Why keeping us safe from terrorism is so hard

(CNN)The attacks Tuesday morning on the airport and a subway station in Brussels were highly probable, and at the same time almost impossible to disrupt. That oxymoron is what makes this wave of terrorism the most dangerous in history, and so different from even the days of al Qaeda.

Key clue in Brussels attacks: The explosives

As I wrote last week, the arrest on Friday of Paris terrorism suspect Salah Abdeslam was never going to be the end of the fight. Indeed, it could possibly be the beginning of a new wave from ISIS.
Because Abdeslam was captured alive, European authorities were likely going to be able to get him to speak and perhaps reveal details about the network that had aided him in the Paris attacks last year and that let him remain in hiding for months after. And even absent any willingness to speak on his part, Belgian officials would have been more than willing to make it appear as if he were cooperating. Indeed, as early as last weekend, Belgian officials were saying that Abdeslam was “collaborating,” a word that makes it sound like he was more than happy to turn over information.
Juliette Kayyem
Juliette Kayyem
Those in his network, or those who support ISIS, were likely to feel the pressure and launch attacks if they had the opportunity. So why, many are asking, wasn’t Brussels ready for another attack?
Basically, no country is ever completely ready.
Security has three fundamental attributes in any open society: minimizing risk, maximizing defenses, and maintaining freedom. Officials minimize the risk by sharing intelligence, infiltrating terrorist groups and trying to disrupt them. They maximize defenses by deploying more police and counterterrorism officials, managing borders, and hardening targets as much as practicable.
The challenge is always the third variable. Open societies that rely on the flow of people and goods, that are plugged into the global market and engaged with the world, can’t possibly harden every place where people gather. As a result, they are constantly balancing the need for counterterrorism efforts against the freedom of movement.
Key clue in Brussels attacks: The explosives
Explosives will be key clue in Brussels investigation
This is true even for airports, which are essentially porous until you get to the security check. They are places where people congregate; there are stores and restaurants where people meet and relax. Departure areas such as the one at Zaventem airport in Brussels are meant to allow people to move quickly so that passengers on their way to flights can check their luggage and head to security, allowing for the next batch of people.
In the United States alone, about 2 million people travel each day in and out of our airports. Our system of commerce and travel would come to a standstill if we essentially pushed out security to the highways outside an airport. The same holds for the metropolitan transit systems. Yes, they can be made safer with security checks of passengers, but a 20-minute train ride would then become an hour or more. And the public, despite the threat of attacks, would simply not tolerate this.
The focus on airplane security that we have seen since 9/11 was of course aimed at ensuring that the airplane itself was not utilized as a weapon of mass destruction. But an airport does not have the same luxury, not if countries want and need to maintain their commitment to movement. That is why the only option for Brussels right now is the exact opposite: a total shutdown of the airport. It’s like an on/off switch, with little room in between.
The reality is that there are just too many soft targets in Europe and the United States to ensure complete security. It is the way our system of freedom has been built. Sadly, as the attacks Tuesday showed, that is also why it is so vulnerable.