Up to and including the twentieth century, superior firepower and advanced technology won wars. Ideas, however, win the hearts and minds of men and women, for better or for worse. In the Information Age, knowledge cannot be suppressed, and denial can be deadly. Cultists, bombers, terrorists of every race, religion and creed will rise from amidst society to threaten the security and freedom of all. They will do so by exploiting assumptions, expectations and advancements towards their own ends.
Security that relies on technology that is more complex and more advanced than the enemy's has worked in the past, but will not work in the future. Progress has already led to the discovery and deployment of the most horrible means for devastation, weapons of mass destruction, nuclear, biological and chemical, opportunities for efficient and quick genocide, each yielding results that are "more than we can bear," to quote NYC mayor Rudolph Guiliani. Further technical wizardry will just be subverted in ways not yet imagined.
Relying on secrecy is not an option, either. The genie cannot be put back into the bottle. It's not just that the twentieth century gave rise to leaks, underground communications, brainwashing and propaganda and conspiracy theories. But the simplistic notion of a "them" against an "us," in the current case Al-Qaeda terrorist cells waiting to be "activated" (the metaphor of robots being but the latest method of dehumanizing the enemy), will not save anyone from "inside" jobs, individuals who go berserk, the black market, traitors and turncoats, even just people trying to make a quick buck without realizing who they are selling their wares or knowledge to. And in the modern world of telecommunications, bad news, and bad knowledge, travels fast and far and has a half-life but never disappears.
Relying on strength, on brute force, power and might, may slow or delay an attack, but also will not stop destruction, especially as there are clever ways to turn such might to a terrorist's own use. In a world of suicidal maniacs, where "death by cop" is unconsciously pursued by lonely depressives and martyrdom is glorified and rewarded, physical properties like mass, bulk, and rigidity can abet rather than impede the worst intentions. Agility, flexibility and the ability to evacuate are restricted when security is built up rather than built in.
A free society must rely on that which distinguishes it - the fact that it is free - to imagine and invent and "design in" security in ways that counter the assumptions of the past. Instead of complex technologies, codes that attempt to be too computationally hard to crack, artificial intelligence, counter-counter-measures and sophisticated toys, devices should be simple and obvious, uncrackable or noticeably cracked, like wrappers on aspirin bottles. Instead of relying on secret knowledge, specialized training and restricted access, devices should be open, inspectable, with the whole of human attention focused on continuous improvement, like open-source software. Instead of relying on the strength and size and power of materials, devices should be fragile and easily defeated, like breakaway road barriers.
If a terrorist is going to try to turn new technologies to evil ends, then those technologies must be reliable and safe but easily disabled. Devices that are inherently dangerous must contain security mechanisms that can only be defeated by disabling the device itself. Dangerous materials must be transported in containers that are always traceable and in ways that are always identifiable. It should be immediately apparent if tampering, theft or concealment is attempted.
New designs cost money, new products cost money, the enforcement of new techniques and monitoring, verifying and responding to breaches cost money, so, in a practical world, there is only so much that can be achieved. Certainly, the most money must be expended in response to the most dangerous threats. Luckily, the deadliest threat, nuclear weaponry, is the hardest to create, the hardest to gather materials for, the most dangerous to handle and the most difficult to conceal. Large chemical threats are hard to manufacture, handle, and transport, also. The more serious the biological threat, the more likely the terrorist attempting to obtain and deliver the threat will be killed instead. But that doesn't mean that what has been imagined will not be attempted - the threat is real and serious.
Money must be spent elsewhere as well. Airplanes can be designed with secure cockpits, breakaway wings, embedded flight pattern tracking, even remote controllability. Weapons can be created that broadcast their location, that can only be operated by the owner, that can be remotely disabled by the police, or a judge. Goods and materials can be tagged, placed in transparent trucks, and tracked.
Just as built-in safety devices in motor vehicles and along roads do not hamper mobility, built-in security improvements of this sort will not restrict freedom, but the money needed to institute the improvements must be spent. And just as safety improvements can be shown to save thousands of lives but can never completely prevent accidents, so, too, will new designs save the lives of thousands, but can never provide guarantees of security to every individual.