What can we learn from the detailed exegesis of Carl von Clausewitz for the study of strategy? Based on a detailed reading of Clausewitz’ book On War, this paper proposes that Clausewitz’ reflections on strategy unfold along two parallel arguments. First, he explores the principal difficulties of a positive theory of strategy. This critical inquiry shows how quantities and qualities influence each other in war; how events emerge rather uncontrollably from the interplay of action and reaction; and how the fog of war puts a veil of uncertainty over all information. Clausewitz’s fundamental critique leads him to the conclusion that a normative theory of strategy is impossible.