There are many who participate in the SWJ discussions more qualified than I to give the "correct" book answer, but in my view and losely paraphrasing Clausewitz the essential nature of war is that it is a large scale duel, where each dualing party uses force/violence to compel the other side to submit to their will.

I think you would find this hard to argue with, this is exactly what is happening in Afghanistan and Iraq, and in many other places in the world today. It is also what happened in the World Wars and any other conflict we choose to examine.

In addition to the dual nature, I think Clausewitz's trinity also falls under the nature of war. It consists of 1. primordial violence, hatred, and enmity, 2. the play of chance and probability, and 3. each opponent is subordinate to a rational policy. In the case of AQ they're subordinate to their interpretation of Islamic doctrine and their political aim of re-establishing the caliphate.

The character of war is the way in which it is waged, whether through terrorism, insurgency, maneuver warfare, nuclear warfare, a bombing campaign (think Kosovo from our view, think something very different from the Serb's view).

In summary, war is war, you can't argue it is a dual where each side uses force to compel the otherside to its will, and I don't think you can argue the trinity aspect that is present in all conflicts.

Every great theorist as allowed for significant differences in the character of war, no general officer would confuse an insurgency with a tank battle. This that the nature of war has changed is misleading and not supported intellectually.