The aim of this paper is to understand the nature of Clausewitz’s philosophical thinking in On War and its two-fold relation to the catastrophic event of the Napoleonic Wars and to Clausewitz’s own reformation of a theory of war. In distinguishing between a philosophical and a military register of thinking—between a reflection on the nature of war and a theory of war—this paper examines Clausewitz’s critique of 18th century military thought, the dialectical progression of this thinking in On War, and the “grammar” of war in its linkage of different definitions of war into a systematic whole. Against Raymond Aron’s proposal to sever the connection between the three substantial definitions of war in the evolution of Clausewitz’s thinking, this paper examines the conceptual progression of these three definitions within On War’s opening reflection on the nature of war. A closer examination of Chapter 1, Book 1, demonstrates the originality of Clausewitz’s philosophical manner of thinking through the essence, or nature, of war from its apparently “straightforward” definition of war as a duel to the Platonic resonance of its concluding image of war as a “paradoxical trinity.” In this manner, Chapter 1 establishes a philosophical space for Clausewitz’s development of a theory of war in the subsequent books of On War.