When Anders Behring Breivik targeted a main government building and a youth camp for the country’s Labor Party last week — two outposts of tolerance and multiculturalism — he forced Europeans to confront an unbearable question: Seventy years after the Holocaust, why do racial and religious extremists continue to haunt Europe? For the most part the problem is not anti-Semitism — although that exists, too, most markedly in France and pockets of Eastern Europe, and it is often tied to anti-Zionism. These days the targets are more likely to be immigrants and their children: French North Africans, English Pakistanis and Caribbeans, German Turks and Armenians, Belgian Congolese, Dutch Indonesians, and yes, Norwegian Muslims. In fact, Breivik called for “a Christian war to defend Europe against the threat of Muslim domination.”

By history and conviction, Jews look with concern on intolerance toward any ethnic or religious minority, and we ask, why still? Why now?

On the surface, Western Europeans have built one of the most openly multicultural societies in the world. In reaction to the Holocaust and Soviet anti-Semitism, countries across the region enacted hate crime and hate speech legislation, protected religious freedoms, and, to a much larger extent than the United States, promoted the transmission of minority cultures.

Although post-war Germans debated whether to grant Turkish “guest workers” a path to citizenship, the legal system eventually extended citizenship to the children of the immigrants (although not to the immigrants themselves). The Council of Europe has enacted human rights laws that are far more extensive than those of the United Nations or the American Bill of Rights, and the laws are enforced by much stronger courts. Today, Protestants study the Talmud in Berlin, the muezzin is heard in Paris, and most pubs in London serve curry chips.

Yet despite the apparent openness, the story of multiculturalism in Europe continues to be written in blood. Unlike the United States, which was conceived from the start as a land of immigrants, European kingdoms were conceived as homogeneous lands of lineage. Familiar American images that celebrate the plural society — the melting pot, the salad bowl, the symphony, the quilt — have historically not been engrained in European consciousness. The reigning image there has been of a single tree with deep roots.

That image never fit the reality: religious minorities have always complicated claims to a unified national collective, and starting in the late-19th century a powerful new challenge arrived when the colonized peoples of the empires began migrating to the imperial centers. In every country, native-born citizens found that they now had thousands of new neighbors who brought home unfamiliar colonial languages and customs.

For decades there has been a so-called “monoculturalist” reaction to increasing diversity in many European countries. French law prohibits the public displays of religion, including Muslim women’s veils. When Ayaan Hirsi Ali, the Somali-Dutch writer and activist, was elected to the Dutch parliament, some politicians called her citizenship into question, and rather than submit herself to the kind of indignity President Obama has endured by the “birthers,” she resigned. Much of the xenophobia is driven by economic fears and the surge of nationalism that followed the disintegration of the Soviet bloc and the 9/11 terrorist attacks.

European Jews rebuilding after the Holocaust have looked on the debates regarding the status of minorities from former colonies with ambivalence. Sensitized by their own suffering, many have objected, often publically and vehemently, to discrimination against other groups. On the other hand, European Jews have largely risen to the middle classes in countries where anti-Semitism has dramatically decreased, and they see their futures bound up with the majority. In other words their sympathies sometimes lie between the natives and the immigrants. They see themselves both as insiders and outsiders. American Jews might notice some similarities to themselves.

Like their fellow Jews in Europe, American Jews must always fear those who preach hatred and violence: you never know when a fanatic might go off the rails, and Jews are all too visible as targets. We can be proud to live in a country that has so publically embraced its plural identity, where a Jewish poet’s words (“Give me your tired, your poor, your huddled masses yearning to be free”) are engraved on the Statue of Liberty.

But the news from Norway reminds us that we cannot afford to be complacent. Anti-immigrant legislation, Tea Party demagoguery and assassination are here, too. Norway has sent us a message that now is the right time to reach out to our neighbors, the ones who are different from us, and make common cause. That is not a sentiment but a call to action.

Michael Galchinsky is the co-editor, with David Biale and Susannah Heschel, of “Insider/Outsider: American Jews and Multiculturalism,” and the author of “Jews and Human Rights: Dancing at Three Weddings.” He teaches English and Jewish studies at Georgia State University in Atlanta.