And so the burden of assimilation has fallen on what Leiken calls "marginal man" — the post-migrant Muslim of the second generation. This is why, in Europe, Muslim extremism is most likely to come not from migrants but from their children, like Mohammed Siddique Khan, ringleader of the 7/7 bombers.
Khan was suspended between two cultures: the tribal, clan-like folk Islam of his father with its imported imams from Mirpur and a cousin in Pakistan awaiting his hand in marriage; and the secular, modern fast-changing world around him. Neither tribal nor urban, traditional nor modern culture, offered Khan a sure footing and he ended up with a series of dead-end jobs. Radical Islam offered him the chance to revolt against both.
Whether you call this a clash of civilisations or of philosophies is beside the point, which is that it happened on ground that had become alien and shaky because the political establishment saw virtue in diversity even at the cost of national identity.
So the young Khan, who had sought out the in-crowd at school, became a Benefits Agency admin assistant and welfare worker, then turned from the inside towards the outside, and into the arms of the mosque at Finsbury Park. Originally set up by Saudi Arabia and opened by the Prince of Wales, it was far from a genteel cosmopolitan centre of study. It purveyed raw, rampant bigotry, from literature like Al-Wala' wa'l Bara which held Muslims to be a superior breed and urged enmity towards the disbelievers: "Do not take Jews and Christians for Friends. They are friends of one another and whoever of you takes them as friends is one of them."
The lost soul of post-migrant "marginal man" was back in with an in-crowd — albeit one that had come from the outside.
Saudi pietists and Afghan veterans also knocked on France's door. But in contrast to Britain's dalek-like command that diversity must be celebrated, the French said "non": for their civic nation is paramount and religious identity is allotted no space in the public sphere.
The French dream hasn't always measured up to the reality, as the riots in the banlieues showed. And yet it was not Islamism but nationalism that motivated France's post-migrant protest at their treatment by the police — and because they were being ignored. They were, as Leiken says, demanding to come under the drapeau tricolore — not the green banner of Islam, as so many commentators said at the time.
The recent shooting of three Jewish schoolchildren, a rabbi and three paratroopers in Toulouse is France's first mass casualty Islamist attack since 1995. It may not be the last, however. As in Britain, the Left has begun to play politics with Muslim identity in its quest for a new proletarian vanguard.
Leiken also fears for Germany, torn by two forces — nationalism and multiculturalism — and unable to sort out where its ugly ethnocentric past ends and where national prides survives.
But his main focus is Britain and the ingredients which have turned it into the centre of European jihad. All are woven with skill, precision and dispassion into a requiem for Britain's lost confidence about nationhood. One statistic stands out: a 2006 Pew Research Center Survey of Muslim and non-Muslim attitudes in Europe shows that where Muslim identity was most distinct and most widely tolerated, as in Britain, Muslims also felt most alienated. Or, as Leiken puts it, there was less conflict between Muslims and non-Muslims in the European country least tolerant of Muslim identity (France) than in highly tolerant multicultural Britain. Those seeking to blame MI5 for failing to stop 7/7 should probably look elsewhere.