In the 52nd chapter of his Decline and Fall of the Roman Empire, Edward Gibbon posed one of the great counterfactual questions of history: If the French had failed to defeat an invading Muslim army at the Battle of Poitiers in AD 732, would all of Western Europe have succumbed to Islam?

“Perhaps,” speculated Gibbon with his inimitable irony, “the interpretation of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mahomet.”

When those words were published in 1788, the idea of a Muslim Oxford could scarcely have seemed more fanciful. The last Muslim forces had been driven from Spain in 1492; the Ottoman advance through Eastern Europe had been decisively halted at the gates of Vienna in 1683.

Today, however, the idea seems somewhat less risible. The French historian Alain Besancon is one of a number of European intellectuals who detect a significant threat to the continent’s traditional Christian culture. The Egyptian-born writer Bat Yeor has for some years referred to the rise of a new “Eurabia” that is hostile in equal measure to the United States and Israel. Two years ago, Pat Buchanan published an apocalyptic book, titled The Death of the West, prophesying that declining European fertility and immigration from Muslim countries could turn “the cradle of Western civilization” into “its grave.”

Such Spenglerian talk has gained credibility since 9/11. The 3/11 bombings in Madrid confirm that terrorists sympathetic to Osama bin Laden continue to operate with comparative freedom in European cities. Some American commentators suspect Europeans of wanting to appease radical Islam. Others detect in sporadic manifestations of anti-Semitism a sinister conjunction of old fascism and new fundamentalism.

Most European Muslims are, of course, law-abiding citizens with little sympathy for terrorist attacks on European cities. Moreover, they are drawn from a wide range of countries and Islamic traditions, few of them close to Arabian Wahhabism. Nevertheless, there is no question that the continent is experiencing fundamental demographic and cultural changes whose long-term consequences no one can foresee.

To begin with, consider the extraordinary prospect of European demographic decline. A hundred years ago—when Europe’s surplus population was still crossing the oceans to populate America and Australasia—the countries that make up today’s European Union accounted for around 14 percent of the world’s population. Today that figure is down to around 6 percent, and by 2050, according to a United Nations forecast, it will be just over 4 percent. The decline is absolute as well as relative. Even allowing for immigration, the United Nations projects that the population of the current European Union members will fall by around 7.5 million over the next 45 years. There has not been such a sustained reduction in the European population since the Black Death of the fourteenth century. (By contrast, the United States population is projected to grow by 44 percent between 2000 and 2050.)

With the median age of Greeks, Italians, and Spaniards projected to exceed 50 by 2050—roughly one in three people will be 65 or over—the welfare states created in the wake of World War II plainly require drastic reform. Either today’s newborn Europeans will spend their working lives paying 75 percent tax rates or retirement and “free” health care will simply have to be abolished. Alternatively (or additionally), Europeans will have to tolerate more legal immigration.

But where will the new immigrants come from? It seems very likely that a high proportion will come from neighboring countries, and Europe’s fastest-growing neighbors today are predominantly if not wholly Muslim. A youthful Muslim society to the south and east of the Mediterranean is poised to colonize—the term is not too strong—a senescent Europe.

This prospect is all the more significant when considered alongside the decline of European Christianity. In the Netherlands, Britain, Germany, Sweden, and Denmark today, fewer than 1 in 10 people now attend church once a month or more. Some 52 percent of Norwegians and 55 percent of Swedes say that God does not matter to them at all. Although the social and sexual freedoms that matter to such societies are antithetical to Muslim fundamentalism, their religious tolerance leaves these societies weak in the face of fanaticism.

What the consequences of these changes will be is very difficult to say. A creeping Islamicization of a decadent Christendom is one conceivable result: While the old Europeans get even older and their religious faith weaker, the Muslim colonies within their cities get larger and more overt in their religious observance. A backlash against immigration by the economically Neanderthal Right is another: Aging electorates turn to demagogues who offer sealed borders without explaining who exactly is going to pay for the pensions and health care. Nor can we rule out the possibility of a happy fusion between rapidly secularized second-generation Muslims and their post-Christian neighbors. Indeed, we may conceivably end up with all three: situation 1 in France, situation 2 in Austria, and situation 3 in Britain.

Still, it is hard not to be reminded of Gibbon—especially now that his old university’s Center for Islamic Studies has almost completed work on its new premises. In addition to the traditional Oxford quadrangle, the building is expected to feature “a prayer hall with traditional dome and minaret tower.”

When I first glimpsed a model of that minaret, I confess, the phrase that sprang to mind was indeed “decline and fall.”

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