What did Clausewitz mean by friction in war?

In his classic strategic treatise On War, the Prussian general Carl von Clausewitz used the term "friction" as a metaphor borrowed from mechanics to describe the inherent difficulty of executing military plans.21

Clausewitz argued that while military operations may appear simple on paper, they are incredibly difficult to carry out in reality. Friction represents the countless minor, unanticipated events—such as errors, muddle, uncertainty, and the enemy's own counteractions—that accumulate to slow down or frustrate even the most straightforward plans.21

Because war is a deeply uncertain endeavor, Clausewitz believed that friction is a constant, inescapable reality that distinguishes the actual conduct of war from theoretical models. It serves as a fundamental reason why strategy in war is rarely a predictable or rational process, as the "fog of war" and the nonlinear logic of combat ensure that things frequently go awry.21