Book VI, the longest book in On War, is entitled “Defense”. It is also the one which is most closely tied to the specific military practices of Clausewitz’s day. Hence, it contains the most material which has become outdated, and is usually skipped over by people who study and teach from On War in our era.
I decided to read it anyway. There is, in fact, a lot of valuable material in Book VI.
The chapter shows the amount of intense, hard thought that Clausewitz applied to the various types of defensive warfare, which must have been derived from both personal experience, discussion with other soldiers, and reading. In its day, portions of it could probably have been used as a manual for commanders who were going to be fighting on the strategic defensive, and for their opponents who were going to have to dislodge and defeat them. That function no longer pertains, due to greatly changed conditions. Therefore, much of Book VI can at best be suggestive in its relation to current practice.
It occurs to me that Chapter VI is similar to the kind of “critical analysis” that Clausewitz described in Book II. In Clausewitz’s usage, “critical analysis” is the process of taking a given body of historical facts and the “tracing of effects back to their causes.” This is followed by “investigation and evaluation of the means employed. This last is criticism proper, involving praise and censure”. While his analysis of defense is not based on a single historical example, it is certainly an analysis of effects and causes, and an evaluation of the means the defending commander can employ, and to some extent suggests grounds for praise and censure where known and knowable features have been missed or disregarded.
I will offer up some speculation.
The only way for Clausewitz to reach the penetrating level of analysis that he captured in Book I was to march through the nitty-gritty details like those in Book VI. Clausewitz was an inductive thinker. He did not start out with a theory. He looked at the mass of facts, tried to cull out the truth, and then derive a suite of principals, that overlapped, that offered guidance, that were generally sound based on historical experience, but which could not ever serve as a “cheat sheet” for military command. The process of writing Books VI was part of that mental priming, that immersion in the tangible reality. Book VI is a synthesis of lessons learned, arranged categorically, e.g. defense in mountains, fortresses, swamps, rather than by campaign or commander. In other words, his chapters in Book VI are perpendicular to the time line, not running along the time line. By taking a right angle cut at his total knowledge — personal, hearsay, and book-derived — he reshuffled his mental deck, and was thus able to see the deeper principles (e.g. friction, the tendency of war toward either stasis or toward absolute war, the trinity of forces at play in any war) more clearly.
In other words, while we may choose not to read Book VI, Clausewitz could not have arrived at the parts of On War we value more highly today if he had imposed on himself the discipline of writing it.
Reasonably enough, our current interests tend to be in the sort of wars we see happening today. Open war between great powers has been in decline for decades. But warfare at a lower level, guerilla war, or war amongst the people, whatever you may call it, is alive and well. Mars may have shrunk from his mountainous size, and his appetite for blood may have shrunk since the days of the Somme and Kursk and the forgotten struggles against the Japanese on the Chinese mainland. But even in his reduced state the bloody-handed God of War is vicious, sneaky and likely to do great harm if he is given the chance.
So, as we read Book VI we are continually struck by the few sections where Clausewitz talks about guerilla-type struggles and popular resistance, particularly Chapter 26, “The People in Arms”. I think a good short book could be written which simply took Clausewitz’s main principles, and then culled out the discussions of popular resistance, and then looked at a range of representative historical cases.
I noticed, to pick one example, in his section on fortresses, the observation that a fortress can be the “focal point of a general insurrection”, because it can be stocked with supplies, and be a refuge for the wounded and for the civil authority. In our current age of “radically lethal” firepower, fortresses can be obliterated by artillery or air power. But the exact same functions can be carried out by concealed fortresses, specifically, complexes of tunnels, which are hidden from sensors and to some degree immunized from firepower. The Vietnamese communists used large tunnel complexes, and al Qaeda in Afghanistan used tunnels as well. In a later chapter Clausewitz says “[i]n our opinion, a defensive position approaches the ideal the more its strength is masked, and the more it lends itself to taking the enemy by surprise in the course of the action.” This is truer than ever now.
Clausewitz’s Chapter 25 is entitled “Retreat to the Interior of the Country”. This was the defensive strategy employed by the Russians against Napoleon. Of course, one prerequisite for this is a country large enough to retreat into. Clausewitz mentions three other “favorable circumstances” for the defender: “(1) A sparsely cultivated area, (2) a loyal and warlike people, (3) severe weather conditions”. I note that Afghanistan fits these criteria pretty closely. In particular, the Pashtuns really are “warlike”, which is consistent with Clausewitz’s comment that “poor men, used to hard, strenuous work and privation, are generally more vigorous and more warlike.” This is still true.
Near the end of Chapter 25, Clausewitz makes a remarkable comment:
This discussion has been less an objective analysis than a groping for the truth. The reason is that this sort of warfare is not as yet very common; those who have been able to observe it for any length of time have not reported enough about it.
To the regret of the armies of Britain, France, Israel, Russia and the USA, among others, this sort of warfare became very common indeed in the Twentieth Century, and continues to evolve in the Twenty-first.