If the Xenonauts are performing well in shooting down UFOs, the Aliens will dispatch UFOs to locate and assault a Xenonaut base. The UFO will search a region before making a bee line towards a detected Xenonauts base. Searching UFOs will prioritise older Xenonaut bases over newer ones.
Fortunatly, individual UFOs will not share information on the location of a Xenonauts base. Meaning once a searching UFO has been destroyed, or a attack has been repulsed, no further Base Attack Missions will be launched until provoked.
Alien Base Attack Missions are available from the third month of the invasion.