Like American universities and with similar results, much of Europe has been in the grip of a self-defeating multiculturalism in which liberal ideals are turned toward the illiberal ends of Islamic extremism. Yet many of the countries that embraced multiculturalism, such as Denmark, Holland, and Germany, are now in the midst of a reconsideration triggered in part by the violent Muslim reaction to the publication in the Danish press of cartoons depicting the Muslim prophet Muhammad. Wolfgang Schauble, the German interior minister, made it clear that he had no intention of caving into Islamist pressure, while Danish prime minister, Anders Fogh Rasmussen, insisted, “Freedom of expression is not an issue for negotiation.”

But because of its virtues as well as its vices, Britain – even in the wake of the July 7, 2005, suicide bombings carried out by homegrown jihadists – has been slow to judgment. No major British publication reprinted the cartoons, and Jack Straw, the British foreign minister, surrendered to the rioters’ veto when he attacked the publication of the cartoons as “disrespectful.” Now, the London journalist Melanie Phillips has written an impassioned and well-argued book, “Londonistan” (Encounter Books, 213 pages, $25.95), in the hopes of moving Britain to reconsideration.

Ms. Phillips asks why British authorities have been so slow to respond to the radical Islamists in their midst. Part of her answer is that the British security services were focused for so long on IRA terrorism that they were slow to react to the threat of Islamism. Stunningly, the MI5 office that dealt with radical Islam was disbanded in 1994. The British were similarly slow to recognize that their justly famed tradition of providing a haven for political exiles might be turned against them.

But there are more disturbing reasons, as well. Early on in her readable but chilling book, Ms. Phillips notes that “U.K.-based terrorists have carried out operations in Pakistan, Afghanistan, Kenya, Tanzania, Saudi Arabia, Iraq, Israel, Morocco, Russia, Spain, and the United States.” Richard Reid, “the shoe bomber,” is the product of London’s Finsbury Park mosque, while Ahmed Omar Saeed Sheik, the planner behind the beheading of journalist Daniel Pearl, comes out of a British university education.The French, American, German, and Israeli governments urged Downing Street to act, but it refused, according to Ms. Phillips, on the basis of a “gentleman’s understanding” that, in return for sanctuary, the radicals would not turn their bombs on Britain When Reda Hussaine, an Algerian journalist, warned MI5 about the radical Finsbury Park mosque preparing young men for jihad, he was told that “we are giving these people a roof over their heads, food, free health care – and the security of Britain will be very safe.” With a few exceptions, such as the Labour Party’s David Blunkett, the British authorities, almost all secularists, couldn’t take the speeches about “flying the Islamic flag” over Downing Street seriously.

Worse yet, the police, having been singed by a series of racially charged scandals, proved themselves every bit as politically correct as any run-of-the-mill anti-Orientalist university professor who was sure that any criticism of Islam had to be based on Western racism and imperialism. In the aftermath of the Dutch filmmaker Theo van Gogh’s murder by an Islamist, the head of the London Metropolitan police, Ian Blair, took consolation in the fact that “there were lots of fundamentalists who didn’t shoot him …” Not even the attacks of July 7 moved the police. On the day of the first London transit bombing, a deputy police commissioner publicly argued, “As far as I am concerned, Islam and terrorists are two words that don’t go together.”

Part of the intellectual warrant for these claims came from the late Columbia University literature professor Edward Said. A hardline supporter of Palestinian Arab irredentism, Said couldn’t speak Arabic yet made himself the foremost interpreter of the Arab world for politically correct Westerners. The core principle of multiculturalism, as it emerged in 1960s America, was that not only white people but even white liberals did not and could not ever know anything about black people, who were wholly “other.” Said used the rhetoric of Michel Foucault to apply this argument to the Arab world. He famously insisted that “every European, in what he could say about the Orient, was a racist, an imperialist and almost” always wrong.

For Said the idea of the Arab-Islamic world, with its distinct culture and characteristics, was a creation of Western Orientalists who were, wittingly or not, the handmaidens of Western imperialism. The problem, as the respected scholar Robert Irwin notes in his new book,”For Lust of Knowing: The Orientalists and Their Enemies” (Allen Lane, 416 pages, $43.65), is that both parts of Said’s proposition can’t be true. How could the Orientalists have been instrumental in the success of Western imperialism if they provided their soldiers with a false map of the region?

Unlike many of Said’s earlier critics, Mr. Irwin does not write out of sympathy for Israel. He takes pains to make it clear that he shares Said’s political views. Rather he writes on behalf of the Orientalists, many of whom were deeply sympathetic to the Arab world and have thus, as he sees it, been defamed by Said.

In the introduction, Mr. Irwin, who generally writes with a balanced tone, describes Said as a “charlatan.” In the course of profiling the leading Orientalists of the West, he goes on to show in detail why Said has been “incoherent” and “slippery,” and an “absurd” purveyor of “fantasy.” Mr. Irwin points out that in his most famous book, “Orientalism,” Said insisted with “breathtaking ignorance” that Muslim armies conquered Turkey before they had subdued North Africa. Worse yet, in subsequent editions, Said never acknowledged any of his numerous errors of basic fact.

Said responded to his critics Stalinist-style by denouncing them as “racist,” “imperialists,” or Zionists while never addressing their specific charges. Yet Said became one of the most influential academics of the last 30 years. Mr. Irwin has trouble explaining why. The answer in part is that like George Sorel, who was an important influence on the development of fascism, Said dealt in myths that were congenial to the generation of 1968 that had entered the academy. Though they had been defeated in politics, Said’s arguments about literature as an essential tool of imperialism transformed them (at least in their own eyes), Walter Mitty-like, from inconsequential scribblers to players on the front line of the fight against imperialism. Like the Stalinists of the 1930s, they became apologists for terror in the name of the highest principles.

In part because of Said’s influence, most professors of Middle Eastern studies, doing penance for Western imperialism, insisted before (and many after) September 11, 2001, that there was no connection whatever between Islam and terrorism. But while the damage done by Said’s version of multiculturalism has been merely to marginalize academia further, what Ms. Phillips writes about is far, far more consequential. Noting that the September 11, 2001, hijackers were radicalized in Germany and England, a State Department official testifying before the Senate Foreign Relations Committee warned recently that Europe’s failure to integrate its Muslim population is becoming a security threat to the United States. As Ms. Phillips notes in her timely book, this means the alliance in England between multiculturalism and radical Islam poses a threat to the “special relationship” between the United States and Britain.

Mr. Siegel is a professor at the Cooper Union for Science and Art in New York and a senior fellow at the Progressive Policy Institute in Washington.