London

SIX years ago today, on July 7, 2005, Islamist suicide bombers attacked London’s transit system. They blew up three subway trains and a bus, killing 52 people and leaving a nation groping for answers.

In one sense the meaning of 7/7 is as clear to Britons as that of 9/11 is to Americans. It was a savage, brutal attack intended to sow mayhem and terror. Yet whereas 9/11 was the work of a foreign terrorist group, 7/7 was the work of British citizens. The question that haunts London, but that Washington has so far barely had to face, is why four men brought up in Britain were gripped by such fanatic zeal for a murderous, medieval dogma.

British authorities have expended much effort in seeking to understand how the 7/7 terrorists acquired their perverted ideas and became “radicalized.” In the immediate wake of the attacks, much ink was spilled over the role of extremist preachers and radical mosques. More recently, the focus has shifted to universities as recruitment centers for terrorists.

But this obsession with radicalization misses the point. The real question is not how people like Mohammad Sidique Khan, the leader of the 7/7 bombers, came to be radicalized, but why so many young men, who by all accounts are intelligent, articulate and integrated, come to find this violent, reactionary ideology so attractive. To answer it, we need to look not at extremist preachers or university lecturers but also at public policy, and in particular the failed policy of multiculturalism.

Multiculturalism has become a fraught issue throughout Europe in recent years. A rancorous chorus of populist politicians, like Geert Wilders in the Netherlands and Jimmie Akesson in Sweden, have made major electoral gains by stoking fears about multiculturalism. Mainstream politicians have joined in, too. Prime Minister David Cameron of Britain and Chancellor Angela Merkel of Germany have recently made deeply critical speeches, and the Dutch government decided last month to dump a decades-old policy of multiculturalism.

The real target of much of this criticism, however, is not multiculturalism but immigration and immigrants — especially Muslims. Mr. Wilders, leader of the Freedom Party, the third largest in the Dutch Parliament, has campaigned for an end to all non-Western immigration, a ban on mosque building and the outlawing of the Koran. Mr. Akesson, whose far-right Sweden Democrats shocked the nation by winning 20 seats in last year’s parliamentary elections, denounces immigration as the biggest threat facing Sweden since World War II. Centrists have responded not by challenging such prejudice but by appropriating the right’s arguments in an effort to hold on to votes.

Part of the difficulty in thinking about multiculturalism is that it has come to have two meanings that are rarely distinguished. On one hand, it refers to a society made diverse by mass immigration, and on the other to the policies governments employ to manage such diversity. The failure to distinguish between these meanings has made it easier to use attacks on multiculturalism as a means of blaming minorities for the failure of government policy.

Mass immigration has been a boon to Western Europe. It has brought great economic benefits and helped create societies that are less insular, more vibrant and more cosmopolitan. But the policies designed to manage immigration have been largely a disaster. To see why, one needs only to look at the experience of Britain and Germany. Both have adopted multicultural policies, though they have taken different paths. The consequences, however, have been similar.

Thirty years ago, Britain was a very different place than it is now. Racism was vicious, visceral and often fatal. “Paki bashing,” the pastime of hunting down and beating up Britons with brown skin, became a national sport in certain circles. I remember organizing patrols on the streets of East London during the 1980s to protect South Asian families from rampaging racist thugs. Workplace discrimination was endemic and police brutality frighteningly common. Anger at such treatment came to an explosive climax in the riots that rocked London, Liverpool, Birmingham, Bristol and other cities during the late 1970s and early 1980s. It was in response to this rage that Britain’s multicultural policies emerged.

The British government developed a new political framework for engaging with minority groups. Britain was now in effect divided into a number of ethnic boxes — Muslim, Sikh, Hindu, African, Caribbean and so on. The claims of minorities upon society were defined less by the social and political needs of individuals than by the box to which they belonged. Political power and financial resources were distributed by ethnicity.

The new policy did not empower individuals; instead, it enhanced the authority of so-called community leaders, often the most conservative voices, who owed their positions and influence largely to their relationship with the state. In 1997, the Islamist groups that had led the campaign against Salman Rushdie’s “Satanic Verses” during the 1980s helped set up the Muslim Council of Britain. Its first general secretary, Iqbal Sacranie, had once declared death “too easy” for Mr. Rushdie. Polls showed that fewer than 10 percent of British Muslims believed that the council represented their views, yet for more than a decade the British government treated it as their official representative.

Politicians effectively abandoned their responsibility to engage directly with minorities, subcontracting it out to often reactionary “leaders.”

If the prime minister wanted to get a message to the “Muslim community,” he called in the council or visited a mosque. Rather than appealing to Muslims as British citizens, politicians preferred to see them as people whose primary loyalty was to their faith and who could be politically engaged only by other Muslims. As a result religious — and Islamist — figures gained new legitimacy in their own neighborhoods and came to be seen by the wider society as the authentic voice of British Muslims.

More progressive movements became sidelined. Today “radical” in an Islamic context means someone who is a religious fundamentalist. Thirty years ago it meant the opposite: a secularist who challenged both racism in the streets and the power of the mosques. Secularism was once strong within Muslim communities, but it has been squeezed out by the new relationship between the state and religious leaders.

Many second-generation British Muslims now find themselves detached from both the religious traditions of their parents, which they often reject, and the wider secular society that insists on viewing them simply as Muslims. A few are drawn inevitably to extremist Islamist groups where they discover a sense of identity and of belonging. It is this that has made them open to radicalization.

A similar process has taken place in Germany. Postwar immigrants, primarily from Turkey, came not as potential citizens, but as “gastarbeiter,” or guest workers, who were expected to eventually return to their native countries. Over time, immigrants became transformed from a temporary necessity to a permanent presence, partly because Germany continued relying on their labor, and partly because they — and especially their children — came to see Germany as home.

The German state, however, continued to view them as outsiders and to refuse them citizenship. Unlike the practices in Britain, France or the United States, German citizenship is based on blood, not soil: it is granted automatically only to children born of German parents. Germany has nearly four million people of Turkish origin today, many of them born there, but fewer than 25 percent have managed to become citizens. Instead, multiculturalism became the German answer to the “Turkish problem.”

In place of citizenship and a genuine status in society, the state “allowed” immigrants to keep their own culture, language and lifestyles. One consequence was the creation of parallel communities. Without any incentive to participate in the national community, many Turks became dangerously inward-looking. Today, almost a third of Turkish adults in Germany regularly attend mosque, a higher rate than elsewhere in Western Europe and higher than in many parts of Turkey. The increasing isolation of second-generation German Turks has made some more open to radical Islamism. The uncovering last year of German jihadis fighting in Afghanistan should therefore have come as no surprise.

In Britain, the promotion of multicultural policies led to the de facto treatment of individuals from minority groups not as citizens but simply as members of particular ethnic units. In Germany, the formal denial of citizenship to immigrants led to the policy of multiculturalism. In both cases this has resulted in the creation of fragmented societies, the scapegoating of immigrants and the rise of both populist and Islamist rhetoric.

IN neither Britain nor Germany did multiculturalism create militant Islam, but in both it helped clear a space for it among Muslims. The challenge facing Europe today, therefore, is how to reject multiculturalism as a political policy while embracing the diversity that immigration brings. No country has yet succeeded in doing so.

In principle, the French assimilationist resolve to treat everyone as a citizen, not simply as an inhabitant of a particular ethnic box, is welcome. Yet as evidenced by police brutality against North African youth and the state ban on burqas, France continues to tolerate, and even encourage, policies that polarize society in the name of colorblindness. And although the relationship between Muslims and the state is healthier in America than in most European countries, the furor over a proposal to build an Islamic center and mosque near ground zero in New York reveals that the same fears and problems that haunt Europe exist in the United States.

There is no off-the-shelf solution. But the anniversary of 7/7 should remind us of how much is at stake in finding one.