But in the same three decades as Ulster's "Troubles," the hitherto moderate Muslim populations of south Asia were radicalized by a politicized form of Islam; previously formally un-Islamic societies such as Nigeria became semi-Islamist; and large Muslim populations settled in parts of Europe that had little or no experience of mass immigration.

On the Continent and elsewhere in the West, native populations are aging and fading and being supplanted remorselessly by a young Muslim demographic. Time for the obligatory "of courses": of course, not all Muslims are terrorists — though enough are hot for jihad to provide an impressive support network of mosques from Vienna to Stockholm to Toronto to Seattle. Of course, not all Muslims support terrorists — though enough of them share their basic objectives (the wish to live under Islamic law in Europe and North America) to function wittingly or otherwise as the "good cop" end of an Islamic good cop/bad cop routine. But, at the very minimum, this fast-moving demographic transformation provides a huge comfort zone for the jihad to move around in. And in a more profound way it rationalizes what would otherwise be the nuttiness of the terrorists' demands. An IRA man blows up a pub in defiance of democratic reality — because he knows that at the ballot box the Ulster Loyalists win the elections and the Irish Republicans lose. When a European jihadist blows something up, that's not in defiance of democratic reality but merely a portent of democratic reality to come. He's jumping the gun, but in every respect things are moving his way.

You may vaguely remember seeing some flaming cars on the evening news toward the end of 2005. Something going on in France, apparently. Something to do with — what's the word? — "youths." When I pointed out the media's strange reluctance to use the M-word vis-à-vis the rioting "youths," I received a ton of emails arguing there's no Islamist component, they're not the madrasa crowd, they may be Muslim but they're secular and Westernized and into drugs and rap and meaningless sex with no emotional commitment, and rioting and looting and torching and trashing, just like any normal healthy Western teenagers. These guys have economic concerns, it's the lack of jobs, it's conditions peculiar to France, etc. As one correspondent wrote, "You right-wing shit-for-brains think everything's about jihad."

Actually, I don't think everything's about jihad. But I do think, as I said, that a good 90 per cent of everything's about demography. Take that media characterization of those French rioters: "youths." What's the salient point about youths? They're youthful. Very few octogenarians want to go torching Renaults every night. It's not easy lobbing a Molotov cocktail into a police station and then hobbling back with your walker across the street before the searing heat of the explosion melts your hip replacement. Civil disobedience is a young man's game.

In June 2006, a 54-year-old Flemish train conductor called Guido Demoor got on the Number 23 bus in Antwerp to go to work. Six — what's that word again? — "youths" boarded the bus and commenced intimidating the other riders. There were some 40 passengers aboard. But the "youths" were youthful and the other passengers less so. Nonetheless, Mr. Demoor asked the lads to cut it out and so they turned on him, thumping and kicking him. Of those 40 other passengers, none intervened to help the man under attack. Instead, at the next stop, 30 of the 40 scrammed, leaving Mr. Demoor to be beaten to death. Three "youths" were arrested, and proved to be — quelle surprise! — of Moroccan origin. The ringleader escaped and, despite police assurances of complete confidentiality, of those 40 passengers only four came forward to speak to investigators. "You see what happens if you intervene," a fellow rail worker told the Belgian newspaper De Morgen. "If Guido had not opened his mouth he would still be alive."

No, he wouldn't. He would be as dead as those 40 passengers are, as the Belgian state is, keeping his head down, trying not to make eye contact, cowering behind his newspaper in the corner seat and hoping just to be left alone. What future in "their" country do Mr. Demoor's two children have? My mother and grandparents came from Sint-Niklaas, a town I remember well from many childhood visits. When we stayed with great-aunts and other relatives, the upstairs floors of the row houses had no bathrooms, just chamber pots. My sister and I were left to mooch around cobbled streets with our little cousin for hours on end, wandering aimlessly past smoke-wreathed bars and cafes, occasionally buying frites with mayonnaise. With hindsight it seemed as parochially Flemish as could be imagined. Not anymore. The week before Mr. Demoor was murdered in plain sight, bus drivers in Sint-Niklaas walked off the job to protest the thuggery of the — here it comes again — "youths." In little more than a generation, a town has been transformed.

Of the ethnic Belgian population, some 17 per cent are under 18 years old. Of the country's Turkish and Moroccan population, 35 per cent are under 18 years old. The "youths" get ever more numerous, the non-youths get older. To avoid the ruthless arithmetic posited by Benjamin Franklin, it is necessary for those "youths" to feel more Belgian. Is that likely? Colonel Gadhafi doesn't think so:

There are signs that Allah will grant Islam victory in Europe — without swords, without guns, without conquests. The fifty million Muslims of Europe will turn it into a Muslim continent within a few decades.

On Sept. 11, 2001, the American mainland was attacked for the first time since the War of 1812. The perpetrators were foreign — Saudis and Egyptians. Since 9/11, Europe has seen the London Tube bombings, the French riots, Dutch murders of nationalist politicians. The perpetrators are their own citizens — British subjects, citoyens de la République française. In Linz, Austria, Muslims are demanding that all female teachers, believers or infidels, wear head scarves in class. The Muslim Council of Britain wants Holocaust Day abolished because it focuses "only" on the Nazis' (alleged) Holocaust of the Jews and not the Israelis' ongoing Holocaust of the Palestinians.

How does the state react? In Seville, King Ferdinand III is no longer patron saint of the annual fiesta because his splendid record in fighting for Spanish independence from the Moors was felt to be insensitive to Muslims. In London, a judge agreed to the removal of Jews and Hindus from a trial jury because the Muslim defendant's counsel argued he couldn't get a fair verdict from them. The Church of England is considering removing St. George as the country's patron saint on the grounds that, according to various Anglican clergy, he's too "militaristic" and "offensive to Muslims." They wish to replace him with St. Alban, and replace St. George's cross on the revamped Union Flag, which would instead show St. Alban's cross as a thin yellow streak.

In a few years, as millions of Muslim teenagers are entering their voting booths, some European countries will not be living formally under sharia, but — as much as parts of Nigeria, they will have reached an accommodation with their radicalized Islamic compatriots, who like many intolerant types are expert at exploiting the "tolerance" of pluralist societies. In other Continental countries, things are likely to play out in more traditional fashion, though without a significantly different ending. Wherever one's sympathies lie on Islam's multiple battle fronts the fact is the jihad has held out a long time against very tough enemies. If you're not shy about taking on the Israelis and Russians, why wouldn't you fancy your chances against the Belgians and Spaniards?

"We're the ones who will change you," the Norwegian imam Mullah Krekar told the Oslo newspaper Dagbladet in 2006. "Just look at the development within Europe, where the number of Muslims is expanding like mosquitoes. Every Western woman in the EU is producing an average of 1.4 children. Every Muslim woman in the same countries is producing 3.5 children." As he summed it up: "Our way of thinking will prove more powerful than yours."