Unsurpassed classic of its field: On War is essential reading for the professional military and for historians, and is of great value to those with an interest in public policy.
That said, it is not easy to read. There are three primary reasons for this:
First, it is unfinished. The first chapter (book as Clausewitz called it) is sharp, well-organized and focused, other chapters are so-so, and still others are almost formless collections of notes.
Second, Clausewitz is thinking philosophically. Most people, including many or most in his target audience, are unaccustomed to thinking this way, and find it difficult to re-orient themselves.
Third, parts of it are firmly locked in a particular time and place. The reader must work to determine what (if any) lessons in those parts are of enduring value and must understand references that, however clear they would have been to his contemporaries, are today obscure.
So, given all of the above, it is fair for the reader to ask why he should bother. The reason is the power of Clausewitzs answers to:
(1) What is the nature of war itself?
(2) What is wars relation to the larger world in which it exists?
(3) How can success in war be achieved?
Clausewitzs answer to question (1) is that war in itself is a duel on a large scale, which unless acted on from the outside, tends towards the maximum possible amount of violence. This discussion of pure war has probably been responsible for more mis-interpretations of Clausewitz than anything else he wrote. He is writing philosophically - trying to understand the nature of the thing, and some readers mis-read him as writing prescriptively - that because pure war (or ideal war) tends towards maximum violence, that those conducting war should employ maximum violence.