The most ambitious of all eight books in On War, Book VI is more than triple the length of the other books – equally any three of them in sheer volume. In this book, entitled simply “Defense”, Clausewitz offers practical lessons for the 19th century warfighter: operations on a flank (with diagrams), defensive mountain warfare, entrenched positions, and – prescient of France’s Maginot Line of the early 20th century – the importance of a network of interlaced cordons to a nation’s security.
However, Clausewitz emphatically reasserts his attritionist bent. While defense consists of “waiting for the blow”, “prepared to parry” for the “preservation of one’s own state and the defeat of the enemy’s…”, he quickly puts defense in the context of war in general: the fundamental tenet of war, even in the defense, is the destruction of the enemy. Therefore, offensive actions in a defensive campaign receive the most attention: “A sudden powerful transition to the offensive – the flashing sword of vengeance – is the greatest moment for the defense.”
Clausewitz deftly grows the scale of his argument in Book VI, from the importance of concealment, the convergence of vectors in the attack, the importance of interior lines, to his culmination in the defense of a theater of operations. Throughout Book VI, he reminds us of the central role of the population: noting the “collective influence” of a populace in Chapter 6, even beyond the preservation of “the integrity of each individual state” in favor of “the life of the system as a whole”, to Chapter 26 entitled “The People in Arms”.
As our host Lexington Green eloquently observed in his post on Book IV, Clausewitz observed a fundamental change in the nature of warfare that would persist in perpetuity: “…the conventional barriers have been swept away in our lifetime by the elemental violence of war.” Introducing the concept of a “center of gravity” (an idea he fully develops in Book VIII), he underscores the “unity in which a single center of gravity can be identified…” as “… the place where the decision should be reached: a victory at that point is in its fullest sense identical with the defense of the theater of operations.”