Just like a perfect storm, the Tunisian beach attack was perfect terrorism.
The essence of terrorism is that it is war-conducted, not against an enemy but against innocent by-standers whose destruction robs everybody of a sense of safety, of security, of living in a world that can be relied on to be reasonably predictable.
The world has tried to make order out of chaos - chaos being the method and objective of terrorism - by a focus on historic locations.
If 9/11 attracted international attention and fear by the use of passenger planes as weapons, the logical response was to beef up security at airports. The end result has been to change, for ever, the experience of air travel.
The law-abiding air traveller has been relentlessly re-trained. Everything liquid into tiny bottles. Boots off. Belt off. Computer into the plastic tray. And the traveller, walking through X-ray and other scanners, developed the reflex: they're ready to stand, arms out, crucifix-style, to be frisked if they fail the electronic examination.
We are like the drunk looking for his dropped car keys under a lamp-post because the light is better there, even though the keys fell somewhere else. Once our attention, our money and our systems are concentrated on airports, terrorists move to another location.
The location in last Friday's case was a beach, because holidaymakers on a beach have their guard down. They are unprotected. They are so sun-sodden and slowed by the sound of the waves in the distance that a killer toting a machine gun and caring nothing about his victims' ages, gender or country of origin can march unchallenged towards his death-dealing task.
Transforming a picture of contentment into a bloody screaming horror can be achieved by one armed man. Those who can, run, calling out warnings, barricading themselves in hotel bathrooms. But at the same time they engage in collusion with the terrorist.
Fifteen years ago, it took time and mainstream media to alert other countries and other people to an attack. Now witnesses use their phones to capture pictures of the killer and beam them around the world, putting him centre-stage, quickly followed by the photos of towels pulled up over the faces of the death.
Social media transmits precisely what the terrorist wants transmitted, creating precisely the effect sought. Unmediated, unedited in its horror, the message is clear: nobody is safe. Dead or alive, people who hate what has happened become the tools of its wider promulgation.
In addition to the pictures come the verbal details telling of a decapitated employer whose head is displayed in a public place with messages written on the forehead in indelible marker.
In medieval times, the heads of those executed were placed on poles, as a lesson, where everybody could see them for months. That has now been updated with text and the capacity to share worldwide.
Terrorism seeks to use the fear engendered by spectacular attacks to shape policy. Within hours, we in Ireland know when one of our own - or more than one - has been mown down. We are speedily inundated with information about which we can do nothing, which, in itself, is part of the perfection of the crime.
We are developing a learned helplessness, a shrugging impotence born of the knowledge that if someone decides to pump bullets into holidaymakers at a beach, none of the normal responses or methods of prevention apply. The pictures repeat themselves from dozens of sources like a fairground hall of horror mirrors, and we blink at the blood on the sand.
Thereafter, we do what humans do when faced with repeated meaningless cruelty - we disengage.
We develop an extra self-protective skin. We talk and talk and talk - and then we forget.
It's all we can do - and that's why the Tunisian attack was perfect terrorism.