Fog of War is a special condition in which a dark fog envelops the battlefield.
Games with Fog of War enabled prevent units from seeing, as well as attacking, enemy units beyond their collective vision range.
In addition, moving units into tiles beyond vision range carries a risk of being "ambushed" by a hidden enemy unit occupying any tile in the movement path. Upon encountering the hidden enemy unit, the ambushed unit's actions are immediately interrupted at the first point of contact, and the unit is unable to move or complete any other actions for the remainder of the turn.
Woods and Reefs are always hidden from vision range, unless an allied unit is directly adjacent to these tiles. Mountains boost vision range of Infantry and Mechs by 2 when placed on them. Units on allied Properties are always visible, except for hidden Stealths and Subs.
Rainy weather reduces vision range by 1 while it's active.