Although as a commander, you can see the entirety of the map at all times, details will be hidden from you. The buildings and units of other nations will be entirely invisible until you have spies or troops of your own in place to observe them.

All units have a visibility radius, within which they can perceive and identify other units or buildings. This is split between the inner and outer radius - the outer radius can only identify that an army (for example) is present, but not what that army is comprised of. Usually, this army’s icon will be greyed out and show a question mark.

The inner radius can show more detailed information - army compositions, specific structures in a province, and so on. This always requires the unit in question to be closer than the outer radius.

There are exceptions to these rules, of course. Some units have the Stealth capability and can remain hidden from regular armies even when in close visual range. To counter this, other units may have the ability to Detect Stealth, as long as the hidden unit is not a higher level than them.

Take advantage of scouting units with wider visibility, and be sure to guard your territory with units that can detect hidden armies, because it’s always the threat you don’t see that catches you out.