This chapter examines strategic theory and how it provides a conceptual understanding of the nature of war. It begins with a discussion of the logic of strategy and how it applies not only in wartime, but also in peace. It then considers some of the most valuable concepts in strategic theory as articulated by Carl von Clausewitz in On War and compares them with Sun Tzu's ideas found in Art of War as well as in the military writings of Mao Tse-tung and jihadist writers. Clausewitz's views on war as a ‘paradoxical trinity’ — composed of violence, hatred, and enmity — and his understanding of the nature of a war, limited versus unlimited warfare, the rational calculus of war, and friction are explored. The chapter concludes with a commentary on the debate over whether classical strategic theory is obsolete.