"My friends, brief as your sojourn in the Afghan kingdoms has been, you cannot have failed to notice that we are fighting, here, a different kind of war. You may feel, some of you, that this is not what you signed up for. These are not the fields of glory of which you dreamed. Understand: The actions we take in this campaign are as legitimate as those enacted in any other. This is not conventional warfare. It is unconventional. And we must fight it in an unconventional way.

"Here the foe will not meet us in pitched battle, as other armies have dueled us in the past, save under conditions of his own choosing. His word to us is worthless. He routinely violates truces; he betrays the peace. When we defeat him, he will not accept our dominion. He comes back again and again. He hates us with a passion whose depth is exceeded only by his patience and his capacity for suffering. His boys and old men, even his women, fight us as combatants. They do not do this openly, however, but instead present themselves as innocents, even as victims, seeking our aid. When we show compassion, they strike with stealth. You have all seen what they do to us when they take us alive.

"Please note, my friends, that I have made good and generous offers to the native peoples. I intend them no harm. I would make them our allies and friends. I abhor this kind of fighting. If an alternative existed, I would sieze it at once. But the foe will not have it. We have seen his methods. We have no choice but to adapt to them."

For two years I've been researching a book about Alexander the Great's counterguerrilla campaign in Afghanistan, 330-327 B.C. What has struck me most powerfully is that that war is a dead ringer for the ones we're fighting today -- even though Alexander was pre-Christian and his enemies were pre-Islamic.

In other words, the clash of East and West is at bottom not about religion. It's about two different ways of being in the world. Those ways haven't changed in 2,300 years. They are polar antagonists, incompatible and irreconcilable.

The West is modern and rational; its constituent unit is the nation. The East is ancient and visceral; its constituent unit is the tribe.

The tribe is the most ancient form of social organization. It arose from the hunter-gatherer clans of prehistory. A tribe is small. It consists of personal, face-to-face relationships, often of blood. A tribe is cohesive. Its structure is hierarchical. It has a leader and a rigid set of norms and customs that define each individual's role. Like a hunting band, the tribe knows who's the top dog and knows how to follow orders. What makes Islam so powerful in the world today is that its all-embracing discipline and order overlay the tribal mindset so perfectly. Islam delivers the certainty and security that the tribe used to. It permits the tribal way to survive and thrive in a post-tribal and supertribal world.

Am I knocking tribalism? Not at all. In many ways I think people are happier in a tribal universe. Consider the appeal of post-apocalyptic movies such as "The Road Warrior" or "The Day After Tomorrow." Modern life is tough. Who can fault us if now and then we entertain the idea of going back to the simple life?

The people we're fighting in Iraq and Afghanistan live that life 24/7/365, and they've been living it for the past 10,000 years. They like it. It's who they are. They're not going to change.