Warning: Long post is long post (again. again.)
One Specific Case: The Montauks
Introduction:
So for this post, I'll be building a scenario in which we have to face an alien invader from start to finish. I'll have differences in gameplay elements and take asides to explain reasoning as I move on through each step. I will begin first with a description of the alien species we are dealing with and what motivations they have for invading our fair planet rather than turning it into a barren rock floating in space. I will then go through the process of their formation of such a force, an explanation as to why they would decide that this is a good idea, and then move on to the actual game phase itself, though not the conclusion. I'll be making a fair number of assumptions and I'll point those out as I go. I'll be inserting convenient Codices too
Part A: Invader
We are dealing with an alien species known as the Montauks, they are a six-legged primarily predatory species that are hyper-aggressive due to the fierce evolutionary competition on the planet. They respect strength and cunning and abhor weakness. They believe in the right of rule by might. However, they also know not to go after tough customers except in groups, and not take risks when there is abundant easier prey around. Montauks are obligate carnivores, subsisting on a meat-based diet, while there are a few plants they might eat it is strictly for taste as they are unable to derive substantial nutrients from these plants.
Materially their world is quite well satisfied, they have advanced away from non-renewable resources towards things like advanced solar and hydrogen fusion technology. Any raw material requirements they might have are easily taken care of via asteroid mining. They have developed advanced iron and nickel based alloys that they use for most production, whatever rarer materials they need can be found in their own planet, and the few asteroids that they capture with said rare materials. They have sufficient water from asteroid mining operations throughout their solar system. They have also developed FTL drives that they have used for colonization efforts.
They are a monarchy similar in the nature to the Mongol hordes. The strongest amongst them is the King, and his word is law unto death when a new King is chosen. Even when weakened by age, the current King is strong because the Montauks respect both physical and mental strength. A cunning old King is just as strong as one who can break walls. He will receive no challenges so long as he is still cunning.
Part B: The Invader's Motivations
The Montauk have run out of space on their home world. The nearby planets have also all developed strong enough Internal Forces to stand off the Montauk External Forces. And there are no nearby unclaimed habitable planets. They have hit the practical limits of their agricultural technology at this point and are a food-importing star-nation, such weakness cannot be tolerated. More importantly, it makes the Montauk vulnerable to blockades, a leveraging point that has been used against them multiple times. Therefore the Montauk need to acquire a new food source, and somewhere to settle some of their population would be nice too. If they could have found a habitable planet without inhabitants (especially ones that shoot back) in practical range, they would have taken that one instead of ours. Unfortunately, those don't exist.
They are constrained in that their FTL and shipbuilding technology is not well developed yet and so they are practical range-limited. That is their ships can only carry so much, and the structural integrity can only sustain so much stress before collapsing at acceleration. They have navigational issues as well, and any more fuel would make cargo carrying capacity very tiny. So the only planet within their range that is habitable and does not have a Navy is ours.
Whoopee.
--Codex: Space Warfare
Space is the ultimate high ground, it is also the ultimate ocean. The influence of sea power upon history is without question, even moreso in space. Warships in space can carry weapons of unimaginable power, they are not constrained by the atmosphere or by gravity, merely their own structural integrity. In traditional space warfare, the only thing that matters is the naval battle. Should a defending star-nation lose a naval battle, the planet that the battle was fought at is compelled to surrender, to not do so means the start of orbital bombardment, typically with thermonuclear weapons until all effective military resistance is annihilated before the ground invasion begins. In order to spare planets from the inevitable defeat, the defenders typically simply surrender the planet once they have lost the naval battle.
--Codex: The Tachyon-FTL Drive
The first of the practical inertia-less FTL drives, it's a stepping stone to newer better things. Much of the fuel consumed by the tachyon-drive is done getting into FTL, for which is need quite the acceleration distance. In order to support these drives a super-clean 'hyper-launch-lane' needs to be constructed, cleared of microasteroids, and then pointed in the correct direction. Once done, the ship will burn somewhere around 45% of its total fuel getting into FTL speeds, using 5% to sustain the drive and then exit at its target. The tachyon-FTL drive has potentially unlimited speed, though powering it is the primary issue, followed by the exotic materials used to fuel it for FTL. Constructing the drive itself is actually really cheap, and they're routinely featured in everything, even missiles. The infrastructure necessary to support such a drive system is very expensive as well. Tachyon-FTL space works very differently from normal space, it is almost impossible to navigate. Instead navigation is done before the flight, complex calculations and simulations are run in order to determine the destination, they are input into the computer systems and then the ship runs on automatic, in flight adjustments are impossible. After a certain distance, these equations fall apart and are no longer able to accurately plot their destination and so ships must 'drop-out' of FTL at a midpoint to get a fix on their bearing and then once again re-run the calculations to their final destination. This is extremely expensive however, as most of the fuel is spent getting 'into' FTL, and the launch infrastructure necessary to make travel possible is also extremely expensive.
However, Tachyon-FTL drives have another use, called the 'stutter-acceleration' which is a method in which they can use normal fuel to provide near inertia-less acceleration at very high efficiency numbers. While this is still constrained by the speed of light, it is extremely useful for getting from A to B within a system.
--Codex: Military Organization with tachyon-FTL Drives
Civilizations that have developed the very expensive tachyon-based superluminal travel system have two distinct branches of military service. They have an' Internal Forces' that composes the bulk of their forces. These are beings that crew manned warships. They are responsible for early detection of enemy invasion forces and neutralization of said force. The Internal Forces typically have access to substantially more firepower than their opponents, should they find the invader, they are capable of mounting a crushing counter-attack early on. Making an invader's life very difficult and enforcing a fragile peace. As even weaker star-nations are capable of standing off much larger star-nations due to the expense of FTL limiting the attacking force's size.
There is also the External Forces, these are the invaders. They travel using tachyon-FTL in order to reach their target in secret. Once they have arrived, they attempt to utilize the resources present within the system in order to secretly build an invasion force that they can throw at the defenders. Their first attack must inflict catastrophic damage against the enemy, as once the defenders are alerted, the industrial power of the defense will be mustered. The first attack must cripple both industrial and counter-attack capabilities. If successful, the invaders will fight a war of attrition to destroy the enemy Navy and seize control over the planet's orbitals.
Part C: Specific Goals
1. Control over the Earth. They need to control the planet and obtain the agricultural capacity of the Earth. Trade is not an option as that is precisely what they were trying to escape. They must have absolute control their own food production. This isn't just "I'll build farms here that I own." This is "My rights here are absolute, this is my territory." They want sovereignty and will settle for nothing less.
2. Humans are optional. Humans are not really that necessary for the functioning of this system. We are a truly secondary consideration to the Montauk, however a number of other factors shape this decision. First, we like our sovereignty (they want it), so we're going to fight for it. Second, we are another competing food eater, and every extra human is another mouth to feed. Third, humans are trouble, the natives are usually restless and they'll sabotage operations should they live. Fourth, Genocide is Cheap, this is something that people don't appreciate very much, but genocide is always the cheapest option when it comes to subjugation, shooting them is preferable to having the take them in and reeducate them and policing them and dealing with dissidents and feeding new generations and slowly crushing rebel ideas, that's just too much of a bother, shoot them all, be done with it. Whatever carrying capacity you free up can be used for worthier pursuits, like your people. Besides, it's not like humans are people after all, they're alien and other. (See: Australian colonists to Aborigines) Also it's unlikely they'll contribute in labor, the invaders built their own invasion fleet far away from home, their automation technology is far beyond human labor capacity.
3. Establishment of colonization and agricultural operations. They intend to set up their own colony on Earth, obtaining food supplies and shipping it back to the home world. They will preserve agricultural space whenever possible, but they'll be an awful lot less restrained in areas they can't turn into farmland. Mountains and cities can expect terminal doses of instant sunshine.
Part D: Us
We're still us. Smart, savvy, handsome (if I do say so myself) and bloodthirsty and vicious. We are living in a mostly stable political order with the United States now competing with the European Federation as the leading economy in the world. Europe has a slight edge this year, America might be back in first place next year. Space exploration is now cheaper than ever before now that we can recycle our rockets and we've built a space infrastructure reflective of that. We have substantially more satellites in space and we've been making an effort to clean up our orbital space. We're still dependent on fossil fuels, though we're slowly weaning ourselves off of that too. All in all, the future of humanity looks bright.
--Codex: The European Federation
The EF is a logical extension of the EU, it is an actual ever-closer union, though still with squabbling nation-states involved. They share a common military, fiscal, monetary, and foreign policy. However certain national elements still get brought to the fore from time to time. Like that time terrorists smashed Tour Total, Tour First, and Societe Generale Twin Towers in La Defense in France.
That hadn't been pretty, especially when the French reminded everyone that Napoleon Bonaparte was French and his legacy was very much alive.
Part E: The Galactic Order
The galaxy now exists in a state of liberalism. That is it exists in a state in which star-nations value mutual ties to each other and continued interactions in interstellar organizations rather than power-centric maneuvers. This galactic order has been mostly enforced by the Myrmidon Empire, which has calmed down from its genocidal imperialist phase (oh thank god, the rest of the galaxy breathed) and is mostly successful. There are occasional rogue states that the Imperial Navy needs to have a 'conversation' with (said conversations involve copious numbers of thermonuclear weapons.) The edges of the unexplored frontiers now exist in a state of carefully regulated spaces. All activities must be done with the sanction of a State, and occasional proxy wars fought over frontier planets are not uncommon. Habitable planets are not uncommon, however, and generally unless technically-limited or there is a strategic requirement, there are plenty of uninhabited habitable planets to go around. Invading inhabited planets that are not part of the interstellar order is still illegal. Trade with these planets must also be carried out in compliance with the general interstellar order, failure to adhere to these rules will result in embargoes, not just by the Empire, but by the rest of civilization, with a possible escalation up to a visit by the Imperial Navy.
--Codex: The Imperial Navy.
"Nothing Withstands the Might of the Imperial Navy"
The Imperial Navy boasts of being the most powerful naval force in existence. It is not an idle boast. Its capital ship fleet is six times larger than all the rest of the capital ships of all the rest of the navies combined, they are more technologically advanced to boot. More importantly, the Imperial Navy has access to the Imperial Jump Gate Network, and also advanced fold-space FTL drives that allow them to enter FTL for next to no fuel costs and easy navigation. So the Imperial Navy is no longer split into "Internal" and "External" forces seeing as they can reach anywhere for little cost. The drives are stupidly expensive though, very maintenance intensive, and very classified, making them only sensible aboard warships. The drives are such a protected secret that the Empire has committed genocide and world-destruction operations to ensure that no examples of the technology are captured.
--Codex: Space Fighters
Don't exist. They make no sense when facing dreadnoughts armed with long-range laser systems that can just make the fighters go away. Larger, faster anti-ship missiles with massive thermonuclear warheads are the preferred way to fight. Any 'fighter' capable of carrying weaponry heavy enough to actually damage a dreadnought would have to be so big that it'd simply make more sense to build a proper warship.
Part F: The Plan
1. Establish a forward staging base. For that production, yo.
2. Gather necessary intelligence, both through passive and active measures. Passive as in watch our TV. Active as in infiltration into our data networks and maybe even the occasional abduction if it can be pulled off silently.
3. Gather allies. Fracture order. The objective will be to prevent a united planet against the invading forces as that would cause complications.
4. Establish Casus Belli. Invading inhabited planets in wars of aggression is strictly illegal and will bring the might of the Imperial Navy down upon the Montauk homeworld. (which would be a bad thing) They want to do this as late as possible to give us as little warning as possible. It'd be like what Germany did with Poland. They'll manufacture an incident. They don't need anything ironclad, just barely legal enough that they can tie the Assembly up debating it forever rather than immediately call for a police action.
5. Active scouting. Much of the information gathering is done, but actively probing defenses works gives details that can't be found by reading and watching TV.
6. Invade. Invade. Invade. Genocide. Genocide. Genocide.
Part G: The War
This is when I drop into a more narrative storytelling style.
In 2035, astronomers working in the VLA discover a very curious signal from the edge of the solar system. They initially dismiss the signal until it reoccurs a few times, they can find no other source and the strength is such that is seems to be artificial and from that particular point which it was detected. They call around to other radio telescopes and they triangulate the point to just beyond Pluto. They check around further and find that the Sudbury Neutrino Observatory and MiniBooNE at Fermilab all detected huge surges of neutrinos, far, far, far above the norm. Now other telescopes, X-Ray and Gamma Ray for example are all reporting that something is wrong and it's all coming from that one point source. Soon all of the higher energy telescopes have reported,
something is very wrong.
At this point the XCOM generation has come into power, and they're genre-savvy enough to hope for the best and prepare for the worst. A UN General Assembly resolution calls for further investigation. The world begins to build up its military strength, slowly and gradually, without much of a hurry, though.
Scientists send repeated coded transmissions at the point source. Nothing.
As much as they might be curious, they can't find anything there with their telescopes and nothing is responding. It's something they're going to investigate once they get the chance.
By 2040 no invasion has materialized, and so the world gets distracted again, terrorists crashed a number of planes into La Defense and the French demonstrate exactly why Europe was terrified of them for a very long time and why they keep getting called the Big Blue Blob in grand strategy games.
North Korea loses its mind and crosses both of its borders. China is invading the North from the North currently while a combined South Korean and US force is pushing up from the South. The world order starts breaking down as paramilitary groups in the Middle East and Africa start toppling governments and expanding their control.
Popular protests in Venezuela evolve into a full blown ultra-nationalist revolution that is spreading very rapidly throughout South America. Fascists are coming into power at an alarming rate. Greece, Spain, and Italy fall to Fascist revolutions and the European Project is starting to fray.
Scientists announce the a discovery in 2042. There's something out there. They've discovered strange atmospheric readings during the testing of their new radar systems. The things are of consistent size and performance profiles, they clearly enter and exit the atmosphere at which point they can no longer be tracked (since what they're actually tracking is disruption of air flow cause by moving objects.)
The first attack hits a suburb of Turkey. Massive chaos is reported, as well as substantial destruction as local military forces rush to deal with what they think is a terrorist attack. They arrive to find nothing, though. In the fray a pair of dead bodies are found, riddled with bullets, they are not human.
Radio telescopes all over the world receive an encoded message. It is written in a dozen different language. It reads: "You have killed our peaceful envoys. This is an act of war."
The second attack occurs almost immediately afterwards, it hits the Middle East, an active military forces immediately respond, engaging in a sustained firefight against the invaders. They report that they are fighting robotic enemies of some sort, because the patrol unit that made the first encounter is completely destroyed. The QRF that responds is similarly destroyed. By the time a larger scaled armored unit is sent, the invaders are gone. Rather more and more are striking all over the world. In China, in Russia, in Europe and in the US. They seem to be striking completely at random wherever they can.
Within two weeks though, space-tracking radar detects massive contacts moving into orbit, and thousands of high speed objects inbound from all directions. They strike missile silos and destroy naval bases, blow apart military-industrial centers and military stockpiles. Tens of thousands more contacts are raining down now, in the midlands of the United States, deep within Russia and China, dead center of Germany and France. Exactly the areas where the military is not.
To make things worse, many of the paramilitary and revolutionary states in Africa, South America, and the Middle East had been puppets of the aliens all along, they had been spared the orbital bombardment. They had exchanged service for power in the New Order. Already the aliens have begun to build bases within these client states.
The Battle for Earth had begun.
Conclusion
And that concludes this episode of: "So how (and why) did the Earth get invaded this time?" My objective here was to flesh out a realistic scenario with understandable motives. It's not the be all and end all, but rather a way to explore an alien invasion from conception to execution. I try and detail all the pressures involved in making this scenario work, and all the motivations that are necessary. I've provided some extra information as well, to provide even more context. This is me going completely mad