So far, at least, neither the carrot nor the stick has worked. Politicians talk of tighter immigration controls. Yet the reality is that a Europe in demographic freefall needs more, not fewer, immigrants if it is to maintain its prosperity. Tony Blair just proposed new laws allowing the deportation of radical mullahs and the shutting of mosques and other sites associated with Islamic extremism. But given the sheer size of the Muslim population in England and throughout the rest of Europe, the security services are always going to be playing catch-up. Working together, and in a much more favorable political and security context, French and Spanish authorities have, after more than 20 years, been unable to put an end to the terrorism of the Basque separatist group ETA. And there are at least twice as many Muslims in France as there are Basques in Spain.

At the same time, it is difficult to see how the extremists' grievances can ever be placated by conciliatory gestures. It is doubtful that the British government's proposed ban on blasphemy against Islam and other religions will have a demonstrable effect. (What would have happened to Salman Rushdie had such a ban been in force when "The Satanic Verses" was published?) Meanwhile, the French government has tried to create an "official" state-sanctioned French Islam. This approach may be worth the effort, but the chances of success are uncertain. It will require the enthusiastic participation of an Islamic religious establishment whose influence over disaffected youth is unclear. What seems clearer is that European governments have very little time and nowhere near enough knowledge about which members of the Islamic community really are "preachers of hate" and which, however unpalatable their views, are part of the immigrant mainstream.

The multicultural fantasy in Europe -- its eclipse can be seen most poignantly in Holland, that most self-definedly liberal of all European countries -- was that, in due course, assuming that the proper resources were committed and benevolence deployed, Islamic and other immigrants would eventually become liberals. As it's said, they would come to "accept" the values of their new countries. It was never clear how this vision was supposed to coexist with multiculturalism's other main assumption, which was that group identity should be maintained. But by now that question is largely academic: the European vision of multiculturalism, in all its simultaneous good will and self-congratulation, is no longer sustainable. And most Europeans know it. What they don't know is what to do next. If the broad-brush anti-Muslim discourse of Jean-Marie Le Pen's National Front in France or the Vlaams Belang Party in Belgium entered the political mainstream, it would only turn the Islamic diaspora in Europe into the fifth column that, for the moment, it certainly is not. But Europeans can hardly accept an immigrant veto over their own mores, whether those mores involve women's rights or, for that matter, the right to blaspheme, which the Dutch filmmaker Theo van Gogh so bravely asserted -- and died for.

Figuring out how to prevent Europe's multicultural reality from becoming a war of all against all is the challenge that confronts the Continent. It makes all of Europe's other problems, from the economy to the euro to the sclerosis of social democracy, seem trivial by comparison. Unfortunately, unlike those challenges, this one is existential and urgent and has no obvious answer.

THE WAY WE LIVE NOW: 08-14-05 David Rieff, a contributing writer, is the author, most recently, of "At the Point of a Gun: Democratic Dreams and Armed Intervention."