Initially, the majority of the screen is covered with a gray fog The Fog of War that obscures the location of your enemy’s buildings and troops, but reveals terrain and city locations. If you move your armies into an area, the fog cover recedes, revealing the presence of enemies. Once your units leave an area, the area covers once again with gray fog. In some scenarios, the entire map may be covered with what is called black fog. This thick black fog fully blocks your view of the underlying terrain and cities. Once the black fog has been explored by a friendly regiment, it will not return. You can adjust your view in the gameplay screen by dragging your mouse cursor to the edge of the screen, and you can zoom in and out with either the mouse wheel or the [Page Up] and [Page Down] keys. To rotate the view, you can either press the middle mouse button and move the mouse, or use the number pad [4] and [6] keys.



Source: Axis and Allies Official Manual