|Carl von Clausewitz|
Der Krieg ist [...] ein Akt der Gewalt, um den Gegner zur Erfüllung unseres Willens zu zwingen.
("Vom Kriege", Carl von Clausewitz, 1832, Book I, Chapter I, §2)
It's not a perfect definition, of course (even though I have certainly used it many times before).
Let's attempt to improve on a classic:
Problem (1): The scale is not given, even though he wrote "unseres" ("our"). A clash of two pimps and their loons, would meet the definition as well.
Solution (1): Let's add "organised" in front of "violence"; that's not perfect, but at least short.
Problem (2): This is the real problem. CvC was writing as an officer of a great power of his time, his definition does not describe war well for small powers. Let's look at Finland in 1939/40, defending its freedom (independence) against Stalin's Soviet Union. Was the Finn's point to compel Stalin to do fulfil their will? That would have been somewhat over-optimistic, as what the wanted was most likely the status quo ante. The Finnish motivation was another one IMO.
Solution (2): Scratch "compel our opponent to fulfil our will" and replace it with "reach an acceptable outcome in a conflict."
"War is an act of organised violence to reach an acceptable outcome in a conflict".
Yes, the stroke of genius of pointing at the importance of will would be lost. I believe it's still a more accurate definition. It works for both aggressor and defender, superior and inferior power.
There's no hint at the importance of will any more, but another important hint; war has to end once an acceptable end can be reached. Greedy warmongers will probably have a very high threshold for acceptance, but for some others the alternative definition will point the way towards the minimum net damage.
The "in conflict" ending might furthermore provoke some thoughts about violent/non-violent conflict. Yes, non-violent ones would not be war, and assuming the others don't turn violent it's usually better to pursue an acceptable outcome the non-violent way. Some people need a reminder about this as well (I don't really believe it helps with these people).
Why did I write this?
Repeated reading of more or less accurate CvC quotes in English left an impression on me that people think too aggressively. Too many people who discuss war and peace issues have the state of mind of a bully who wants to subdue someone else with superior strength and force him to surrender. Yes, that includes to break his will, but it is not what war is about on the receiving end. It's not an understanding of war, but only of at most one side of the coin.
On top of that, many wars aren't even about compelling others, some are simply about eradicating others. The Afghanistan conflict (OEF) with its de facto intent of eradicating Taliban and AQ is just one example. It wasn't about the will of the Jews on Masada when the Romans besieged them or about the will of the Phoenicians of Tyre when Alexander besieged them. In both cases it was about establishing a reputation in order to compel a third, bigger party. The razing of Grozny might be such a case as well. The "acceptable outcome" definitions does fit in these cases (an end of the siege would have been the acceptable outcome for the besieged), while CvC's "compel" part does not fit.
Wars of (semi-)nomadic people as described in the bible or known from Asian steppe, African or Native American tribes don't fit well to CvC's definition. These wars were quite often simply about booty. The invader sometimes didn't care about the other's will or behaviour; he just grabbed booty, killed or maimed who resisted and then left. Sure, sometimes they (especially the Asiatic steppe tribes) aimed more at making tribute arrangements, but other times it was about no such thing.
Again, "acceptable outcome" fits well. You just need to keep in mind that the greediness of the people raises the threshold for what's acceptable.
Maybe people would discuss war with more self-restraint if they used my alternative definition of war. CvC's definition lends itself too easily to extreme war goals and is simply not comprehensive and not exclusive enough at the same time.