Slitherine

Couldn't find anything on this topic online.

Can somebody explain to me how the aquisition of new regions (specifically those unoccupied regions in the east) in the 550 BC scenario works for Persia?

They seem to conquer those regions automatically without any sufficient army.

Thanks and greetings to all of you FoG:E enthusiasts out there

Persia absorbs regions randomly from neighbours that are not stable, ie Elam, Media, Lydia, Babylon, Egypt(?)
I haven't seen anything about the unoccupied.