Scouts of various sizes will attempt to locate your base. If you shoot them down, they will be unable to do so. A battleship will appear at the end of this wave. If your base was detected, the Battleship will zoom in at top speed and immediately land, making it only possible to intercept with an Avenger, coming from head-on. If you managed to stop the scout's search efforts, the battleship will appear, fly around for a while, (as if challenging your interceptors to a fight) then fly off again into space.
Sometimes the very first UFO you encounter in a brand new game will be a Retaliation Scout. If it locates your base then you might be attacked almost immediately which is likely to be a tough mission.
Shooting down the retaliation scouts may prevent a base assault, but shooting them down close to your base almost guarantees an attack, presumably as they report your base if they come down close enough for your base to be in its radar range. Therefore, shoot down retaliation scouts with great care.
Aliens do not directly score for this mission. Losing a base will give you negative points depending on what was in the base.
Retaliation missions consist of the scout that looks for your base. When found, it is followed up by a Battleship dropship carrying a team of aliens that are to be inserted into your base. The assault team consists of aliens that you can find manning Battleships and Alien Bases. The Battleship vanishes once it has inserted the troops - presumably escaping into the atmosphere during the attack.
Each retaliation wave consists of a single group of aliens that are seeking to destroy your base. Each group, which can be of the same species, will independently target your base for retaliation. Once your base is located, they will continuously send Battleships at the base until one breaks through your defences. They will not stop until this happens.
Once they manage to drop a retaliation squad into your base the group sending the battleships will have a sudden case of amnesia and forget the base location. Why this is a bit of mystery, but what's important is that you will not have to worry about another attack until the retaliation scouts appear.
If you are being targeted by several groups of aliens at the same time, you will have to crush both (or all) of the assault squads before the aliens stop attacking your base.
First, each wave of retaliation ends when the retaliation squad is inserted into the base - then the base is forgotten.
Second, an impenetrable base is highly discouraged as it will never be left alone until the battleships are let through. As base defenses can either completely stop an attempted attack or effectively do nothing, this means that base defenses are of very limited value in most circumstances, except for the Mind Shield.
|X-COM: Enemy Unknown/UFO Defense|
|Alien Missions:||Alien Origins • The Martian Solution • Cydonia or Bust • Alien Research • Alien Harvest|
|Alien Abduction • Alien Infiltration • Alien Base • Alien Terror • Alien Retaliation • Alien Supply|