The world's security and intelligence organisations have had a patchy record in their long war on terror.
There is little doubt that plots have been foiled and lives saved, even if the details are too rarely shared with those protected. It is also true that failures, conspicuously the loss of New York's Twin Towers, have been catastrophic.
Sometimes things go wrong: that's just a fact of life. What's less excusable are the abuses of power by those claiming to defend liberty, from Guantanamo to the antics of GCHQ, from rendition flights to the outsourcing of torture. Those who expect, even demand, our trust are not always to be trusted. This is a problem, to say the least, for democratic societies when the threats are real.
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How do you react, though, when the kids are eager for their Florida holiday and "credible" evidence is produced by American agencies of serious plots against airlines? Most people, it seems, will sigh or fume, accept what they are being told, accept another round of irksome tightened security, and understand that to behave otherwise would be irrational. I'd be put off air travel. However reasonable their explanations, those who run airport security make assumptions, let's say, that I'm not content to share.
In the United States, and increasingly in Britain, intrusive and sometimes insulting behaviour towards travellers is taken for granted. But many of the millions who fill the skies for the sake of business or families have no choice in the matter. Therefore, says common sense, they have to accept much of what goes along with flying in this troubled world.
It seems clear from events in recent years that Islamist bombers are striving to become more ingenious. It is obvious enough, too, that they are wedded still to the tactic of the murderous "spectacular" against civilians. Those who plot also seem devoted to the notion that paralysing air travel is a perfect strategy in their nihilistic war. So when the Americans claim there is a new "credible threat" from al-Qaeda types in Syria and Yemen, the world takes notice.
If it has any sense, however, the world also asks a few questions. Is it simply established that, when Washington in effect issues an order for heightened security, countries and airports everywhere simply obey? That has certainly been the case in Britain and France. Yet while passengers are subjected to all new manner of technology, the UK sees no reason to raise its terrorist threat level from the present "substantial" level. That doesn't add up.
Then there is the considered opinion of Nick Clegg, the Deputy Prime Minister. He has let it be known that the latest restrictions are likely to be permanent, that we must just get used to them because "this is the world we now live in". But how does one announcement of a suspected threat become transformed into a permanent state of affairs?
You could note that security services have a bad, long-standing habit of taking more power over people's lives than is strictly necessary and refusing to contemplate its return. Relentless terrorism, forever undefeated, forever devising new challenges, can be too handy to those who run a security apparatus. They have taken increasing control over the Western world since 9/11 and show not the slightest sign of relinquishing an iota.
Perhaps you could call that paranoia. People have been murdered by scores and hundreds in several countries, after all. Bomb plots have been uncovered. You don't have to be as demented as Tony Blair to realise that various versions of Islamist ideology incite mass killing. As security people and politicians often observe, those who whine about civil liberties are the first to complain when an outrage is not prevented.
Mr Clegg's statement cannot be explained so easily. Is he saying that this war on terror has become a war without end, a perpetual conflict? If so, he doesn't inspire much optimism. He also casts a light on various western "interventions", not least those staged by Mr Blair and George W Bush. Follow the logic of Mr Clegg and it seems that a bad situation has been made very much worse.
If the world is to be placed under an unending security lockdown, one fact is worth remembering: that's exactly what Osama bin Laden hoped to achieve. Part of his ugly scheme was to provoke the West into repression, the repression of young Muslims above all. So how many of them have been flocking to Syria and Iraq? Call them deluded or deceived. They hardly count as evidence that this war on terror, overt since September 11, 2001, is being won.
That might be another conclusion to draw from Mr Clegg's comments. Plainly, he sees no prospect that the terrorists can be defeated anytime soon. Why is that? Because the West's leaders have no viable strategies of their own? If that's the case, someone might mention it to the populations placed under threat and deprived of their liberties "for their own good".
It might be that we are relearning one of the lessons of anti-terrorist campaigns and anti-insurgent wars. Sometimes they are generational, stumbling to a bloody close not because one side or another has won, but because those involved are exhausted, tired of sacrificing their young people year upon year. Islamists who see no moral problem with suicide bombings might not submit to that kind of logic quickly but, equally, they do not subscribe to Mr Clegg's bleak acceptance of perpetual war.
Those who wage that war on behalf of democracies are entitled to point out that they operate within the limits of the possible. If terrorism is real, what is to be done? The avoidance of arrogant, ham-fisted interventions might be a start. Refraining from backing (or "advising") military juntas with a taste for torture might be another. But such arguments are not quite sufficient to answer all the questions posed by the Islamist bomb-makers. Where victims are concerned, any Westerner will do. If Muslims die, they say, so be it. There's the challenge.
It does not begin to justify Mr Blair's appetite for the endless war that Mr Clegg accepts with apparent equanimity. But the risk of being dragged to the level of your adversaries, inch by inch, year by year, always in the name of safety and security, is growing. It looks very like defeat. When those who talk a lot about freedom ask "What is to be done?" they should deal with that paradox. Or are they simply not too fussed when the state engages in the mass surveillance of people who have committed no crimes?
Unless work obliges, I won't be going out of my way to encounter Homeland Security or its European equivalents. That's not much use, I grant, to a business traveller who has no real choice to exercise. It simply strikes me that co-operating with each new demand is creating one set of problems for our society while failing to solve another.
It keeps numerous people from "the security community" in business, however, and it would be unwise to forget the fact.