They are more motivated than we are.
That's a seven-word answer for why airport security never will be totally secure.
Terrorists, mainly radical Islamic jihadists, want to destroy our way of life. That is their sole motivation and their sole aim. As a jihadi Web site promised after the Christmas Day attack on an airliner en route to Detroit, more terrorism is planned.
We worry about thwarting those attacks. However, we also worry about things such as making sure flying isn't too inconvenient, which would harm the economy. We worry about making sure airport inspections are fair and equal so nobody is singled out by race or ethnicity. We wring our hands and debate endlessly over whether to try terrorists in civil courts or military tribunals, and we castigate ourselves endlessly over interrogations that turned into torture sessions.
It's not wrong that we do this. Our motivations are geared toward civilization; theirs is toward destruction and theocratic rule.
If you invite a puppy into your home, you can work hard to puppy-proof your house. But you can bet the puppy will find something valuable to chew to pieces anyway. Your motivations include living comfortably. The dog has one motivation — to chew on something.
Civilized nations are ruled by laws, not passions. Some of those laws are suspended in times of war, in order to protect the nation. But this is no ordinary war with enemies in uniforms representing foreign governments. It is a fight against phantoms, and they always are looking for ways to make sure we are fighting the last war.
In warfare, generals often fight "the last war," meaning that they assume the next war will use the same tactics and weapons as the last.
Now we will have full-body scanners at airports. Does anybody believe that will make us totally safe?
Before anyone answers that question, it may be helpful to do a little review. The attacks of 9/11 exploited a system geared toward stopping the last terrorists — hijackers with guns and other conventional weapons. After 2001, airport security went through several upgrades and encountered pushback from the traveling public, which grew understandably weary of long lines.
For a while in 2002, each car entering the second or third levels of the parking garage at Salt Lake International Airport had to be hand-inspected to make sure it wasn't carrying explosives. Trunks were opened and passenger compartments checked. This didn't last long. It was too inconvenient and unlikely to prevent a crime. Next, security began demanding our shoes, then our water bottles.
And yet Umar Farouk Abdulmutallab was allowed an American visa and safe passage on an airplane despite paying cash for his ticket; having no checked baggage (on an international flight); having spent time in Yemen, which is known as a hangout for terrorists; having a Nigerian banker father who called authorities to warn them his son had been radicalized and might be dangerous; and having his name on a database of people who might pose risks.
And yet all the system could think to do was check him for water bottles.
It seems incredible that no U.S. official had thought to add all these factors, or at least a few of them, together and at least ask a few questions before issuing a visa. But then, each piece of information probably belonged to a different bureaucracy.
This isn't an argument against full-body scans or water-bottle checks. All of those things are necessary and have, no doubt, thwarted attacks. But all of them together won't keep the airways completely safe.
Civilized societies have learned there is a trade-off between freedom and safety. Total safety would come at the expense of a lot of things that make us who we are.
To be sure, we can be smarter and more effective. We should never give up. But we should hope we won't ever be as motivated as this enemy.