"All terrorists are politely reminded that this is London and whatever you do to us, we will drink tea and jolly well carry on. Thank you."
It was hard not to smile at messages such as this that appeared online in the wake of Khalid Masood's murderous rampage through Westminster. How ineffably British. The stiff upper lip. Keep calm and carry on.
Yet, I found myself increasingly uneasy as details of Masood's life began to come out. Adrian Elms was his real name. A former neighbour in Luton recalled a "polite, shy" and "quite portly man" who liked gardening and playing with his children. Then we read of the racism he suffered in "the quiet Sussex village of Northiam", where he was one of only two non-white male residents.
Wait. First, the guy was a violent criminal, who was jailed twice for knife attacks. Second, his path from crime to Islamist terrorism was a familiar one: the conversion to Islam probably in jail, the spell in Saudi Arabia, the relocation to Luton - the home town of several jailed extremists. Third, another familiar story: known to the authorities for "violent extremism", but no longer under surveillance.
London Mayor Sadiq Khan was right when he said last year that the threat of terrorist attacks is "part and parcel of living in a big city". But this is not the time to downplay what is happening in Britain.
Yes, I know, the victims of Islamist terrorism in Britain have been far fewer than the victims of Irish republican terrorism. And yet, as researcher Hannah Stuart shows in a meticulous new study for the Henry Jackson Society, there have been 135 terrorism-related cases since 1998 resulting in 264 convictions. The frequency of terrorism offences has roughly doubled since 2010.
As Ms Stuart shows, the perpetrators of terrorist offences are mostly male and "home-grown". Converts are disproportionately involved (they make up 16 per cent of offenders but fewer than 4 per cent of British Muslims as a whole). Nearly two-fifths of terrorism offenders have police records. And "lone wolf" attacks are growing more common.
Let's not be parochial. The world is in the grip of an epidemic of Islamist terrorism. Of the past 16 years, the worst was 2014, with 93 countries experiencing attacks and close to 33,000 people killed. The second-worst was 2015, with more than 29,000 victims. In that year, four radical Islamist groups were responsible for three-quarters of all deaths from terrorism: Islamic State in Iraq and Syria (ISIS), Boko Haram, Taleban and Al-Qaeda.
Although Muslim-majority countries suffer the most from Islamist terrorism, the West is increasingly under attack. There were 64 ISIS-affiliated attacks in Western countries in 2015, including the massacres in Paris (130 killed) and Orlando (49 killed). Thus far, Britain has got off lightly.
No doubt there will be whining about security lapses. But the constant vigilance of the British security services has prevented many more people from being killed in the past dozen years. In 2014 to 2015, there were more terrorism-related arrests in Britain than in any year since 2000. Even this intensified effort cannot pre-empt every terrorist.
Why not? The answer lies in the transformation of Islamism - I use the clumsy term to distinguish the political ideology from the religion - that we have seen since the killing of Osama bin Laden in May 2011.
ISIS was quite differently organised from Al-Qaeda. In the Middle East, it aspired to become a territorial state. But in the West, it created a kind of open-source network of Islamists, attracting the most ardent to come and join it in Mosul and Raqqa, and encouraging others to carry out indiscriminate attacks in Western cities.
The term "lone wolf" is a misleading one. No one becomes an Islamist all by himself just by watching beheading videos. As my wife, Ms Ayaan Hirsi Ali, argues in a powerful new report, jihad is always preceded by dawa - the process of non-violent but toxic radicalisation that transforms the petty criminal into a zealot.
The network of dawa takes many different forms. In the United Kingdom, a key role used to be played by the organisation Al-Muhajiroun (the Emigrants), which the jailed Islamist preacher Anjem Choudary led before his arrest. But there are many less visible organisations - Islamic centres with shadowy imams - busily spreading the mind poison.
To see how this poison works, read the recent Policy Exchange study of Britain's Muslim communities, Unsettled Belonging. At first sight, the news is good. Altogether, 90 per cent of those surveyed condemned terrorism. Most British Muslims, we read, have "fundamentally secular interests and priorities". Only 7 per cent said they did not feel a strong sense of belonging to the UK.
But read on. Nearly half said they did not want to "fully integrate with non-Muslims in all aspects of life", preferring some separation in "schooling and laws". Asked whether they would support the introduction of syariah, 43 per cent said yes. And one in 10 British Muslims opposes the prohibition of tutoring that "promotes extreme views or is deemed incompatible with fundamental British values".
Worst of all, nearly a third (31 per cent) of those surveyed believe that the American government was responsible for the 9/11 attacks. Get this: "More people claimed that the Jews were behind these attacks (7 per cent) than said it was the work of Al-Qaeda (4 per cent)."
After the July 7 attacks in London, the government's anti-terrorism strategy was designed to "Prevent" people from becoming terrorists or supporting terrorism. The Counter-Terrorism and Security Act 2015 even placed a duty on the police, prisons, local authorities, schools and universities to stop people from "being drawn into terrorism".
When she was home secretary, Mrs Theresa May vowed to "systematically confront and challenge extremist ideology". For this, she was denounced by the usual suspects, notably the Muslim Council of Britain, Hizb ut-Tahrir, and the Islamic Human Rights Commission. But the reality is that Prevent has not prevented enough.
The problem is that it is very hard to stop a network such as this from flourishing when it can operate even in jails. Figures published by the Ministry of Justice show the number of Muslims in prison (for all types of offences) more than doubled to 12,255 between 2004 and 2014. One in seven inmates in England and Wales is a Muslim. Guess what goes on inside. Clue: It is not like an episode of Porridge.
This problem is not going away. Ask the French. About 8 per cent of the French population is Muslim, which is roughly the proportion the Pew Research Centre projects it will be in Britain by 2030. The French authorities estimate that they have 11,400 radical Islamists. And about 60 per cent to 70 per cent of the French prison population is Muslim.
If you have not read Mr Michel Houellebecq's Submission, about a Muslim government in France, now might be a good time. Alternatively, you can "drink tea and jolly well carry on" - though it is hard to do that when your head is in the sand.
THE SUNDAY TIMES, LONDON
•Ms Ayaan Hirsi Ali's report, The Challenge Of Dawa: Political Islam As Ideology And Movement And How To Counter It, is published by Hoover Institution Press.
