In Tom Clancy's "The Sum of All Fears," terrorists obtain a nuclear bomb, ship it to Baltimore and detonate it. Policymakers in Washington want to make sure that piece of fiction doesn't become fact.

Some politicians want to require inspectors to look inside each container before it's shipped to U.S. ports. Supposedly, this would prevent terrorists from smuggling in a weapon of mass destruction or a "dirty" bomb (a large, conventional explosive laced with radiological material). But in reality, we'd be wasting our time and money.

While it's true that a terrorist could put a bomb in a box, it's neither likely nor logical. In the case of all but a nuclear device, it would be easier and more certain to just build the weapon here.

That's especially true for conventional explosives. Biological weapons can be produced with materials and equipment bought off the Internet or shipped here via any number of cargo delivery services. Potential chemical weapons surround us: chlorine tankers, gasoline trucks, pipelines and storage facilities. All a terrorist group needs for a dirty bomb is some low-grade radioactive material stolen from a hospital or a watch factory. Even the machines used to scan containers have radioactive material.

Besides, if terrorists had a nuclear weapon, it's not at all clear why they would risk allowing it to leave their control. After all the time and trouble required to build a bomb, would they really wave good-bye and hope it gets to the right place? The terrorists would be far better off to hide their bomb in a private vessel (if they can afford a nuclear weapon, they can afford a boat to carry it in), a truck coming across from Canada, or a small tramp ship operating out of the Caribbean destined for, say, the Port of Richmond.

If terrorists wanted to target a port, they would more likely use a truck, train or small boat. A McVeigh-style truck bomb, constructed domestically, would do the trick. And it would be much easier to approach a port from the land than from the sea.

Finally, if foreign ports did attempt to screen every container of sneakers coming to America, they would likely fail. There aren't enough people and computers to scrutinize the millions of records that would be produced in real time before the containers reach their destination. It also isn't clear if any technology is fast, accurate and cheap enough to do the job with any degree of confidence.

Nevertheless, terrorists might try to slip bad things or bad people through a port. We can stop them by investing in the defenses that get terrorists, break up their networks, interdict their operations and deter their activities. In the maritime domain, building up the grossly underfunded U.S. Coast Guard would be a good place to start.

At the same time, we should insist the private sector do reasonable things to make the global supply chain safer.

A reasonable measure provides a useful amount of security at a modest cost. Based on that definition, container seals, tracking devices, smart tamper-proof containers, RFID tags, X-ray machines and radiation detectors (that aren't very effective) are not reasonable. Nor are more guns, guards, fences and gates.

Together they represent a hugely expensive investment for which there is no legitimate business case. They also represent an ineffective response, largely because they don't and can't address the actual threat -- a handful of determined and resourceful people who will strike when they can where America is weakest and most vulnerable.

The better answer, as the 9/11 Commission noted, is to connect the dots.

Customs and Border Patrol evaluates all the cargo coming here and determines which shipping containers need checking. About 5 percent of all cargo ends up being scanned by X-ray machines or opened and searched. Their assessments can be improved.

Manufacturers, shippers and other commercial entities already produce data on who ordered a cargo, where it was made, to whom it will be sent, as well as of anyone who paid for, touched or moved the goods. Using this data to better assess the risks would represent a reasonable effort to improve what's already being done. Combined with aggressive policing and counterterrorism efforts, it would deter terrorists who want to target America's ports.

When it comes to homeland security, we can't do everything. We have to focus on doing the big things well.

Instead of trying to protect ourselves from unlikely threats by inspecting everything and every person that comes here (at a cost of billions of dollars), let's focus on smarter ways to make us all safer.

James Carafano is senior fellow at The Heritage Foundation and co-author of "Winning the Long War: Lessons from the Cold War for Defeating Terrorism and Preserving Freedom." Robert Quartel is a former member of the U.S. Federal Maritime Commission and an internationally recognized expert on trade security.