As details emerge of the paranoid, Islamophobic rantings of Anders Breivik, it seems as though the Norwegian mass-killer thought a Muslim invasion of Europe was imminent. This is arrant nonsense, but the theory is not without its historical underpinnings. The Umayyads did conquer Spain, it is true, and Ottoman armies advanced through Eastern Europe and laid siege to Vienna several times. But centuries have passed since "Christendom" (now more appropriately known as Europe) faced such a threat. Time enough, one would have thought, for antique fears of marauding "Mussulmen" to have thoroughly dissipated, especially in far-off Scandinavia.
If it were only Breivik who obsessed about this history, it would be easy to paint this as a continuing emotional reaction to September 11, to the other acts of "Islamist terrorism" that have followed, and as part of the climate of hysteria these have engendered. But the truth is more worrying. Over a period of decades, a narrative has been growing in which Islam is presented as being, and having always been, voracious, repressive and monolithic.
And this narrative has been taken on board not only by far-right extremists, but by powerful figures in the European and American mainstream, politicians and commentators. Neo-Cons and those on the liberal left who supported the Iraq war redefined themselves as aggressive defenders of Enlightenment values and hardened their opposition to any system they deemed fell short of their liberal ideal - which, they believed, included almost any strand of Islam.
The work of the Egyptian Jewish writer Bat Ye'or has been particularly influential in this, not least in popularising terms such as "dhimmitude". By this account, Jews and Christians - the "dhimmis" - did not enjoy a protected status in Muslim empires at a time when European states were enthusiastically slaughtering their Jewish populations. Instead, Ye'or argued that they were subject to a "condition of inferiority, spoliation and humiliation".
Those who have taken up Ye'or's ideas suggest that Muslims today really want to impose their will on other countries where they are currently in the minority. Never mind that most are tolerant, community-minded citizens dedicated to improving themselves and the lot of their neighbours. The only real Islam, goes the line, is that of the most literalist and warlike interpreters. They have a plan: to turn Europe into "Eurabia ... a cultural and political appendage of the Arab/Muslim world ... fundamentally anti-Christian, anti-western, anti-American and anti-Semitic."
Now one might have thought that such an obviously one-sided, outlandish view could hardly be taken seriously. But the above quote, which appears as a summary at the end of Bat Ye'or's 2005 study Eurabia: the Euro-Arab Axis, is followed by garlands of praise, not only from the distinguished historians Sir Martin Gilbert and Niall Ferguson, but also the well-known American commentator Daniel Pipes.
Ye'or's analysis is regarded as perfectly reasonable by many in the West, as is the thesis that Muslim minorities in Europe will take over the continent by stealth, through immigration and the higher birthrates prevalent in their communities. That, too, one might think far-fetched; Muslims currently make up only around 4 per cent of the EU population. But no less an authority than the Princeton academic Bernard Lewis, described by Britain's Spectator as "our greatest living expert on Islam" when I interviewed him for the magazine last year, doesn't think so. "They are advancing, no doubt," he told me. "If present demographic trends continue, they will become the majority."
In his books Lewis, who is now 95, writes with both admiration and affection for Islam's past. About the present, however, he is startlingly alarmist. "Either we bring them freedom, or they destroy us," reads one sentence in his latest collection of essays. As an adviser to US administrations, from Jimmy Carter to George W Bush, his opinion has consequences.
These messages are loud, carry the respectability of academic advocates, and are attractively simple. More nuanced voices, such as that of the French Islamic scholar Olivier Roy, let alone those of the millions of moderate Muslims who deplore hatred and violence, struggle to be heard above the din. If you argue that co-existence is not only possible but also desirable, that there is much peoples of different faiths can learn from each other, and that Christians, Muslims and Jews should celebrate their common Abrahamic heritage, you are, as I have found, condemned as an appeaser.
The fear is constantly stoked and encouraged, and has led to absurdities like the Swiss ban on minaret construction (even though there are only four in the country) and the French and Belgian laws against the wearing of burqas and niqabs - which may well contravene human rights legislation.
It is a very big leap indeed to go from apocalyptic words to apocalyptic acts. Nevertheless, it should be noted that in his justifications, Breivik frequently quoted the writings of Bat Ye'or and her acolytes, even using the term "dhimmitude"; he was clearly familiar with their arguments. One cannot hold them responsible for the criminal behaviour of a man whom even his lawyer says is insane. One can, however, accuse them of having helped create an atmosphere of irrational suspicion, xenophobia and Islamophobia to which troubled minds may be all too susceptible.
Those who preach about the "dangers" of Islam warn of a clash of civilisations. Too often it appears that is exactly what they want.
Sholto Byrnes is a contributing editor of the New Statesman