Bomb Bloke, on 11 October 2016 - 06:08 AM, said:
Just to expand on this:
An alien "mission" consists of UFOs of certain types being spawned into the world map according a time table. There's simply no way to foil an alien mission; it'll run to completion no matter what you do, but you can stall the arrival of the next UFO in the sequence by shooting down prior ones.
Whilst all alien missions run to completion, most of them can be foiled.
Alien Research and Alien Supply don't score, so there's nothing to foil.
Alien Abduction scores 50 points every time an Abductor takes off after having landed; shooting down the Abductors, or Ground Assaulting them, will prevent the aliens from scoring.
Alien Harvest scores 30 points every time a Harvester or Battleship takes off after having landed; shooting them down, or Ground Assaulting them, will prevent the aliens from scoring.
Alien Terror primarily scores via generating Terror Sites (1000 points if you don't attend, or 30x however many civilians they kill if you do); you can prevent the generation of Terror Sites by shooting down the Terror Ships. It also scores 10 points every time one of the scouts takes off after having landed; you can prevent that as well by shooting them down or Ground Assaulting them.
Alien Retaliation doesn't directly score points for the aliens, but it does harm you by destroying a base; you can prevent this by shooting down the scouts or mounting a successful Base Defence.
Alien Base and Alien Infiltration are the only ones that always
succeed, inevitably scoring 50 (base) or 150 (infiltration) points and generating an alien base.
Because missions can be delayed, it's quite possible to have multiple base-building missions complete within the same month (even if they started during different months). That's just if they're in different regions of the world, though - only one mission of each type can run within each area at a time.
Actually, for most missions, only one of that type can occur in a given area per game
. Every time a Research, Abduction, Harvest, Base, or Infiltration mission is performed in a given zone, that mission is permanently removed from the table of possible missions for that zone. If all of them have been performed, that zone is permanently removed from the table of possible zones the aliens can attack. That's why Infiltration missions repeat until all countries in the zone have been infiltrated; a truly new
Infiltration mission actually can't happen.
The exceptions are Alien Terror (which will always happen once per month in a random inhabited zone), Alien Retaliation (which triggers reactively in response to X-Com shooting down UFOs) and Alien Supply (which triggers at random from established alien bases).
(Yes, this means that if you play until 2003 and the aliens have performed every mission in every zone, they'll stop doing anything outside of Supply, Terror and Retaliation.)
Your fighter craft and transports have their own radars built-in, so don't be surprised if alien bases pop into view while you have them flying about. If you see a lot of UFO activity somewhere (or even if your graphs just tell you its there!), it often pays to have your ships patrol a few hours in that zone.
Worth noting that base detection only happens during "Patrol" - NOT when the craft are flying to a destination.
Normally, if one of your units is spotted during the alien's turn, or an alien is successfully but non-lethally attacked by one of your units, all aliens will know the location of that unit until a number of turns have passed (the amount of which varies depending on the "intelligence" stat of each alien, which goes up to eight at the most). Among other things, this allows them to fire psi-attacks at that unit whenever they like, regardless of LOS.
After turn 20, all of your units count as spotted all the time. At that point most aliens will start to make a beeline to your location - the ones which don't are typically either in a mad panic, or hiding around a corner so's they can get a reaction shot off when you go around it. Against Etherals it's often a good idea to try to mop things up before then.
Actually, that's not quite accurate. The aliens get a free look at all your units on turn 20, but they still only remember it for <Intelligence> rounds.