This year, thousands of teenage girls, very few past the age of consent, will arrive in Germany from Turkey for arranged marriages and lives of domestic servitude enforced by tradition, isolation and fear.

It’s a thriving one-way trade that has been going on for more than three decades, and it sits at the core of Europe’s greatest predicament today: the widening gulf between an increasingly postmodern society and its often premodern immigrants.

The subject of foreign brides broke wide in the German media last year, when a 28-year-old Turkish man took his 11-year-old wife to a registry office in Düsseldorf to get her an ID card. On that occasion, the girl was detained by the authorities and deported to Turkey. But according to the Turkish-born German sociologist Necla Kelek, that is more often the exception than the rule. Ms. Kelek, 48, is one to know: In two bestselling books, "The Foreign Bride" and "The Lost Sons," she has exposed Germans to the lives of their 2.6 million-strong Turkish community in a way few of her German-born peers would have dared.

This week, the German parliament is set to debate legislation, conceived by Ms. Kelek and supported by Chancellor Angela Merkel, that would require foreign brides (from outside the European Union) to learn German before their arrival and bar entry to those under 21. "The goal," says Ms. Kelek, "is to ensure that those who come are willing to integrate."

This isn’t just an academic or political issue for Ms. Kelek. It’s a telling fact that the most prominent Muslim critics of contemporary Muslim societies–Ayaan Hirsi Ali in Holland, Irshad Manji in Canada, Seyran Ates and Serap Cileli in Germany–are women. "It’s the women who have felt the relapse into Shariah the most," explains Ms. Kelek. "The boys might be slaves to their families, but on the streets they are free, and besides they can always look forward to a wife they can suppress. It’s the women who explode."

Ms. Kelek herself came to Germany as a child in the late 1960s, along with a family that, initially at least, sought to integrate into German society. She learned German, made German friends, respected what later would be called, controversially, the German Leitkultur, the "lead culture."

But things changed in the 1970s. Previous Turkish immigrants had generally come from cities and were relatively secular, but later arrivals were overwhelmingly from the countryside and traditional in their outlook. The rise of fundamentalist Islam also had an effect. Religion became the primary marker of individual identity. Codes of family honor and standards of female purity, to which Ms. Kelek’s family had once been relatively indifferent, became important.

When Ms. Kelek was 17, she locked herself in her room in a fit of adolescent rebellion. Her father knocked the door down with an ax. Instead of beating or killing her, he abandoned the family for good. It was, she says, one of the happiest days of her life: "We turned on all the lights and played music. We were free."

A similar scenario between a rebellious daughter and her Turkish father might work out differently these days. There have been 55 honor killings in Germany in the past six years. Most of the victims were "fallen" girls who had broken from their families and were living "like a German." Usually the perpetrator is a brother, acting at his father’s behest. The Turkish community tends to treat these young killers as heroes.

Such violence is integral to what Ms. Kelek calls the Turkish community’s "organized self-marginalization." The tender age of the foreign brides, for instance: That isn’t just a matter of depraved sexual tastes. "They want a girl with ‘closed eyes,’" Ms. Kelek explains. The younger the bride, the more likely she is to be submissive to her husband, dependent on his family, ignorant and terrified of the world outside.

Today, every second Turkish woman who has a child in a German school is herself a foreign bride. Two-thirds of these children arrive in school not speaking a word of German. The German educational system bends over backward for them, providing religious instruction in Turkish or Arabic and excluding girls from physical education, sex ed and other subjects where Islamic mores might be offended. The results have been dismal: 60% of Turkish children leave school without any kind of certificate. "The distance between Turkish youngsters and German ones increases every year," Ms. Kelek says.

The Turkish community is not the only party at fault, however. Until last year, few Turks, including those whose families had lived in Germany for generations, could obtain German citizenship. Successive German governments compensated for their refusal to facilitate citizenship procedures by allowing the Turkish community to do more or less as it pleased. Thus the 11-year-old bride: With a parent’s consent, Turkish law will allow even a 9-year-old girl to marry. Had German law applied, the age threshold would have been 16.

There’s a deeper problem here, though, which goes to the heart of modern Germany’s problematic notion of goodness. Germans, Ms. Kelek says, "want to do everything right that they previously did wrong. This is especially the case with the Muslim community because it’s such a different culture, such a different religion. Germans are trying to prove to themselves just how tolerant they are."

No surprise, then, that Ms. Kelek’s legislation is being hotly opposed by the Social Democrats and the Green Party. For too many self-described progressives, limitless tolerance of "the other" has replaced the defense of individual liberty as proof of virtue.

Ms. Kelek sees it differently. Europe, she says, "has to fight for its values," not least by putting some hard questions to its increasingly alien and belligerent Muslim communities: "’Why aren’t your women free? Why aren’t your children free?’ If we don’t ask those questions, this will only continue."