In his fourth book of On War, Clausewitz “ … now turn[s] to the essential military activity, fighting.” Following the same logical construct as Books I and II, where he first defines something then elaborates on its nature, Clausewitz gives us a description of “the nature of battle today” before providing general truisms of “the engagement”.
It is in Book IV that we see how dated some portions of On War have become. For instance, when describing “the nature of battle ‘today’” (i.e., in the early 19th century, shortly after the Congress of Vienna concluded the Napoleonic Wars), Clausewitz opines that “[d]arkness brings it to a halt: no one can see, and no one cares to trust himself to chance.”
Although Clausewitz has occasional rejoinders to prevent us from “ … creating fashions and fragmentary systems through which theory became elevated far above everyday practice,” it is also in Book IV that he asserts that, “… in all the most important cases, the destruction of the enemy’s forces must be the main objective.”
While correctly asserting the importance of simplicity in a plan (since complex plans take time – time that the enemy has to launch a simpler attack that will “wreck the grand design”), Clausewitz shows some incomplete rationalizations regarding the nature of simplicity and the application of force. For instance, Clausewitz offers a seemingly half-hearted assertion, “It seems to us that this is proof enough of the superiority of the simple and direct over the complex.”
Clausewitz continues to emphasize the essential nature of attrition in war in Chapter Four: “… [I]f we have lost proportionately as many men in the process as he did, no trace of this so-called victory will show up in the final balance-sheet of the campaign. … Nothing remains, therefore, but the direct profit gained in the process of destruction.” Physical destruction, in Clausewitz’s mind, was the only lasting gain – “ … the only advantage that will be permanently his.” Morale will recover, materiel will be rebuilt, courage will return. And here we see the crux of his disagreement with Sun Tzu.
While Clausewitz clearly favors simple approaches aimed at the destruction of the enemy’s forces, it does not mean he favors direct clashes – despite war being “nothing but mutual destruction.” Rather, he elaborates in Chapter Five on “The Significance of the Engagement”, offering a matrix perspective of both “offensive engagement” and “defensive engagement”.
The remainder of Book IV provides some specific parameters of the engagement: its duration, its decision, its use, the “Strategic Means of Exploiting Victory,” and the retreat after a lost battle. But the first four chapters make Clausewitz’s point quite clear: simple, direct plans aimed at the destruction of the enemy’s forces are paramount to victory.