Sometimes it seems like the people in charge of homeland security spend too much time watching action movies. They defend against specific movie plots instead of against the broad threats of terrorism.

We all do it. Our imaginations run wild with detailed and specific threats. We imagine anthrax spread from crop dusters. Or a contaminated milk supply. Or terrorist scuba divers armed with almanacs. Before long, we're envisioning an entire movie plot, without Bruce Willis saving the day. And we're scared.

Psychologically, this all makes sense. Humans have good imaginations. Box cutters and shoe bombs conjure vivid mental images. "We must protect the Super Bowl" packs more emotional punch than the vague "we should defend ourselves against terrorism."

The 9/11 terrorists used small pointy things to take over airplanes, so we ban small pointy things from airplanes. Richard Reid tried to hide a bomb in his shoes, so now we all have to take off our shoes. Recently, the Department of Homeland Security said that it might relax airplane security rules. It's not that there's a lessened risk of shoes, or that small pointy things are suddenly less dangerous. It's that those movie plots no longer capture the imagination like they did in the months after 9/11, and everyone is beginning to see how silly (or pointless) they always were.

Commuter terrorism is the new movie plot. The London bombers carried bombs into the subway, so now we search people entering the subways. They used cell phones, so we're talking about ways to shut down the cell-phone network.

It's too early to tell if hurricanes are the next movie-plot threat that captures the imagination.

The problem with movie plot security is that it only works if we guess right. If we spend billions defending our subways, and the terrorists bomb a bus, we've wasted our money. To be sure, defending the subways makes commuting safer. But focusing on subways also has the effect of shifting attacks toward less-defended targets, and the result is that we're no safer overall.

Terrorists don't care if they blow up subways, buses, stadiums, theaters, restaurants, nightclubs, schools, churches, crowded markets or busy intersections. Reasonable arguments can be made that some targets are more attractive than others: airplanes because a small bomb can result in the death of everyone aboard, monuments because of their national significance, national events because of television coverage, and transportation because most people commute daily. But the United States is a big country; we can't defend everything.

One problem is that our nation's leaders are giving us what we want. Party affiliation notwithstanding, appearing tough on terrorism is important. Voting for missile defense makes for better campaigning than increasing intelligence funding. Elected officials want to do something visible, even if it turns out to be ineffective.

The other problem is that many security decisions are made at too low a level. The decision to turn off cell phones in some tunnels was made by those in charge of the tunnels. Even if terrorists then bomb a different tunnel elsewhere in the country, that person did his job.

And anyone in charge of security knows that he'll be judged in hindsight. If the next terrorist attack targets a chemical plant, we'll demand to know why more wasn't done to protect chemical plants. If it targets schoolchildren, we'll demand to know why that threat was ignored. We won't accept "we didn't know the target" as an answer. Defending particular targets protects reputations and careers.

We need to defend against the broad threat of terrorism, not against specific movie plots. Security is most effective when it doesn't make arbitrary assumptions about the next terrorist act. We need to spend more money on intelligence and investigation: identifying the terrorists themselves, cutting off their funding, and stopping them regardless of what their plans are. We need to spend more money on emergency response: lessening the impact of a terrorist attack, regardless of what it is. And we need to face the geopolitical consequences of our foreign policy and how it helps or hinders terrorism.

These vague things are less visible, and don't make for good political grandstanding. But they will make us safer. Throwing money at this year's movie plot threat won't.

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Bruce Schneier is the CTO of Counterpane Internet Security and the author of Beyond Fear: Thinking Sensibly About Security in an Uncertain World. You can contact him through his website.