard enemy aurfields, vehicles, etc. The US millitary investigated something like this awhile back (dropping a bar of tungsten from orbit to level a small area). They determined that it wasn't cost effective, but that won't be a problem for you. Moreover, modern icbms attack from orbit, and are reasonably accurate and could be fitted with convengional munitions. This capability makes conventional resistance impossible, as any strong points could be reduced from orbit prior to the ground attack. Morover, while camouflaged and underground bases would survive the initial attack, orbital strikes would be called in as soon as the position was engaged.

The solution is for the defenders to scatter their forces and wage a guerilla campaign. This would work best in jungle or forested areas where satellites couldn't track them. Alternatively, they could hide in the civilian population, like isis is doing in Mosul at the moment. Both strategies can keep resisting for a long time, so the attacker would need to recruit a local force to patrols the planets vast areas. Moreover, they would need to engage in a Hearts And Minds campaign to win back the populace and hopefully make the resistance put down their arms. Or just use Agent Orange to clear the forests and the let Orbital weapons platforms do the rest.

• modern icbms have [120 m possible error][en.wikipedia.org/wiki/LGM-118_Peacekeeper] so conventional munitions couldn't destroy undergound base – ADS Apr 22 '17 at 21:51
• @ADS good point, but keep in mind that this advanced future civilization will have far more accurate ones than the current (already antiquated) designes. Also, they'll have bigger, more penetrative bombs. – Bert Haddad Apr 22 '17 at 22:52
• The last time somebody tried a Hearts and Minds campaign (here on planet Earth) it failed dismally. Using Agent Orange is the subtle approach (yes that's sarcasm at work). For an invasion to truly succeed it needs a political solution not an exercise in brute force. – a4android Apr 23 '17 at 4:08
• @a4android Agreed, but I wrote this under the assumption that a political solution was impossible. HAM isn't always the failure it's made out to be, as while it never ends all the fighting, it usually gets a significant number of locals on your side. – Bert Haddad Apr 23 '17 at 6:05

See how it done in Civilisation:

1. After defeating the army of defenders your army will begin to suppress resistance
2. Amount of resistance depends on how hostile population against your army
3. The bigger your army the faster it will suppress resistance
4. During suppression it's possible that population has been reduced and/or infrastructure has destroyed

In real world some resistance forces could act many years after invasion. But in fact they couldn't do anything significant because of

• their negligibility
• willingness of most population to live in peace.

As game designer you could add details to make process more demanding on the players. But it may cause that players to prefer not to invade planets just because it's too boring.

• The willingness of populations to acquiesce to alien occupation may depend on whether the planetary population and the invaders are the same species. – a4android Apr 23 '17 at 4:00
• @a4android This thought is used in Civilisation: same species and cultural influence significantly reduce the seristance – ADS Apr 23 '17 at 9:44

I think the best option is not to have outright invasions and simply have a game of diplomacy for the negotiation complete with intelligence assets, special ops, surgical strikes with orbital weapons, and so on. Less destructive and more interesting than an invasion.

Use the fact that controlling space means controlling air and surface movement. This gives invader an absolute advantage in mobility. So the planet would be given a number of strategic targets which would then be either destroyed by orbital bombardment or assaulted by a heavy concentration of elite troops with orbital bombardment preventing any counter-attacks or support by defenders outside the immediate area. This would be countered by digging in strategic targets (to defend against orbital bombardment), automated defences (to counter the concentration of force), and possibly self-destruct devices powerful enough to destroy the attacking force.

Strategic targets would typically be command infrastructure and logistics. There might also be psychological targets. Once defender loses enough command structure, logistics, or morale the resistance will collapse. Brute force probably should not be the only method here. Agents might be able to persuade some defenders to defect and hand over a target intact. Promises or threats backed by appropriate reputation allow morale warfare.

An invader with excellent diplomacy, strong reputation, and powerful network of agents should be able to invade without actual fighting. There should be (ideally) a smooth transition between negotiation and invasion. Negotiation minimises the need to fight, assaults improve negotiating position.

If an enemy nation, race, empire, ect had achieved domination over a planet's orbital space, and the planet refused to surrender, conquest would be incredibly difficult if technology is roughly equivalent.

Let us assume that Planet A is being invaded by Empire Z. Planet A's space navy has been utterly destroyed and all ground-to-space weapons have been eliminated through strategic orbital strikes.

1. The first main question is: what are the logistics behind the invading force? How far is the supply line, and approximately how long can the invading force last without further resupply?

2. The second main question that must be asked before an invasion begins is, "Is there a space elevator?" If there is a space elevator, the first step in conquest would be seizing control of the space elevator which DRASTICALLY improves the logistics of supplying ground forces on the planet's surface.

3. Logistics aside if there is a space elevator the invaders will launch a smaller elite invasion force onto territory near the space elevator and then blitz to secure the space elevator's base. Think marines taking a beachhead and then sprinting to capture a port. Once the space elevator is secure all other ground forces will be sent to the planet's surface via the elevator. All major supply routes will stem from the elevator. From this point on the goal of the invader force is to capture all strategic objectives ranging from military bases, major cities, resources nodes, ect. Warfare would likely be very conventional with the one nuance being the availability of tactical orbital strikes on enemy formations and bases.

4. Without the space elevator the invasion could be drastically different. Without a supply method to defeat the gravity well of the planet's surface the amount of troops that could be deployed would be severely limited. The goal of orbital invasions would establishing "beachheads" on the surface, and then capturing areas that can produce supplies for you armies. Newly found supplies can be used to expand the invader's ground presence, but increasing the number of men on the planet is risky. If momentum is lost and the invading forces cannot capture any more supplies, than the army may collapse from supply constraints and evacuation might become necessary or else the invaders let millions of their soldiers die.

Generally speaking however let us say a planet has one billion people. They can, when push comes to shove, mobilize around around 25% of their population. When I say mobilize I mean that they can put guns in mens' hands. Their standing army would be much smaller, but it is likely that they could drastically outnumber any invading force, because it seems that putting tens of millions of soldiers on space ships and keeping them fed is a pretty difficult feat.

For an invasion force to negate these differences it should be expected that significant orbital strike support will be needed from the fleet in orbit. So expect large amounts of "rods from the gods" being sent down to devastate the planet's ground forces. Casualties will be high no matter what and infrastructure damage will likely be severe.

For the purposes of your game space elevators will make a drastic difference on invasion difficulty, and if the player decides to invade it should require a large investment of manpower and resources.

• What's so difficult about dropping supplies from orbit? You just need to put them into a container with a heat shield and a parachute. Give them a slight nudge to get them on a suborbital trajectory and the atmosphere will do the rest. – Philipp Apr 23 '17 at 1:57
• Well to be frank I forgot about just dropping stuff and letting it fall to the surface. I was thinking of the invaders' space shuttles heading down the the surface and then heading back out into space. I would assume however that they have a limited number of containers, heat shields, and parachutes suited for atmospheric entry. I doubt that they would be able to drop all supplies in that manor, but that could not be the case. One question though would be, could armored vehicles be dropped on the planet's surface in that way? Could a 70 ton vehicle be dropped to the surface? – ntchapin Apr 23 '17 at 15:44
• We are talking about a quite high tech-level here. An armored vehicle might not even need a heat shield. Its usual armor and shielding might be sufficient. Stopping 70 ton with a parachute would require a really, really large parachute. But single-use retro-rockets are also a way to land softly. When dropping right into a battle, retro-rockets would likely be preferable because you spend far less time being target practice for the enemy's anti-air weapons. – Philipp Apr 24 '17 at 1:43
• You can intercept RVs / parachutes quite easily if you have space faring technology. And they are very vulnerable (and sensor-blind, too!) while reentering. – Michał Jastrzębski May 5 '17 at 11:14
• add-on, because edit time limit: Not being a target practice is not that easy - consider that modern anti-air missile systems can go well beyond mach 3, while pulling 50+ G in turns! Current ballistic defence missiles KEP warheads have less maneuverability, but still pull serious loads, while hypersonic. As for 70 ton vehicles - shuttle orbiter had 68 metric tons of EMPTY weight alone. And it could land with cargo onboard too. edit: The From wikipedia: Space Shuttle orbited a combined 122,534 kg (270,142 lb) when launching the Chandra X-ray Observatory on STS-93 – Michał Jastrzębski May 5 '17 at 11:20

I would suggest covertly landing a small well-trained force that will sabotage ground to space defenses in the LZ and causing general chaos. Sleeper agents could conceivably be used for this if you absolutely can't have a craft land before the defenses and detection systems are taken out.

Ok, so I personally believe people forget certain things such as:

• atmosphere differences from homeworlds
• alien diseases
• climates
• etc.

But for invading it's pretty easy to be the invader. Well, unless the defender knows how your species attacks or ticks or something along those lines.

Let me make it easier:
The plan to invade an alien world is mostly a logistical problem and health and safety (other than the killing bit), so you will need to know what the alien world is a ocean world.

But what is its gravity? Or the heat? Like, are the oceans boiling or are they freezing? What is the atmosphere? Is the Alien population massive (easier to hit, but more likely to absorb that damage)?

What are your weapons? Heat based? Light based? Mass based? Are the weapons affected by the planet? Like mass is affected by gravity, light is affected by mist, fog, heat by cold or heat resistance.

And so far we haven't got into the invasion yet. So now that we've got the basics done:

An alien world with the same technology has a few space ports, and a defence fleet. The fleet is destroyed, but here is the thing... ALWAYS DESTROY ALIEN SHIPS no matter what. And I mean it: one ship could be a world ending disaster (if it is based of real life). One ship the size of a skyscraper (small I guessing) could destroy a city and the surrounding area. So destroy them. Hopefully the debris will go into the planet and cause havoc.

Now let's say all the logistics is done. How are your troops doing? If they have mechs or something anyway, you would planetary bomb them, destroy mostly defence things or even terror tactics. So let's say we want to conquer it with an occupational force. You would drop your main army in civilian areas, where their military might not be. Then you would drop special forces in military zones. Marines would combat drop anywhere on a small notice, support units, such as supply drops from space ships, drop pods, drop ships etc. or conquer the moon (if it has one).

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