"Fog of war" is the name given to the lack of knowledge that occurs during a war. Most importantly it refers to each side's uncertainty about the enemy's capabilities and plans, but it also includes the chaos that can occur in one's own forces, for instance when a unit misinterprets its orders, or takes a wrong turn and becomes lost.

The Prussian military analyst Karl von Clausewitz wrote: "The great uncertainty of all data in war is a peculiar difficulty, because all action must, to a certain extent, be planned in a mere twilight, which in addition not unfrequently like the effect of a fog or moonshine gives to things exaggerated dimensions and unnatural appearance."

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from http://www.free-definition.com/Fog-of-war.html
Carl Von Clausewitz, On War, eds. Michael Howard and Peter Paret, Princeton University Press, Princeton, New Jersey, 1976, p. 138.