Interdiction, aimed at seizing drugs and smugglers on their way to the United States, was the centerpiece of the early days of that war. In 1988 Congress passed a resolution calling on the Department of Defense to "seal the borders" against drugs, a mission for which the then-secretary, Frank Carlucci, and the services had little enthusiasm. As Carlucci said in congressional testimony, the military knows how to defend a perimeter against anyone who attacks; it is less good at working out which civilians should be allowed across and which shouldn't.

The U.S. Customs Bureau commissioner at the time, William von Raab, a drug war enthusiast given to blunt statement, said that his agency could indeed seal the borders. Doing so would require that every ship and truck halt 10 miles from the border to be completely searched and every passenger frisked. Since the nation would not tolerate that, it would have to tolerate a lot of cocaine and heroin crossing its borders.

Indeed, that is the case. A surprisingly large share of the roughly 400 tons of cocaine shipped to the United States each year is seized, at least one-quarter and perhaps as much as 40 percent. But the cost of replacing a kilo of cocaine is low enough ($1,000 to $2,000 in Colombia, compared with $20,000 over the border in Texas) that simply putting 100 kilograms into a cargo container and launching it off to the United States is worth doing, even with a high seizure rate.

For those interested in biological warfare agents, the experience with another drug, heroin, is more relevant and even more chilling. No more than 10 percent of imports are seized. Most heroin seems to enter the country in small quantities, one to 10 kilograms. Some enters with "body-packers," who swallow about a pound of the drug distributed among dozens of latex packages; these burst often enough that there is a small medical literature on the "body packer syndrome."

Most drug smugglers end up being caught, but that is because they smuggle frequently. Any one trip is low risk. Fifty low-risk events produce some very high risks. Terrorists need make only one or two entries to accomplish their task. Moreover, for some purposes terrorists do not need to be carrying something illegal. Unlike drug smugglers, it is they themselves who represent much of the danger.

Maintaining a list of known drug smugglers has had limited success in catching or deterring them. Given the intensity of the current anti-terrorism intelligence effort, a list of terrorists might be more effective than usual, but there's not a lot of basis for optimism.

Why has the United States been unable to protect the borders against cocaine and heroin? It is not ambivalence about the task; polls repeatedly show high support for drug interdiction. But drug interdiction competes with other considerations. Delays for travelers at the Mexican border (where about three-quarters of a million people cross every day) bring loud complaints. Unfortunately, this is the entry point for most cocaine and much of heroin and methamphetamine coming into this country. Shippers demand that their goods move promptly to allow competition with other nations.

One important factor may make the effort to prevent terrorists from entering the country a bit easier than the drug interdiction program. The scale of drug smuggling is so great that interdiction is a predictable event, like a tax; it makes drugs a little more expensive and not much else. Few people any longer believe that even a big seizure does much to make America a safer nation. That dramatically reduces the public's willingness to put up with much inconvenience.

Catching a terrorist coming across the border can prevent some specific people from dying. Moreover, if there is not an endless supply of suicidal and competent terrorists, each one caught may make a difference. That makes it easier to maintain popular support and keep agents and agencies focused on the task, even in the face of the great tedium of search.

But if success is possible, it will require a great deal more cost and inconvenience in international travel and commerce than the nation has so far been willing to bear to keep out drugs.

Peter Reuter is a professor of public policy at the University of Maryland and co-author (with Robert MacCoun) of Drug War Heresies: Learning from Other Vices, Times and Places.

A version of this article was published in the Washington Post on December 28, 2001.