Each unit has a vision range. In missions with Fog of War enabled, anything that falls beyond that vision range is obscured by the Fog of War and cannot be seen. Units cannot attack units that are beyond their vision range. Note that the A.I. (artificial intelligence) in the first three games is able to see the entire map except for woods and reefs but would ignore any units concealed in them unless it accidentally sighted the unit, crosses over it (in which the A.I. does get ambushed though it's not shown) or used Sonja's CO power. However, The A.I. is affected like the player in Dual Strike.
In Fog of War battles, a unit moving to a space outside its vision range may encounter a hidden enemy unit. When this happens, that unit is ambushed - it stops immediately and cannot accept any more commands until the next turn.
Vision through Fog of War is decreased by one space during rain. Vision is not affected by snow or sandstorms.
Hiding and Properties
In forests, reefs, mist, allied or neutral properties, and ruins, units are able to hide and avoid sight in Fog of War (or rain, depending on the game). Only allied units and units beside those terrains can see them.
Also, in Days of Ruin, two tiles can help for FoW missions: radars and fires. The radar gives to all allied units a +1 vision bonus whilst the fire shows non-hidding tiles in a 3 tiles radius.
Sonja - she has specialty in Fog of War. Her units have an extended vision range of one space. Her CO Power, Enhanced Vision, also increases Sonja's unit's vision range by another space and allows them to see directly through woods and reefs even without being adjacent to them.
Lin - in Days of Ruin/Dark Conflict, she is a Fog of War specialist (to a smaller scale compared to Sonja) . Her CO Power increases units vision by 2 and allows them to see into woods and reefs.
Recon - recons are a must have in Fog of War; they have an excellant vision (5 tiles) and a good mobility.
Flare - Exclusive to Days of Ruin, this unit is specifically made for Fog of War. Whilst they have a smaller vision range than the recon, they have the ability to fire a flare (range of 1 to 5 titles) to reveal what on the tiles around the target (even hidings) in the same pattern as a missile silo.
Infantry and Mechs - Place them on mountain tiles to get a vision bonus of 3 spaces to their vision range.