I've been writing a long piece (or a series) on the War on Terror, and it has me reading Carl von Clausewitz' vom Kreige ("On War") in detail, really for the first time. Clausewitz, along with Sunzi's Art of War (孙子兵法, "Sunzi's Military Rules"), really are the best places to go, I think, for a clear-eyed philosophical understanding of what war is and what it's about.
In the first book, first chapter, Clausewitz offers this definition:
Der Krieg ist also ein Akt der Gewalt, um den Gegner zur Erfüllung unseres Willens zu zwingen.
"War is thus an act of violence to bend the opponent to do our will." On the anniversary of 9/11, it's important to keep this in mind: it was an act of violence intended to bring the United States to heel. It was not a crime, it was not a terrorist incident or a human-caused disaster, it was an act of war.
It was a war then. It is still a war now.