Given current birth rates, it is not impossible that in 25 years France will have a Muslim majority. The consequences are dynamic: is it possible that secular France might become an Islamic state?

The situation is not dissimilar elsewhere in the EU. Europeans may at some young point in the 21st century have to decide whether they wish to retain the diluted but traditional Judaeo-Christian culture of their minority or have it replaced by the Islamic culture of the majority.

In theory, the cultural and legal assimilation of Europe's Muslims would be the ideal. This was supposed to be the notion behind the vision of the French interior minister, Nicolas Sarkozy, of a "French church of Islam" with homegrown imams.

But knowledgeable observers say his "moderate" Council of Muslims has made radical Islam the government-sanctioned norm for all Muslims.

For Islamists, assimilation is contamination since, in Professor Bernard Lewis's words, "Muslims must not sojourn in the land of the infidel". Intermarriage should be another route to assimilation, though in France this usually involves an Islamic male and often the wife converts to Islam.

Meanwhile, the state of Christendom in France is perilous. Catholics may not have reached the secular nirvana of the Church of England's working party that declared the Sunday Sabbath redundant, but French Catholicism, except for little pools of the faithful, is taken with the notion that their Church will be borne forward only if the next Pope is ready to "dialogue" with Islam - a code word that augurs dilution of the faith.

Currently, Islamists are only a fraction of France's Muslim population. In last week's demonstrations against the headscarf law, only 20,000 people turned out. But as in all radical movements, the young are the driving force. As their numbers increase, the militancy of Islam is likely to increase as well.

Europe's chickens are coming home to roost. The Great Powers used the Commonwealth or La Francophonie to continue the fiction of Empire. Large numbers of people were admitted mainly from North Africa.

The borders of mainland France seemed extended to include Algeria. Guest workers arrived to satisfy needs for cheap labour. Unloved by their host country, they were marginalised in shabby living conditions, with no attempt made to assimilate them. Political refugees and asylum seekers moved in.

Early arrivals, such as the White Russians or the Ayatollah Khomeini and his supporters, never intended to assimilate. They were sitting out bad weather before returning home. More recent ones, who arrived because of Nato policies in the Balkans, have been greeted with hostility and distrust.

European countries are not organically immigrant societies. The groups that went to America in the 18th, 19th and 20th centuries did so specifically to become Americans. They wanted to shed their past and, within a generation, they did. America's emphasis today on faith and God is just an echo of the founding Pilgrims for whom Christianity was central.

Their beliefs were reinforced by many Christian groups, from Baptists to Mennonites, all in search of religious freedom. These founding fathers decreed separation of church and state, not to make sure the nation was secular, as in France, but to make sure no state religion could interfere with religious freedom.

European countries have none of this melting-pot principle. You cannot become German or Italian with the same ease with which you become American. Also, into this very different European environment came a very different sort of immigrant - people who had no interest in assimilation at all.

They came as settlers, wanting to establish their own communities; at best they favoured a merger - at worst, a takeover. Their approach was nurtured by notions of multiculturalism, a creed appealing to intellectuals, administrators and enforcers, but having almost zero appeal to the home population.

The cultural abrasions that developed, especially between the rapidly growing Muslim community and the French, became the problem that could not be talked about. All respectable political parties, journalists and academics felt it too volatile and far too politically incorrect. The field was abandoned to extreme Right-wingers and nativists who, by default, established the unpleasant tone of the debate and became exclusive owners of a subject affecting the whole nation.

In the absence of openness, the government's response was a cover-up - or, rather, an uncovering: to outlaw Muslim headscarves, shave beards worn for reasons of faith, or ban crucifixes if too large. In Britain, some school Nativity plays were forbidden.

There seemed to be a genuine belief among governments that they could solve this problem by violating Western traditions of religious freedom and by outlawing their own cultural traditions. Far from alleviating the situation, this only aggravated it. Worse, it gave fodder to the extreme Right.

Tribal friction has only two solutions: groups will either unite in the manner of Normans and Saxons, melding into a society that may have different religious practices but subscribes to the same laws and values - in which case headscarves, beards and demographics don't matter a fig. Or they will follow the pattern of warring tribes throughout history.

The question is not whether French and Muslims can co-exist with each other so long as Muslim schoolgirls are bareheaded. Rather, it is the fundamental question of whether Muslim groups will become part of the French nation. This is not one of those old "querelles gauloises" that Barzini so loved. It is the fundamental dilemma of the new century.