Cohen was criticized from all sides in the days after the murder, but a year later he was being widely praised for his handling of the crisis. Time magazine named Cohen a “European hero” and dubbed him a “hate buster.” In 2006 he was runner-up in a contest for the “World Mayor” award. (The winner was the mayor of Melbourne.)

Nevertheless, the murder continued to shake Dutch society, precisely because it kept the question of national identity — “Who are we as a people?” — in the forefront. When, in 2007, Wilders called for the Koran to be banned in the country, it caused outrage, but at the same time his underlying message — that, as he has declared, “we are heading for the end of European and Dutch civilization as we know it” — reflected a widespread feeling that the country’s way of handling immigration was a disaster.

That way, in a word, was multiculturalism, the reigning dogma of the Dutch left in the 1980s and 1990s. By its logic, the government, far from insisting that newcomers integrate, actually provided money so that immigrant communities could keep up the traditions and language of their homelands, maintaining little Moroccos and Turkeys within the Dutch borders, largely disconnected from the wider society. If multiculturalism had failed, did Wilders represent the only alternative?

In Amsterdam, Cohen kept to his own agenda, at some remove from the national debate. Management of his intensely multiethnic city in the post-9/11 period led him to alter his traditional Dutch liberalism. Immigrants, he held, needed to become part of society, and that included learning the language and respecting the laws, and appreciating what he considers the paramount Dutch value, freedom. Newcomers, he told me, should study a Dutch canon of important historical events and figures. Cohen’s idea seems to have been to move away from the multicultural extreme while also avoiding the anti-immigrant extreme, to fashion a practical inclusiveness. He has repeatedly said that “Keeping Things Together” was his motto for governing the city. “I think he would be the first to say that keeping things together is more a strategy than a philosophy,” says Paul Scheffer, a Dutch sociologist who is one of the leading thinkers of the Labor Party but who has also been critical of Cohen. “What’s behind it is experience. He understands that the relationships between groups in our time are fragile. He has a modest ambition.”

At the same time, after the Van Gogh murder, Cohen realized he needed a guiding theory or doctrine. He sought out Jean Tillie, a professor at the Institute for Migration and Ethnic Studies at the University of Amsterdam, who had been influenced by the Harvard political scientist Robert Putnam, and in particular his understanding of social capital. Putnam had studied the breakdown of community and the increasing individual isolation that modern society engenders, most famously described in his 2000 book, “Bowling Alone.” In 2006, Tillie led a study of the city’s immigrant groups, focusing on what made certain Muslim communities turn toward a violent, radical philosophy, and he issued a report. For the first time, it quantified the threat: about 2 percent of Amsterdam’s Muslim population — 1,400 people in all — were potentially becoming radicalized. The report made certain recommendations. Cohen’s City Hall formalized an antiradicalization plan that has since been copied by several other European cities. “Maybe the most important result of the study was that it showed that radicalization results from social isolation,” Tillie says. “Therefore, building networks in the city among ethnic and religious organizations was an important recommendation.”

Social networks, in this reckoning, come in two types: weak and strong. Business relationships are examples of weak networks, in which people communicate with one another infrequently. An ethnic community in a neighborhood, by contrast, might constitute a strong network, in which participants see and exchange ideas with one another on a daily basis. It may be logical to think that such a strong, localized network would shun the wider society and so be a potential breeding ground for radicalization, so that an antiradicalization plan might seek to weaken it. But according to Tillie, the paradox is that if a strong network is given support, its members will become active participants in society. Thus came the most contentious feature of Cohen’s tenure as mayor of Amsterdam: his decision to give a sympathetic ear even to insular and orthodox Muslim communities. “Drinking tea in mosques” became a term of derision used by those convinced this was exactly the wrong tack. When, in March, Cohen announced his intention to take over his party — and in effect to go straight at Geert Wilders in the coming election — Wilders castigated him as “tea-drinking, multiculti-coddling Cohen.”

That criticism doesn’t seem to bother Cohen’s supporters. “Tea drinking was actually a policy recommendation,” Tillie told me. “We did not really write ‘drink tea,’ but we did say you should talk to people who think the system is not legitimate, and work together with religious organizations.” The city has given support to a variety of immigrant and community organizations, including conservative mosques, with the idea of working together to fight radicalization. As Cohen told me: “The approach is geared toward the individual. You hear that one of these kids has changed in the past few months. Is there someone who knows him, who can talk to him? You put questions: Why don’t you go to school? Should I look for a job for you? If you think life is better in Morocco, go and see for yourself.” Pieter Jan van Slooten, a policy adviser who works on the antiradicalization program, told me that his office now works hand in hand with mosques as well as schools and community groups to determine when young Muslims show signs of falling prey to radicalism.