Book six gives us Clausewitz’ theory of the defense. While he is particularly verbose in this book, Clausewitz lays out for us some timeless concepts that can and should be applied as the basis to any defensive strategy. First, Clausewitz gives us the purpose of the defense. Essentially it is to gain time for the commander to seek a battle that is more advantageous to him (p.370, 380). He makes it clear that the defense is merely a means to an end, a method of war, and not the end result in its self (p.392).
Clausewitz then goes on to give us an extremely detailed treatise on the use of various terrain types, man-made obstacles, and maneuver in order to properly carry out the strategic defense. There is a lot of material in this section, and I hesitate to attempt to summarize, especially given that some of the issues Clausewitz sites have been solved by technology and changes in the tactical application of forces over the years. The prevailing ideas, however seem to be these: Use the resources of the land to your advantage. Let it provide you cover and cause your enemy hardship, but don’t over-estimate its effect. Maneuver or take a stand in such a way as to delay the enemy and cause him the most hardship. This will either force him into battle on terms disadvantageous to him or cause his momentum to deteriorate. This feels like an oversimplification of a much more complex theory, but these two points will be found at the heart of any successful defensive action, including those that Clausewitz’ experience in the 19th century sphere of conflict could not have covered.
In chapter twenty-six, Clausewitz brings forth an idea that is relatively new in his time: that of a loyal armed populous in insurgency as a defensive measure. Clausewitz sees this as a means of support for the main army in defense. While he dismisses the idea of using irregulars in battle proper (p.480), Clausewitz envisions them as fighting guerrilla campaigns against the support structure of the enemy, with only indirect support from the regular military (p.482). While he only discusses the idea briefly, this is an area where Clausewitz may have been on to something bigger than he realized. The 2nd amendment of our constitution carries a similar theme. The resistance forces in Nazi occupied states during WWII carried out operations very similar to what Clausewitz suggests. It could be argued that elements of the Viet Cong also filled this role. Certainly today, insurgency is almost exclusively the form of “defense” that our military encounters, though it could be argued that this form of asymmetric warfare falls outside the defensive value-add that Clausewitz initially intended.