Storyline variant: Deceitful aliens backstory.Several ideas by others glued together by me (thanks folks). A small backstory variant, just to give some support for tech-tree and tactical missions. I think we should leave the rest for fan-fiction.
Reason for InvasionUnknown. Probably warmongery. Also no knowledge about aliens monitoring earth earlier or if they even knew they will find life on this planet. They are complete mystery even after they are defeated...
But I also like the other ideas for Reason for Invasion:
--- Searching for some artifact (There's something on earth that they want)
--- Hunting someone (there's an alien on earth that these aliens want to capture alive)
Both explain why aliens do not just nuke Earth, especially the first one. Moreover, they would like to use humans as the workforce for the search.
Anyway, aliens would like to terrorize humanity and use them as slaves, rather than fight a regular war with humans and then fight a prolonged guerilla war or be forced to nuke Earth. They are intelligent enough to know that diplomacy is much cheaper than war, especially if you have a huge army as a diplomatic argument. However, they are used to rather crude, brute-force diplomacy.
Phalanx BirthWhat aliens sent to Earth is a standard multi-purpose reconessaince/diplomacy/annexation force. Powerful enough to conquer Earth, but slow to prepare for full-scale war. Reinforcements are impossible, because of the distance to their homeworld.
Aliens are unlucky that the approach of their fleet is caught by Earth telescopes several years before they arrive. Phalanx is established by the UN, just in case, because attempts to communicate with the approaching fleet are unsuccessful (for whatever reason, ideas welcome). There is still no base for Phalanx, only organization structure, initial staff, money, and the chief (you). The organization is not secret, but its description is not focused on fighting aliens, so as not to offend them, but rather on research on aliens, doing customs duty service, guarding Earth borders, etc.
After the aliens arrive, they are surprised that humans are aware of their arrival. They hoped to hide and prepare for several years and attack/terrorize/intimidate only after they are ready. Different species are waking from cryogenic sleep with different speed. They have to mine Earth, other planets, moons "in order to amass the materials necessary to increase their robotic army", they are slowly adapting to Earth conditions, such as gravity, level of radioactivity, Solar wind, etc. So right now they can only send limited forces to fight, or hire humans to fight for them.
Therefore their plan is to hold off the invasion, lie about their goals, at the same time establishing foothold on earth, learning about human weaknesses and prepearing for the war. Then they try to terrorize some nations and infiltrate them, while all the time lying about their intentions. "So the aliens try to suppress the humans by winning a psychological victory". For this the aliens have two operation bodies --- the official one declaring peace and blaming incidents on humans or rogue aliens, and the other secret alien forces, experimenting on Earth and humans, infiltrating, terrorizing, and eventually, crushing humans ("liberating them").
So this is a war between the secret service of Aliens and the non-secret UN-governed Phalanx, bound by political restrictions and public opinion reactions. Aliens lie to the very end. "On the other hand we have the Alien "Elite Forces", trying to implement a secret agenda assigned to them by their leaders. Thus we avoid a setting in which a big war between worlds is going on." Big wars are boring when you participate in them on the tactical level (hundreds of thousands of missions would be required and each single mission would be quite unimportant in itself).
Phalanx Duties"As for Phalanx itself, if it was up against such a potentially overwhelming enemy, it seems like its goal wouldn't be so much as to defeat the enemy, but to find out how to beat the enemy (different focus)." However, humans armies are not what they were before, so Phalanx even gradually becomes important militarily and its experiences with aliens are invaluable for reorganization of rebuilding armies. Governments prefer to send Phalanx to deal with aliens so that civilian casualties are lower than when a regular army waltzes into a city, so that more is learned about aliens and so that soldiers are not demoralized/bribed/infected by aliens. Army is too thin to guard all of Earth, so it is concentrated on guarding dams, chemical factories, government seats, etc. Cities are mostly undefended. The army sometimes does an independent action (a big army unit happens to be close to ufo landing) and the player is informed about it. But these are exceptions.
However, the army can always be called in, though they are usually too late so save civilians and the aliens are often gone. The calling of regular army can be represented by the "auto" feature of tactical missions --- they help Phalanx, so the mission will usually be won, even if the player is a newbie and would have trouble winning the mission alone. OTOH, the number of artifacts recovered and civilians saved would usually be lower than what an experienced player could achieve --- army is not as subtle as Phalanx and it arrives late. It would be nice if the army could be called also for hunting the last remaining alien on a big map --- a button "call the army" that concludes the battle automatically, with percent of additional civilian casualties and destroyed alien artifacts based on the number of remaining aliens.
ComplicationsPeacemongering (somebody wants to do it diplomatically) --- Phalanx is not secret service and aliens lie a lot, so there will be a lot of trouble, especially at the beginning (this is represented by "gloves off", by the counter-identification trick when player is angry at stupid politicians that believe in scientific cooperation with aliens, etc.).
I think a nice and important thing is to fool player as to the alien strength throughout the game. I propose the following.
At the beginning aliens are surprised, weak and afraid, but try to make themselves appear more aggressive and powerful, hence Mumbai "incident" caused by "misunderstanding" and "human overreaction", just after aliens' proposal to take over and improve Earth power plants and space programmes was rejected. At this stage the player should think the aliens are fools to reveal their power and ruthlessness, but the player is a fool himself in believing in the brute force of the aliens.
In the mid-game the aliens are already considered a force to be reckoned with and quick to anger, but they are still preparing for the total war and still trying infiltration, etc. so they do not attack frontally and do not reveal the scale of their mounting power, just sampling and experimenting. At this point the player should begin to suspect he can defeat aliens on earth, that they are only bluffing, terrorizing, but they can be overcome.
Finally, the aliens launch the offensive and the player should be totally depressed and exactly at this point the hints at the endgame solution should become too obvious for him to miss.
ENDGAMEAt the end, after the preparations for war are finished, aliens are too powerful to beat on Earth, they bases flourish, nations drop out, so the player has to attack the alien fleet in the "good old Millennium-Falcon-Blows-Up-The-Death-Star-In-The-Nick-Of-Time last gambit" style.
AddendumBelow is an interesting subplot by BloodMagus that is generally compatible with my backstory. There can surely be alternative or additional subplots that can be woven in, but it is important to have the major parts done first.
"A rather ingenious plot I think would be for the Phalanx to start out as a private committee formed by an Anonymous Benefactor. The player is the committee's leader. You're given goals by the benefactor, and get bonuses for completing them. Eventually you'd be come a UN sanctioned organization. Before that, you'd have the option of convince countries to privately fund you.
You could work some interesting ideas, such as having the first goal of the phalanx being to PROVE the existence of aliens & UFO by capturing one. Having the UN explode in you face after finding out what you'd be doing, and my favourite:
The anonymous benefactor being some intelligent alien that the Antareans are after and we were setup to protect him (unbeknownst to us)."