The six years since the 11 September 2001 attacks have changed the way western liberals talk about Islam. In the aftermath of the World Trade Center bombing, the focus was on jihadists, or criminals who cloaked themselves in Islam's mantle to wage a campaign against western global domination. But with Osama Bin Laden still not captured and with fears of terrorism changing western civilisation, from air travel to constitutional freedoms, the discourse on Islam has become increasingly diffuse. As Europe grapples with its evolving status as a region of immigrants, Muslim culture - and not just its perversion in Islamist ideology - has been cast as a threat.

A powerful weapon in Islam's arsenal is women's clothing. A charged symbol of the changing face of Europe, the veil has become as hotly debated a topic as terrorism. At times, the terror threat and the threat from veiled women are neatly braided together, as when a male terrorist suspect fled Britain in a burqa. Veil-bashing is suddenly socially acceptable among not merely tabloid-reading Little Englanders, but also metropolitan sophisticates. Centrist politicians such as Jack Straw voiced disquiet over its effects on the democracy. Salman Rushdie supported him in no uncertain terms: "Veils," he said, "suck."

But British debates about the veil pale next to those in France. Three times since 1989, debate has erupted over allowing girls to wear the hijab in French state schools, culminating in the 2004 ban on "conspicuous signs" of religious affiliation in schools. Though the ban applies as much to yarmulkes and crosses as headscarves, it was designed for Muslim hijabis. The law was passed after a polarising national debate in which the veil was pitted against the "values of the Republic". The then president, Jacques Chirac, described the veil as "a kind of aggression".

Why should a bit of cloth so threaten the French republic? That is the central question posed by a subtle new study, The Politics of the Veil, by the American historian Joan Wallach Scott. Many French commentators cast the debate about the veil as an issue about Muslims, Islam and integration. Scott, a distinguished historian at Princeton's Institute for Advanced Study, shows that it revealed rather more about the French themselves. It challenged France's notion of laïcité, or secularism, as well as its attitudes to sexuality and individualism. The veil debate was, writes Scott, "a way of insisting on the timeless superiority of French 'civilisation' in the face of a changing world". With its beloved 35-hour week and state benefits system threatened, its schools and its suburbs in crisis, its relevance as a global power a dimming memory, it is not surprising that France freighted the issue with hundreds of years of cultural baggage. As Professor Scott writes: "The preservation of a mythical notion of 'France' in its many aspects was a driving force in the affaire des foulards."

The veil debate drew on a history of racism burnished during the colonial era. The French justified their occupation of Algeria, which began in 1830 and ended in humiliation in 1962, as part of la mission civilisatrice - the project of bringing post-Enlightenment values to the Arabs. Civilising the natives meant separating them from Islam, which was widely cast as a cruel and irrational system, "at once a symptom of innate Arab perversity and the cause of it", writes Scott.

Such Islamophobia blinded even Alexis de Tocqueville, that peerless commentator on power and politics. "There are in the entire world few religions with such morbid consequences as that of Mohammed," he wrote in 1843. "To me it is the primary cause of the now visible decadence of the Islamic world." Such logic, observes Scott, points to the "paradox of the civilising mission", one that continues today: a "commitment to change and uplift could be confirmed only in juxtaposition to the permanent inferiority of those it claimed to be civilising".

For the 21st-century French intent on civilising the Muslims in their midst, civilisation meant sexual freedom, and Muslim women must be "liberated" from the veil to enjoy the same sexual freedoms as their Gallic sisters. The veil, commonly read by westerners and Muslim feminists alike as a way of regulating sexual freedom, not only challenged French notions of sexual liberation, it also showed up the limits of the fundamental equalities guaranteed to French citizens.

By covering their heads, Muslim girls were highlighting gender differences, which, as Scott shows, chipped away at the cornerstone of French egalitarianism. Banning the hijab from schools, French officials argued, would remove any sign of women's inequality from the classroom. And equality between the sexes, they said, was the first principle of the republic and a vital tenet of la laïcité. "According to republican political theory, citizens are abstract individuals, indistinguishable from one another." By marking out their difference from other French citizens with veils, Muslim women stick two fingers up at the cosy notions of liberty, equality and fraternity.