Can realistic planetary invasion have any meaningful strategy?

Question

The sea is used as an analogy in space travel, and sometimes it works well. Ships sail on long journeys, between distant lands, across a hostile medium.

But these analogies tend to break down when it comes to planetary invasion. Planets aren't like islands: because an island has a coast that can be defended, and defenders can retreat into the interior of the island. This happened a lot in WW2, particularly in the Pacific. But in terms of planetary invasion... the entire planet surface is the coast, and the interior of the planet (the mantle) is not something which can really be retreated into.

Much more important, bridgeheads do not seem to be important. Spaceships can land virtually anywhere there is a flat surface on a planet. The concept of a front line in terms of planetary invasion seems somewhat irrelevant if you can land an army behind what is the "front" line.

Clearly there must be means to counter spaceships. Spaceships when in close orbit, or in the atmosphere, become subject to the same rules as aircraft. They can be hit by terrestrial missiles and other vessels. But all this means is that air superiority be achieved before invasion, and defences hit from long distance by bombardment.

Which brings me to the last point: why invade at all to destroy an enemy? Most structures of a civilisation are probably going to be on a surface. It is, after all, more difficult to build underground than on a surface. That means that most items of value can be simply blasted from above. Sure, some command points, armament storage and production facilities, and offices of government may be transferred to secure underground bunkers, but this still means essentially conceding the surface to whatever punishment the attacker chooses. There can be no counterstrike, merely a populace putting its heads between its legs and hoping for the best. Perhaps in the event of a siege there could be hope of external assistance, but this still makes it such that the attacked planet is itself left defenseless.

So is there any way for realistic planetary invasion have any meaningful strategy? I have never seen such in fiction - the invasion just "happens" and we generally catch up with the consequences of the invasion. If the invasion is defeated, it is always some Deus Ex Machina that has nothing to do with strategy relating to the invasion itself.

Edit: By "invade a planet" I mean from a strategic, military point of view. The Allied invasion of France during WW2, for instance, wasn't just an exercise to liberate western Europe - the Axis forces needed to be eliminated on the ground (aircraft not used in combined arms offensives were of limited use except in urban bombing, and urban bombing had no significant effect until airbases within mainland Europe were available). The option to float thousands of gunships a mile up to rain down precise fire 24 hours a day simply wasn't available, even when looking at recent historical wars. Had it been available in something like WW2, there simply would have been no Axis forces left to resist invasion.

Edit 2: I should probably have defined the parameters of this question as EITHER "Why would an interstellar civilisation invade instead of attacking a planet" OR "Given that an interstellar civilisation is invading a planet, how would meaningful defence be mounted?". I was kind of leaning in favour of the latter definition of the question, but that's a damn difficult question to answer (if it wasn't we would have seen it in sci-fi settings already).

Answer 1

why invade at all? Most structures of a civilisation are probably going to be on a surface. It is, after all, more difficult to build underground than on a surface. That means that most items of value can be simply blasted from above.

But you might not want to blast them from orbit because these items might also be of value to you.

You might want to avoid damage to the economic infrastructure. Economic assets like factories, resource stockpiles, mining facilities, research labs or transportation infrastructure could be useful for your own empire. You also might want to avoid excessive casualties among the civilian population. Either because you care about public opinion or because you have plans with the population which require them to be alive (for example: enslave, liberate, study, ritually sacrifice, re-educate, economically exploit or eat them).

So if you don't just want to take control of the planet but also of the assets on the surface, then you need troops on the ground.

So the average planetary invasion would likely work as follows:

1. Enter the solar system with your space fleet and neutralize any enemy forces in space (enemy fleets and armed space stations)
2. Enter orbit around the target planet. At that point, any mobile defending troops will likely start to entrench themselves in those areas they know you want to capture intact.
3. Neutralize any ground-to-orbit weapon systems which threaten your fleet in orbit.
4. Neutralize any military targets from orbit which you can safely destroy without hitting valuable assets. Barracks, government districts, military spaceports, ground troops which are still on the move, military production facilities.
5. If there are military assets you can not safely engage from orbit but which you want gone before you start the invasion, deploy aerodynamic precision bombers into the atmosphere to take them out or land small squads of special forces to perform surgical sabotage operations.
6. Land the bulk of your ground troops in areas which you believe to have minimum enemy presence and establish operating bases. Even though you can move troops quickly with space ships, having a permanent presence on the ground can be useful:
   • You establish permanent hold over the surrounding area.
   • Ground fortifications might make it harder for the enemy to attack back.
   • You have a place ground troops can retreat to on their own in case you ever temporary lose air superiority.
   • You have a place aerodynamic vessels can operate from.
   • Your logistics, maintenance and medical personnel might have an easier time doing their job if they work in spacious ground facilities instead from inside a cramped space ship.
   • Depending on how space ships work in your universe, landing from and launching to orbit might be costly, so you might want to limit such trips to a minimum.
7. Originating from those bases, have your ground troops conquer the population centers one by one. When your ground troops encounter resistance from entrenched enemies, aerial and orbital bombardments might be used as the situation allows.

But all of this of course assumes that you actually want the planet intact. If you simply want to obliterate your enemies, then you can just nuke the planet from orbit.

Answer 2

Well, part of the Island Hopping strategy had to deal with the fact that many the islands weren't lone chunks of land, but archipelago chains (Hawaii, the Philipines, Indonesia, Japan are all islands nations/states that include more than one Island... and were also part of the Pacific Theater). So many battles for one piece of territory were spread over multiple Islands... Perhaps use this to make your story... You're not invading a single enemy planet, but an enemy stellar system of planets. This allows for greater diversity. If we look at our own solar system we have the following areas:

Your shore waters would be your Kuiper Belt and Dwarf Planets/Plutoian celestial bodies... Here is where you should consider prepping your fleet for "landing" and taking interior planets. The defender would also likely station patrols and early warning systems here, so at this stage, both the defenses and offenses are have their red alerts and battle stations calls sounded and are mobilizing for battle.

Heading interior, our Gas giants would be our shore (Jupiter, Saturn, Uranus, Neptune). Each of theses are large planets with stormy atmospheres that would hide all manner of fun space station defenses and fleet depots to take. Additionally, all of those planets have multiple moons and plantary rings that can have some fun counter invasion measures. Your radar stations, fuel, scrambled star fighters, gun boats, and other defenses.

Our Asteroid Belt would be the demarkation of the forest line or other change from beach to interior land, and finally the rocky planets and the star (Sun, Mercury, Venus, Earth, Mars) would be the interior of an island. This isn't unheard of as anyone defending a planet in intersteller war will likely have resources spread out into the solar system to stop the advancing ships.... your big ships aren't just the fleet, but the tanks in this scenario.

YOu can see models like this in fiction like Star Trek, where the Solar system does have multiple facilities on other planets that all count to the defense of the Solar system. Headquarters is on Earth, but repair facilities are on Mars, with Jupiter serving as a command center for system movement, coordinating early warning, clearances to enter and leave Earth, and having an air field that can launch patrol fighter shuttles and other equipment. Each of these have ground and space based stations that also have roles to play. While the canon was that Earth was to well defended for a surprise strike, in the show the Dominion pulled it off for a moral victory in DS9 and in Star Trek Online, one of the Klingon missions has you lead a raid on military facilities that end up in a Pearl Harbor like action (basically you blow up ship yards on Mars, crippling repair facilities).

Now, how do we organize this invasion. All ships in a fleet are organized into "Groups". A command structure from the top would be a high ranking Admiral in charge of both the Fleet and lower ranking Admirals in charge of a single group...

What required ships for a group largely depends on your group organization theory and your mission needs... typically, the Group will be the Capital Ship and it's screen ships. Your Capital ship is your big offensive weapon and your screen protects the capital ship so it doesn't have to play defense. World War II is unique in that two different schools of capital ship theory were in different points of their life span: Battleship Theory was on it's way to being phased out, while Carrier Theory was the young hot newness of Naval Combat. Which ever theory you want to go with, keep in mind that the Capital Ship is the offensive ship, and the other ships are making sure that it doesn't die.

In real naval combat, Carrier theory is far superior as it's weapons systems (the planes) are able to better target enemies and from a father range. The Battle of Midway, which was the first true carrier to carrier naval battle, was fought without either side's ship coming within visual range of each other and was fought mostly this way because the U.S. Battle Ships were still recovering from Pearl Harbor and the Japanese developed Carriers to get around crippling restrictions on Battleships that had been placed on it (treaties on Capital ship limitations at the time were written when Carriers were first being tested as a concept at all). Battleship theory has a shorter range but was still used for support of landing forces and while it wasn't accurate, they didn't need to be... those shells still could reach targets out of line of sight and didn't need to hit directly... close was good enough to work.

In Sci-Fi a good Carrier vs. Battleship demonstration is to look at how Star Wars and Star Trek fight ship to ship in space battles. Star Wars uses Carrier Theory as the Star Destroyers (and their prequel sister classes more so) would deploy small fighters and smaller ships that couldn't be targeted by the big guns to fly close and shoot ships. Return of the Jedi has a great "group" mechanic employed where the Capital Ship (Death Star II) has a protective screen of Cruisers (Super Star Destroyers) and Destroyers (Star Destroyers) as well as fighter aircraft (TIE fighters) all engaging the enemy fleet... to the point that for much of the battle, the Death Star II was largely not focused on the battle and it's weapons were actually pretty spot on for its role. As a Carrier, it was much more sucessful as the fighters can be used for offense or defense, while as a Destroyer, the Planet-Blow-Up-Ray was largely scary but ineffective at close ranges. Star Trek is noted for it's lack of carrier ships on screen with the Enterprise almost always being a ship akin to a Battleship (and occasionally submarines, given the nature of both being boats surrounded by hostile environment). Almost any ship to ship engagement is sure to be broadsides with lasers, which suits battleship theory, and the shuttle compliments are never used in engagements. Deep Space 9, which shows fleet actions actually uprooted it completely by using organizations that would be based off of air-force operations, not naval operations (Sisko would repeated call out to "wings" instead of "groups" in the multiship battles).

A typical Carrier "Group" (which in the modern world, is enough) is usually alone and rarely works in a fleet. The United States is the only nation at time of writing with more than 2 carriers... 11 to be exact... and that's only counting the 11 Nimitz and Ford class super-carriers... the American Class "Amphibious Assault" ships aren't "Carriers" in U.S. Navy terms, even though they are bigger than most non-U.S. carriers and allow for air craft to take off and land from them... which is all you need for carriers to be counted as such... The U.S. has 20 of these). Typically the command of the group will be under a Rear Admiral Lower Half. On the carrier, there would be two people of Captain Rank running the shows. The Captain, who is incharge of the ship's operations and the CAG (never addressed as Captain while Underway... Ships have one Captain... his proper rank is Captain though) who is in charge of the ship's Air Wing (in the Navy, a Wing of airplanes is considered a "ship" for the purposes of Command). The Rear Admiral is also in charge of the Groups Cruiser Captains (Usually two, but numbers may differ for mission), the Destroyer Group Captain (all destroyers have a single Captain in charge, but if he or she is on another ship, the CO on your ship is usually a Commander) and the submarine commanders (Submarines usually have a CO of Commander Rank... they also are not ships, but boats... don't ask me why but never call a ship a boat or a boat a ship... it's insulting, for reasons...). If there are two "groups" in a single battle, typically the Admiral might be higher rank (Rear Admiral Upper Half or Vice Admiral) but will be dual hatted with both Group commander and fleet commander... this is because while any capital ships will have space for the Admiral's staff to due their job, they don't have duplicate space. In a carrier's "Island" there are three general levels for each CO onboard. Captain has the Bridge, CAG has the tower control, and Admiral has a level for general coordination between all ships. Battle ships similarly have a bridge and CIC (in the interior of the ship) to handle communications and multi-ship coordination. Also there is a difference between a Capital Ship and a Flag Ship. A Capital Ship is typically the ship with the most firepower while the Flag Ship is the ship with the Admiral on it (Admirals have individual flags that they are authorized to fly on their ship, hence the name. In the Days of Semaphore, this was needed so Captains could readily identify which ship was giving orders). So if the Admiral decides to run his show out of a Cruiser's CIC, then the Cruiser is the Flag Ship, but the Carrier is the Capital Ship.

In space combat, Battleship theory is a bit more viable if you're going to try for realistic, as the guns in space can have the same range as fighers without having the worry of pilots... And actually, a good portrayal would be using Submarine warfare if you go that route as subs rarely engage targets with a visual contact. On scree, this is boring unless you use the silent hunting for drama (Wrath of Khan does this wonderfully, as does the TOS episode with the first appearance of the Romulans). DS9 often used the Defiant's stripped down and limited space status (and cloaking device) to do sub stories in Space. This also works as Subs fight in 3D spaces as a space ship would, where as surface ships fight on a 2D plane (Wrath of Khan also uses this to allow Kirk to get the upper hand on Khan, who is a brilliant commander from his grasp of surface naval warfare... he doesn't think of space as 3D).

Other options are to make a capital ship a Battlestar (from the series of Battlestar Galactica, the titualar ships are Hybrid Carrier/Battleships. These don't work in navies but the contained flight deck of many space carriers means all the guns of a battleship be viable. Carriers don't have many guns as they can't be fired while in launch or recovery of aircraft... which is kinda the point of making a carrier anyway). Typically space carriers will have the flight deck embedded into the belly of the ship, either running stern to bow or port to starboard or off to the sides like wings. Each have an advantage over the real carrier as Launch and Recovery can happen simultaneously. In the latter two options, multiple runways exist, and one is dedicated to take off and one to landing. In the former, the flight deck is so long, it can do both.

As for ground landings, there isn't a lot as most scifi send woefully underwelming man power to planetary invasions... something akin to Helicopter Transport of Marines from ship to shore is generally depicted (consider those really cool looking landing craft in the Clone Wars finale). Star Trek rarely dealt with on screen ground depictions, though they did say they happened. Part of the reason is the book Starship: Trooper is such a big scifi staple that it tends to be dominant for overall planet wide theaters and it's difficult to show a planet wide scope of ground battle. Starship: Troopers is told from a Marine ground combatant and the military employs power armor for it's infantry. The book does describe that a single individual infantry man can typically cover a staggering amount of terrain by himself... at one point the hero notes that a unit formation with individuals spaced a kilometer apart is a very unusually tight formation that presents a threat of friendly fire from being clustered so close.

Answer 3

They want the planet whole

If they just wipe out huge chunks, humans will use nukes. If they capture the planet whole and the civilian population continues under occupation, they would be less likely to use scorched earth tactics.

A planet capable of supporting life isn't likely to be that common so to have the natives turn it into a radioactive cinder would be a tragedy. If you can capture the civilians, they won't want to nuke their own cities when you have seven billion hostages.

Once quelled, you can exterminate the lot but they need to believe they can surrender and live in peace until that point.

It means hand to hand fighting but a life supporting planet is worth the effort.

Answer 4

Here is my take on this.

Moving up/ down from a gravity well is expensive, slow, and prone to catastrophic failure.

Luckily we have space elevators (more realistically space rings) that allow us to use the power of the sun to lift things too/from the surface of the planet, instead of wasteing the unobtanium that powers our handwavium drive.

Space elevators are extremely expensive to build, it's a megastructrue. Not something you would destroy out of hand. As not having a space elevator makes the planet worthless for both the invader and the defender. Therefore neither side wants to destroy the space elevator. Invasions would start out with odsts falling from the sky to secure the base of a space elevator. While marines capture the official super structure.

The landing point than becomes your primary beach head as it is drastically cheaper to get supplies planetside thru the elevator than a hot drop.

But why don't you just glass the planet? Or use bio weapons to clear out the unwanted population?

The solution is that this was done in the past, leaving behind tomb worlds and the shattered husks of planets. The civilized species got together in a galactic convention and decided to lay out the rules of war they expect everyone to follow Breaking this convention, will lead to all other civilized species declaring war on you. Not something you want.

Answer 5

You can still use the island metaphor.

If the defending planet is technologically advanced enough to have their own space force, then they could have a blockade of ships around the planet.

Your front line is the extent of gravitational reach of the planet, or even further away depending on how fast/powerful the ships are. You could even compare artillery hidden in a forest to fortifications hidden in a meteor field.

Retreating inward to the island would be entering the planet atmosphere to take up ground defenses or activate more powerful weapons which have limitations that prevent them from being placed in space. Your island defense warship might have a nice cannon, but typically you have a larger one on the island that just can't fit on the ship.

Invaders may also want to not obliterate the surface of the planet if there is a strategic or otherwise coveted resource which is rare among other planets. You may even want to leave a certain amount of infrastructure in tact to save costs of developing a way to harvest that resource.

So:

If you want to invade a planet, but don't care about it strategically, then you go for shock and awe. A space faring civilization could possibly build a massive bulky spacecraft to use as a metal asteroid and just accelerate it as much as possible to destroy or cripple the planet in one shot.

If you want to invade and keep things, then you might use more precise tactics such as disabling communications, defenses, etc. But you would be more careful about launching weapons that could obliterate an entire continent. You might even want to use ground forces if the defending side can destroy your larger vessels. Alllllsooooo, a massive ship might not want to enter the gravitational pull of a planet, because then they need to spend a lot of fuel to leave again.

If you are a defender, you would want to establish your front line. Have a standing military that can hold a line in space. Use natural satellites and debris to conceal long range artillery. Put long range sensors out in space to detect an incoming invasion force so that you have time to prepare. And of course, make sure you have larger defenses planet-side that you can use while your front line ships hold off the invading force.

Answer 6

The defense: An array of really big satellite turrets.

Our current technology has no effective means of fighting off an attack from space, but unless we are talking about an invasion from a vastly technologically superior species we can presume that as spacecraft technology advanced, so did the defenses needed to defend against those spacecraft.

The first use of aircraft in warfare had no effective defenses, but anti-aircraft artillery quickly evolved to shoot down incoming planes. What we need, then, is anti-spacecraft guns - preferably positioned in orbit.

The key technology necessary for defending against an interplanetary invasion is a really long-ranged gun capable of hitting anything within a large, spherical region - anywhere it is not blocked by the planet itself. This will allow you to set up a defensive array with a relatively small number of satellites. The key to invading a planet, therefore, will be to either destroy one or more of these satellites or sneak past them.

The invasion: a Trojan Horse.

Since there is No Stealth In Space, if you intend to sneak past these satellites the only way to do so will be to pose as something you're not - sneak a bunch of soldiers inside what appears to be a group of benign cargo ships, for instance. This will, naturally, limit the number of troops you can field and the places you can field them.

The objective of the invasion will not be to conquer the planet's surface with infantry, but to take control of the base or bases that control the Planetary Defense Array. Once the satellites are down, the planet will be vulnerable to space invasion, rendering it effectively defenseless.

If you have warp gate technology, the initial invaders could set up a portal on the ground, allowing you to field more troops but restricting their entry point, forcing you to use more conventional ground-based strategy.

Answer 7

I'll summarize vastly different invasion strategies from different SF novels (Many details will be wrong because it's so long ago I read those).

Iain M Banks - The Algebraist: In this space opera, a warlord travels, with a huge fleet to a system to embiggen his empire and go after a Mc Guffin that's hidden somehwere there. His strategy is to achieve space dominance (it's pretty much a curb stomp battle), the threaten the system governemnt into submission by destroying habitats, murdering millions. IIRC, the actual planetary invasion happens afterwards, to secure important points on the main inhabited planet. OP is to clear a landing zone with neutron bombs, land power armored troops with vast air superiority - but these only have to deal with isolated resistance nests (and the fact that some aliens that coinhabit the invaded system, and are part of its military, are more radiation resistant than humans ...).

The drives of the invading fleet show up on the night sky as hundred extra stars as they break from relativistic speeds (the parts that don't fly through the system at relativistic speeds, taking potshots at military targets). Some, especially those with much to loose, offer tehir support to the new overlords.

Meanwhile, a smaller but technologically more advanced fleet is on it's way to fight the invaders (but will be late to the party). Part of their stated strategy is punitive measures against the population and elite of the system, if tehy find the resistance lacking.

Charles Stross - Singularity Sky: An interstellar expedition of post scarcity, post singularity (and post human) civilization comes across a planet whose population is mostly held at mid 20th century level by their dictatorship. The "invaders" are not interested in material goods, they care for interesting ideas, culture, and people to upload into their simulated world n their ship. Their invasion starts with a rain of mobile telefones. Place a call, take to the invaders, make a wish ...

Stanisław Lem - Fiasco is not about an invasion at all. The human explorers want to make peaceful contact. Their attempts at communication are met with silence, the strategy of the explorers is to communicate unambigously (using a powerful laser to write into clouds) and make credible threats (IIRC they blow up a moon) to force contact and communication. The title of the book is somehwat of a spoiler, but it's a good read.

Iain M Banks - Consider Phlebas features planetary invasions as a background event. The Idirans (the civilization taht does the invading) are technologically advanced enough to live a post scarcity life or live in space entirely. They chose not to, the strategy seems to rely on total space dominance, well armed shock troops and local collaborators.

Arkady and Boris Strugatzky - Wayside Picknick No invasion, but a contact with aliens that turns some areas of the world into zones filled with deadly traps, weird phenomena and some trinkets. Noone knows what the goal of the aliens was, maybe it was just a wayside picknick. No discernible strategy, several points on the globe ar hit by something resembling meteor strikes. That's it.

unkown One recentish "invasion" story - I forgot the author and name of the story - has aline nanotech somethings land in Kenya. The somethings build weird structures, change the landscape and are dangerous to touch. This leads to efforts to research them, plunder parts (similar to wayside picknick, except the zone is expanding).

Answer 8

Just as an alternative to the "there is something you want that you shouldnt destroy" idea, the available supplies could prevent this.

Lots of popular science fiction makes the space travel exceedingly simple and almost costless. Just set your location and you can get all the equipment you need to the planet in no time! But what if that isnt the case?

Imagine that you can only carry so much material to a planet. You can use a few kinetic bombardments but only to strike a few targets, its a stragetic weapon to damage key formations, not something to wipe a planet clean. Getting space rocks off-course into the planet/sun takes time, fuel and several space-battles as the planet's owners will send ships to try and get the rock on another course long before it even gets close.

So the attackers have come up with a different strategy: they arrive, use what KE bombardments with Rods From God and nukes to suppress and clear enemy prescenses near your landing zone where one or more ships will land. These ships are equipped with factories that will use the local materials to generate the equipment you need for the invasion (including perhaps 3D printed soldiers?). While you have control of space, you also have to capture an ENTIRE PLANET with the soldiers you brought with you. Likely a lot less people than the entire planet can throw at them. The invaders will need to capture more infrastructure and production facilities (likely something they try to land on top of) to succeed.

The defenders in the meantime will be building up their own forces and create missiles in secretive bunkers. Once enough missiles are created they will fire them simultaneously at any ships in the solar system. Encouraging the attackers to either hide their ships or land most of them on the planet to support the invasion instead of just nuking everything from orbit without resistance.

Answer 9

I have realised the flaws in my old strategy design so here is another that (hopefully) makes sense.

1. Send scouts to report on any of the planets defenses.
2. If the planet has defenses deploy infiltrators to kill and replace engineers/soldiers working of defenses after learning how it works.
3. Infiltrators sabotage defenses and take any engineers/soldiers as hostages(when they are asleep or defenseless)
4. Deploy fleet in orbit and tell defenders to surrender or the fleet carpet bombs the planet.
5. If the defenders refuse to surrender bomb and military targets and if the defenders deploy space fighters bomb civilian targets until the retreat.
6. If they have not surrendered direct the planets moon/s(if it has them) to the planet and or bomb areas vital to their survival(farms roads(or whatever transport they have) and water purification.
7. If the planet has been bombed to the point where nothing lives on it anymore you can begin mining operations or whatever you came there for.

So far i have spotted the flaw of how the infiltrators and scouts make it to the planet which can be fixed with posing as a trade ship. If you see any other flaws please inform me.

Answer 10

It depends on if you want the planet intact. If you want to leave things intact for your usage, especially their infrastructure, then unless you want to destroy some of it to cripple them a la WWII Allied bombings of German railways, you still have to invade on the ground and fight a conventional war. This lets you use the area as either a colony or, if you don't kill everyone, a slave colony, and use the resources potentially more efficiently (at least cheaply) than if you had to transport your own equipment.

If you only want resources, then you can destroy their main urban areas and send down your own harvesting equipment to the areas with the resources, a la Halo. Because planetary natives are secondary to your alien's goals, they can be killed off indiscriminately while they look for resources. Once the resources are located and collected, then the planet can be destroyed so other aliens can't use it.

Answer 11

IMHO it would be more usual to attack a planet than to invade it, or at least to threaten the planet with attack to force their surrender.

"Greetings Earthlings! You will be pleased to learn that the total extermination of your species can still be avoided by total surrender if done quickly enough."

Answer 12

Phil Geusz's David Birkenhead series has one of these that works.

The background is that there's a civil war in an interplanetary empire, and this planet was planning to switch from the loyalists to the rebels. The loyalists arrived first and prevented the betrayal, so the rebels arrive to find a planet against them instead of for them. The rebels have to take the planet, or their massive fleet will run dry. The book goes into plenty of depth about the fighting that results, which, roughly speaking, takes the following shape.

First, there's some fighting in space, which the rebels easily win. Then, the rebels make a few landing attempts; the first is ambushed right after landing and heavily defeated, but the second is successful. The rebels then land a large force on the planet, occupying its cities, but a massive planetwide guerilla war combined with sabotage operations prior to the rebel landing prevent total occupation. The rebel fleet is stuck waiting, which allows the main loyalist fleet to arrive and defeat the weakened rebel fleet.

To answer your question, the purpose here was simple - the planet had supplies that the rebel fleet needed in order to function. Necessity of supplies has been one of the biggest factors in warfare throughout history, and that wouldn't change in the future, short of replicators being invented.

(The series in general has one of the most realistic strategic representations of space warfare I've ever seen. It talks about the problems of defending such a large body as a planet, commerce raiding, psychological warfare, and more.)

Answer 13

TL;DR

Well, the thing is, you can't invade earth, as we have enough firepower to blast any fleet that comes withing range.* Therefore, your aliens must consider alternative options. A viable strategy for them would be to show up in orbit with a mock-up of the Death Star in tow.

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Long answer:

While it would be very easy to get ships to a planet you want to invade, and you could land troops pretty easily, it would be hard to maintain the beachhead. You see, while there would be quite a bit of confusion during the initial landing, governments would quickly get their collective acts together enough to drop a Tsar Bomba, killing anybody unfortunate enough to be in the area. If you try to land elsewhere, they will do it again as many times as necessary. Landing in multiple spots would not fix the problem, as then the earthlings would just use a bunch of smaller nukes (or a few MOABs.)

Destroying bombers / missiles before they reach your troops would not solve this problem, as then militaries would bring weapons like the M65 "Atomic Annie", point defense systems being useless against these.

While your troops are being destroyed on the ground, your fleet is experiencing similar problems. For the first 30 minutes they will be safe, no problems. Minute 31 is when the fireworks start. Missile silos throughout the world will pour forth their deadly contents, they being so profuse and of such diverse types that even the best point defenses will be unable to block all of them. Even if your forces manage to destroy the missile silos in time (unlikely; they are usually well hidden) they will still have to deal with all the missiles being fired from submarines and single-rocket launch points. Regardless, 15 minutes after the missiles are launched they will reach your fleet.

45 minutes after your fleet drops out of FTL it will be destroyed.

Because of these problems, your empire will have to explore other options.

The first (and least risky) is to take the route of the Pierson's Puppeteers from Larry Niven's Known Space series. In other words, instead of wasting time on bootless attacks, just use trade and governmental corruption to bend them to your will. For example, in the novel Ringworld, a Puppeteer named Nessus admits that the Puppeteers manipulated the Humans and Kzinti into engaging in four extremely bloody wars with each other. This had the net effect of killing most of the Kzinti, making them less of a threat for the Puppeteers.

If your hypothetical race is willing to wait a few millenia, they could just nuke the planet. This would kill everything, meaning no resistance. Of course though, it would also make the planet unusable for a few millenia before they can terraform it, but such concerns are relatively minor...

If they are not willing to wait, they could always go with the approach that @M.A.Golding suggested, namely by appearing in orbit with a mock-up of the Death Star and then broadcasting a message to the effect of "Greetings Earthlings! You will be pleased to learn that the total extermination of your species can still be avoided by total surrender if done quickly enough." That being said, this would be a very risky approach, as the Earth governments would probably respond by painting "Bug off!" on the nosecones of their missiles before launching them.

*Nota Bene: Unlike what they show in Stargate: SG1, an energy field would be unable to mitigate the shockwave or radiation from a nuclear weapon.

Answer 14

My advice is split the planet into geographical regions(as in deciding the areas for certain ground forces to take and not political). and send specialised ground forces to take each region.When a ground force finishes conquest(either taking land by force or making the enemy surrender) they check every location resistance fighters could hide in then they leave half their forces to occupy the land while the other half assists ground units that are losing once the planet is taken they clear potential threats to their power(this strategy only works after sending scouts to map out different ecosystems and learning of the political landscape)Edit:the strategy was designed to be superior to just land troops in one area and have them spread out from there