Toward the end of its widely praised report, the Sept. 11 commission offers a prescriptive chapter titled "What to Do?" There, it makes an assertion that is genuinely shocking. It says that in our current conflict, "the enemy is not just 'terrorism,' some generic evil. This vagueness blurs the strategy. The catastrophic threat at this moment in history is more specific. It is the threat posed by Islamist terrorism [the report's emphasis] -- especially the al Qaeda network, its affiliates, and its ideology."

At a stroke, in other words, the members of the commission have tried to rewrite the terms of the global war on terrorism and turn it into a global war on Islamist terrorism alone.

It seems almost incredible that we could have been at war this long without defining precisely who or what we are at war with. But such is the case, and it has never seemed an urgent matter to lawmakers. When I appeared before a congressional subcommittee studying strategies for the war on terrorism in 2002 and suggested that the first step should be the promulgation of just such a uniform definition, the members were momentarily dumbstruck. To their credit, they soon recovered and we began to discuss the issue, but a comprehensive definition of terrorism for the use of the American government and the education of the American people never emerged. Now, however, the president and his supporters are apparently ready to instantly approve the radical definition set forward by the commission.

Terrorism, as defined by military historians, has been a constant, ugly feature of warfare, an aberrant tactic akin to slavery, piracy and genocide. One of the reasons that some of us argued throughout the 1990s for undertaking of genuine war on terrorism (involving the military in addition to intelligence and law enforcement) was the notion that we might finally declare the tactic -- like those other aberrant belligerent methods -- to be out of bounds, for the armed forces of civilized nations and non-state organizations alike.

It's true that both slavery and piracy are still practiced, but only in remote corners of the world; certainly genocide is still with us, but its employment is now cause for immediate sanction and forceful reaction (theoretically, at any rate) by the United Nations. Banning such tactics and actively stamping out their practice has been the work of some of the great political and military minds and leaders of the past two centuries. Now it is time -- past time, really -- for terrorism to take its place as a similarly proscribed and anachronistic practice.

But first we must agree on an internationally acceptable definition. Certainly terrorism must include the deliberate victimization of civilians for political purposes as a principal feature -- anything else would be a logical absurdity. And yet there are powerful voices, in this country and elsewhere, that argue against such a definition. They don't want to lose the weapon of terror -- and they don't want to admit to having used it in the past. Should the United States assent to such a specific definition of terrorism, for example, it would have to admit that its fire-bombings of German and Japanese cities during World War II represented effective terrorism. On the other hand, few Muslim nations want to go up against the power of organized terrorist groups by declaring them de jure as well as de facto outlaws.

In the intellectual arena, meanwhile, the fatuous logic that "one man's terrorist is another man's freedom fighter" keeps left-leaning intellectuals away from the cause of definition. And so its promulgation continues to elude the world, even as we have embarked on a war against the phenomenon itself.

The Sept. 11 commission evidently also came to feel, during its months of sitting, that defining terrorism was too thorny a problem to be undertaken in anything but a partial and temporary manner. Fighting wars against tactics, they announced -- fighting wars over the nature of war itself -- is simply too complicated. We need to fight specific wars about people, not general wars about ideas (the American Revolution, the Civil War and two world wars notwithstanding).

By this token, any and all intellectual or moral meaning is removed from our military undertakings in Afghanistan and Iraq, as well as from the global war on terrorism generally. What began as a war between modernism and medievalism, between progressive ideas of how to reform war and regressive notions of cataclysmic conflicts, will, if the commission's recommendations are fully implemented, become instead a "clash of civilizations" between extremist Western and extremist Muslim values: a simplistic, devastating confrontation. In a terribly ironic but real sense, the final hijacking of Sept. 11 will be the commandeering of the global war on terrorism itself.

What the commission fails to see is that the word "extremist" (or "Islamist") is not what will be heard on the "Arab street," or indeed much of anywhere else in the world, when the new enemy is proclaimed. George Bush initially reacted to the Sept. 11 attacks by calling for a "crusade" against terrorism, but many Muslims heard only one word, "crusade," and they heard it in its historical rather than its rhetorical sense. The West, that word implied, is coming again to take control of Muslim nations and holy places, just as it did after the turn of the last millennium. The president later apologized for his thoughtlessness, but the damage had been done.

And now, when the Sept. 11 commission says that terrorism is no longer the enemy, that Islamist extremism has assumed that role, most Muslims are going to hear the same sort of threatening, generalized message, one constantly repeated by Osama bin Laden: The Americans are not really concerned with terrorism -- in fact, they've practiced it throughout their history; what they are embarked on is a war against Islam itself.

The commission should immediately amend its report, and reassert, rather than deny, that we are indeed engaged in a global war against terrorism, whoever practices it. (They might also think to recommend that, at some point soon, the United States formally repudiate the deliberate victimization of civilians, something it has never done.) Then President Bush, Sen. John Kerry and all national leaders should support the change in message. The war on terrorism began not as a crusade about ideology but as a pragmatic war about war. It must remain such.

The writer is professor of military history at Bard College and the author, most recently, of "The Lessons of Terror: A History of Warfare Against Civilians."