Of course, there is a history of violent campaigns that were genuinely political in their intent. Any number of wars of liberation against colonial powers or foreign occupation have involved the systematic use of violence against civilian communities, with the intention of driving out what was seen as illegitimate domination. As often as not, there was lively controversy at the time about whether those atrocities should be regarded as straightforward criminal activity to be dealt with by enhanced policing, or as political tactics that had to be addressed by negotiation.
We all know how the various stories ended: generally in a settlement involving concessions - either outright independence (India) or some kind of power-sharing agreement (Northern Ireland). But the thing about those previous terror campaigns was precisely that they presented the possibility of bargaining and dialogue. The imperial or occupying forces understood what was being demanded and were in a position either to make offers or to remain intransigent. There was cause, in other words, for regarding the terrorism as something different from run-of-the-mill crime.
How does that work in the present case? What is there to negotiate here? When Western political leaders say that Islamist terrorism is committed to "destroying our way of life", that is not hyperbole. It is the literal truth. This is a self-described movement to crush the infidel. Where are the rational demands that might be met - even "half-way" in the great diplomatic tradition? Should we contemplate abandoning some of our social freedoms, dispensing with our democratic institutions, or the rights of women, or any of the liberties of a modern enlightened society? Even if we did, to whom would we make the offer? And who would have the authority to accept it? Clearly, this is all absurd. There is no comprehensible political programme being proposed with its own legitimising justification to which the West might give some collective answer.
There can be no answer because there is no intelligible question. This movement is a contagious psychosis that has attached itself to a religious cause whose traditional factions are currently involved in a regional war. The attacks it makes outside that region are gratuitous, nihilistic displays of strength calculated to extend its influence within the home territories under dispute. The targets are chosen because they are "soft", and because they are in countries that have been particularly inept at dealing with the communities that breed alienation, not because they have any particular relevance to some ultimate goal. There is almost nothing that is truly strategic in this onslaught - which is why it is so difficult to anticipate.
To say that this phenomenon is irrational does not mean that there are no sane policies, apart from ruthless policing, that might be adopted towards it. First it is certainly not the case that it is totally unconnected to Islam, just as the 19th-century anarchists whose activities it resembles were not totally unconnected to the anti-capitalist revolutionary movements of the time.
But it is a parasitical blight on Islam, using a perverse, wildly selective extrapolation of its texts to inflame youthful fantasy and murderous discontent. To insist that Islam itself not be branded as inherently demonic isn't just politically correct piety. You cannot demand that families and neighbourhoods co-operate with national security - which means informing on their own members (or their own children) - while subjecting them to indiscriminate mass harassment. Either they will be integrated into their host societies - something that will take a conscious national effort - or they won't be, which will mean endless intrusive surveillance that would contravene our famous democratic values and probably prove futile anyway.
In the United States there is an ever greater demand (notably from competing candidates for the Republican presidential nomination) for racial profiling: which is another name for the systematic persecution of certain ethnic groups and a suspension of the presumption of innocence.
The Fourth Amendment to the US Constitution stipulates "the right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures [without] probable cause, supported by Oath or affirmation, and particularly describing the place to be searched and the persons or things to be seized."
In other words, the state must have specific evidence or testimony giving grounds for suspicion (probable cause) in every individual case, before "searches and seizures" may be carried out. It is not sufficient that your racial group, or your religious community, or your neighbourhood is thought to be suspect. The police are specifically forbidden from engaging in mass, indiscriminate investigation of numbers of people against whom there are no identifiable grounds.
Even without a written constitution to spell it out, most free societies incorporate such an assumption in their political culture. If we abandon that principle, what will go next? In the end, what is it that we will be defending when we speak of the threat to "our way of life"?
So where does this leave us? Helpless in the face of pathological unreason, because we refuse to abandon our own ideals? I don't think so. It is critically important to see that there are two problems which are intertwined but not the same. Permitting virtually unlimited migration of people from non-European cultures into Western countries with no plan (or even thought) for dealing with their profoundly different social attitudes has been catastrophic.
If you accept foreign communities, you must absorb them - not simply dump them on the margins of national life. The well-intentioned British name for this dumping is "multiculturalism". In France and Belgium, it looks more like laziness and outright racism. In itself, it is corrosive, but its relationship with jihadist violence is contingent, not inevitable. But the rate and speed of the influx will have to be controlled because deliberate programmes of integration take time.
Lastly, for a normal Muslim family the discovery that a son or daughter has joined jihad is a tragedy. Somehow, European countries will have to find ways to make common cause with that personal loss. Paradoxically, treating the terrorists as criminals might actually make this easier. These people are murderers - not religious radicals on a sacred mission. Perhaps that's the beginning of something we might all agree on.