The problem with terrorists is that they have entropy on their side. The buildings they take down were coming down anyway, the planes they want to crash are perilously hanging in midair, just waiting for a nudge. It is so easy for them to cause so much damage, because they are working with chaos, not against it. People are very difficult to keep alive, requiring at least food, water, shelter, and some semblance of of structural integrity. Terrorists can take any of these away, by taking out a GMO farm, dropping some botulinium in the local water filtration plant, blowing up a few buildings, or rigging up an old-fashioned improvised explosive device in Iraq. When they do, they quickly undo untold years of effort on the part of your parents which made you into the complex and well-rounded individual that you are.
A classic childhood example happens inevitably every fall when a proud father spends a saturday afternoon raking the backyard. A few hours of work took what was a messy (albeit photogenic) ground-covering of leaves and made a neat pile in the middle of the lawn. It will sit there for a night, awaiting the next morning when daddy bags it up, and puts it either in the trash or the compost, depending on his political outlook. Unwittingly, he left it out all night long. It would take no more than one giddy leap from Calvin or Hobbes to return it to its ground state: all over the yard.
It takes a hen a few weeks of dedicated effort to squeeze out an egg (though they generally do have a few in the queue). This egg is a perfectly constructed vehicle for continuing the species. It awaits fertilization, at which point it provides a young chick everything it needs to become posterity; including a tough shell, capable of withstanding impressive situations (ever seen an egg drop competition?). When you crack this egg into a frying pan, you are leveraging heat to take it down to a less useful level: one that feeds you, but could never result in a baby chicken. It costs you maybe a few cents (free if you live in the desert) to fry the egg. Not even with 1000s of brilliant biochemists and billions of dollars could you take your fried egg and take it back to its earlier state. Now, we have plenty of eggs, and they make a delicious breakfast, so the threat from terrorists here is minimal.
A big bridge, however, is not so safe from attack. It takes civil engineers months to years to create a bridge, and that's just the construction. The government-payed corporations who actually do all of this take easily decades to plan out a bridge. It takes massive amounts of planning, resources, and human gusto to put together even the simplest highway overpass. It takes an oafish terrorist a few hours to excitedly pore over the Anarchist Cookbook, a few more to get the parts and supplies, and in a day or two they have a pretty nice little bomb. Strap it to a bridge, and thar she blows.
It took probably 30 people (19 actively in the planes, plus support) to orchestrate the 9/11 attacks. These 30 people took a few years off of the end of their lives to end those of thousands. They also took out two of the most impressive pieces of architecture in the world. Let's say the terrorists spent 300 person-years to pull of their stunt (30 people, 10 years). Ignoring the incalculable worth of the individual lives they ended, the structures they demolished took sometimes 3500 people at a time, and took 4 years to construct, we'll say 8000 man-years. From this example, we can see that to destroy something, it takes at most an order of magnitude less effort than to construct it. This is bad news.
Our entire society is the bastion of centralization. A large number of people get their water, power, and gas from a single, centralized place. Anybody who has played Sim-City knows how precarious this position is: if your water-treatment plant goes south, you get lots of little gray derelict buildings and your approval ratings drop. This is not a problem of patching a few holes in an old canoe: civilization in general is far easier to destroy than it is to create: just ask the Mayans. Ever since specialization of labor, we have become dependent on each other. The more people we depend on for our continued survival, the easier it is to screw us up. "Critical Infrastructure" to an African Tribesman is his spear and the nearby river. The list for the average american is miles long, including water, power, phones, food, gasoline, shipping, and the Internet, in no particular order. Each of these represents a ridiculously complex web of interaction that boils down to a simple service. Just talk to a civil engineer about how under-appreciated they are and you'll find out how many engineer-hours it takes to give you your water bill.
The point here is not that we are unsafe, but that we are looking at the problem in the wrong way. "Anti-Terrorism Measures" mean stricter port-control, and more of those things that beep when you wear a belt or carrying change into an airport. Presumably they beep with more sinister things than that, but I haven't tested that, have you? The secret that the politicians are hiding, is that these don't do a damned thing. This has been the story in Israel for years. They have invested billions of dollars into anti-terror measures that would make Dick Cheney blow his vice-presidential load. What has it done? Nothing. They still have suicide bombers pestering them, and they still get rockets lobbed at them every other day. The only way to keep society safe is to keep people happy. If someone really wants to fry society's egg, it's just waiting to happen, literally. While this is probably an interesting lesson to those studying statistical mechanics, it sucks for the rest of us. What we need to do is stop making people want to kill us.
The money spent on "Homeland Security" would literally be better spent giving books to schools in Iraq, or passing out condoms in Africa, or paying the 'russkies to keep track of their nuclear arsenal. Hell, we'd probably get more results, though admittedly less bang, for our buck if we just dumped greenbacks and love letters out of planes instead of smart bombs. Right now, if you walk up to anybody in Baghdad, statistically speaking, one of their loved ones was killed directly or indirectly by the USA. That's bad. This may sound like hippie drooling, but do the math. "All you need is love" is not quite right: food and antibiotics help, but love's a start. This is a question of monetary scale. We could give every man, woman, and child living in Iraq $150 a month for less than it takes to keep killing them. If you live in a glass house… you better not throw stones, even if you spent the extra money and got yourself reinforced concrete butting.
Still, we shouldn't be scared here because we're still got statistics on our side. The year after the worst attacks in U.S. history, 2002, there were 25% more deaths (across all age ranges, 3770 total) from "accidental drowning" than from terrorism in 2001 (2992 total). In other words… even in a good year for terrorists, they're still offing fewer people than swimming pools and bathtubs.