In the contemporary era, after unlocking and completing all the missions to send satellites up, etc. All enemy combatants are shown on the map. During battle, however, the 'fog of war' rule is still active in several cases even though the enemy unit is clearly visible (due to Satellite coverage). When the unit is visible to the player, the player is unable to attack said unit until they bring a friendly unit within line-of-view (how it's done prior to unlocking/using satellites). Not fulfilling this results in the player being unable to attack but the enemy AI can still attack. Bombing (by support aircraft) still seems to work even if the enemy is not visible though.

Important note: it's not that the friendly units don't have a 'line of sight' to shoot, it's that the friendly units seem to not recognize there's an enemy there at all.