What did Carl von Clausewitz mean by the fog of war?

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strategy Detail of the Bayeux Tapestry depicting an English soldier in combat with Norman cavalry during the Battle of Hastings, wool on linen, 11th-century; in the Bayeux Tapestry Museum, France.

In his seminal treatise On War, the Prussian general Carl von Clausewitz used the term "fog of war" to describe the profound uncertainty under which all military commanders and statesmen must operate.1

Clausewitz argued that war is a deeply unpredictable endeavor, characterized by a lack of reliable information and the constant presence of unanticipated events. Because of this, he maintained that leaders rarely possess sufficient data to base their decisions on certainty. This concept is central to his "pessimistic" view of strategy, which posits that error, muddle, and chance are the norms in armed conflict, rather than the rational, predictable outcomes sought by some theorists.1

The "fog of war" is closely related to another of Clausewitz’s core concepts, "friction," which refers to the tendency for any military plan—no matter how simple—to go awry when put into practice. Together, these ideas emphasize the inherent difficulty of controlling the chaotic nature of war.1