Defence experts are fond of saying that for every measure, there is a countermeasure. Well, let's hope they are wrong about underwear bombs.
Al-Qaida's foiled plot to blow up an American-bound airliner exposed a chilling new reality. Terrorist bomb-makers have designed an underwear bomb that is made completely of chemicals, including the detonator, making it undetectable by the conventional pat-downs and metal detectors used in most of the world's airports.
It is unclear if it would have been detected by advanced imaging screening devices, known as the "naked" X-ray machines. Some reports claim it would have been discovered, while others say it would have cleared security.
But even if advanced technology could have interrupted such a terrorist plot, the problem is many airports around the world do not deploy it, including many in major European cities.
When terrorists struck on 9-11, airport security tightened up dramatically, much to the annoyance of travellers. A year later, a man on a flight from Paris to Miami tried to detonate a bomb hidden in his shoes. Security increased again, and passengers are now accustomed to removing their shoes.
The advent of more sophisticated weapons resulted in newer screening devices, which some travellers claim are an invasion of privacy, since they render them "naked."
The question today is how much farther security can go before travellers are required to remove all their clothing for chemical tests before they board an airplane.
That, clearly, is not a realistic option. But does that mean airport security has reached the end of the line? Unlikely, but then terrorists haven't reached their optimum capability, either.
The best defence against terrorists is well-trained security officers at airports and aggressive intelligence, the kind that exposed the new underwear bomb in Yemen.
In the meantime, the war of measure-countermeasure continues. Hopefully, we will always be one measure ahead.