Last week, I wrote a cover story in The New Republic arguing that the struggle against Islamist totalitarianism should define contemporary liberalism, as the struggle against Soviet totalitarianism defined liberalism during the early cold war ("A Fighting Faith," December 13). This week, I waded through responses--some supportive, some critical, some both. The most surprising came from Kevin Drum, who writes the blog Political Animal at washingtonmonthly.com. Drum said I never proved my key point--that I never explained why totalitarian Islam is so grave a threat that liberals should make it central to their worldview. He urged me to write another article, a "prequel."

In a way, the response confirmed my theory: that many contemporary liberals, including many smart ones, don't see defeating Al Qaeda as a paramount national challenge. And that's a political problem, since most Americans do. Throughout 2004, Americans consistently named terrorism as one of their top concerns and generally felt that President Bush could handle it better than John Kerry. And even those polls, I suspect, understate terrorism's political resonance--since, in both 2002 and 2004, roughly as many voters cited domestic issues, such as the economy, where Democrats enjoyed a big lead. And yet, in both elections, Republicans exceeded expectations.

But just because many Americans consider Al Qaeda a grave threat doesn't mean it is. Drum argues that, "compared to fascism and communism, Islamic totalitarianism seems like pretty thin beer to many. It's not fundamentally expansionist, and its power to kill people isn't even remotely in the same league."

There are lots of problems with this statement. For starters, you don't have to believe Al Qaeda is as grave a threat as the ussr to believe it is the greatest threat to U.S. security and liberal values today. In 1954, segregation was probably the greatest domestic threat to liberal values. But that doesn't mean it was as great a threat as slavery.

If Islamist totalitarianism is less of a threat than Soviet totalitarianism, however, it is far graver than Drum suggests. To say that Al Qaeda's ideology is "not fundamentally expansionist" is wrong. It is true that, unlike communism, which aspired to guide every nation on Earth, totalitarian Islam only recommends itself to the Middle East. But, in Al Qaeda's definition, the Middle East includes every country with a substantial Muslim population or once under Muslim rule. In other words, it extends from Southeast Asia to Europe. In the mid-'90s, Al Qaeda went to war in Bosnia. In February 2003, Osama bin Laden cited Pakistan and Nigeria as ripe for Islamist takeover--although both are far from the Arab world, and Nigeria is only 50 percent Muslim. As University of Michigan professor and Informed Comment blogger Juan Cole has pointed out, bin Laden envisions a unified Muslim state, ruled, as it was in the heyday of Islamic power, by a caliphate. And, as bin Laden pointedly reminded listeners early this year, that caliphate includes "Al Andalus"--Spain.

Is Al Qaeda likely to take power in all the places bin Laden desires? Of course not. But it could do far less and still send the United States into deep crisis. By many accounts, Al Qaeda has long enjoyed substantial support in the Pakistani security services. Were Islamist fanatics to assassinate Pervez Musharraf and ally themselves with the general who succeeded him--in an echo of the military-Islamist alliance that ruled Sudan from 1989 to 1999--Al Qaeda would be within striking distance of a nuclear bomb. Or take Saudi Arabia, where bin Laden is wildly popular. If bin Laden, or his local associates, took control of the Saudi oil supply, the U.S. economy would plunge into depression.

But, even if Al Qaeda never seizes a single government, it still poses a grave threat. By suggesting that Islamist totalitarianism is "thin beer" because, unlike Soviet totalitarianism, it doesn't control states, Drum ironically echoes the neocons, who are so mired in a cold war mindset that they can't grasp terrorism except as an extension of state power.

Drum suggests that Al Qaeda's "power to kill people isn't even remotely in the same league" as the ussr's. But, if you're talking about killing Americans--which Drum is--the fact that Al Qaeda controls no territory makes it more dangerous, as well as less. Yes, the ussr, with its massive nuclear arsenal, had the power to kill more Americans. But, as a government interested in self-preservation, it was also deterred by the threat of U.S. retaliation. And that threat made the ussr cautious about taking American lives. As September 11 showed, Osama bin Laden is not cautious. The prospect of U.S. retaliation does not faze him--in fact, he welcomes it in the hope that it will spawn more Muslim anger and more recruits.

So, while bin Laden's capacity to kill Americans is clearly inferior to the ussr's, for Al Qaeda--unlike the Kremlin--capacity is the only limiting factor. And the spread of technical knowledge and materials makes the destructive capacity of a small terrorist band far greater than it was even a few years ago. This February, then-CIA Director George Tenet noted that Al Qaeda documents discussed how to assemble and use a chemical weapon or a dirty bomb. According to the 9/11 Commission, Al Qaeda has been trying to acquire a nuclear device for ten years-- with the United States "a prime target." No one knows how close they are. But, in his book Imperial Hubris, Michael Scheuer, former chief of the CIA's bin Laden unit, writes, "No one should be surprised when bin Laden and Al Qaeda detonate a weapon of mass destruction in the United States."

If Scheuer's prediction comes true, the consequences for individual rights will be terrifying. Which is to say this: The fight for national security is the fight for liberal values, not merely in the Muslim world, where fanaticism has already blighted countless lives, but also at home, where threats to American safety almost inevitably spawn threats to American freedom. Totalitarian Islam has already damaged both, and unless defeated, the damage could be exponentially worse. What more do liberals need to know before they make this fight their own?