f the unnerving truths arising from September 11, one stands out: The land of plenty offers an abundance of vulnerabilities to terrorists. The illusion of security afforded by two oceans and two friendly neighbors has been blown away. Mobile, diverse, and vast, the United States is finally confronting the fact that many of its strengths–most notably its openness–can be turned against it. "Our society," notes the Bush administration's first "National Strategy for Homeland Security," "presents an almost infinite array of potential targets."

• Click to view larger image

Protecting the homeland has become the No. 1 job, in President Bush's view. In just a year, federal spending on domestic security will have doubled to more than $37 billion. More will follow. With state and local governments and the private sector joining in, a total of some $100 billion will flow into domestic security in the first year after 9/11.

To manage the threat, Bush and Congress have also embarked on the most ambitious government reorganization since the 1940s. A new mega-institution, the Department of Homeland Security, will bring together 170,000 workers from 22 agencies. The aim is to avert attacks at home and, if they occur, minimize damage and speed recovery.

But the administration and Congress haven't waited for a new institution to begin plugging some of the security gaps. The screening of airline passengers has been federalized. Smallpox vaccine for every American is being stockpiled. Patrols at border crossings, monuments, and other public installations have been stepped up, as has security at nuclear plants, waterworks, and ports. And the FBI and the CIA have been pressured into improving their antiterrorism coordination. The wave of security-driven actions has been unprecedented–at least since World War II. "It's impressive how they were able to address a variety of vulnerabilities, each requiring unique steps," says Lynn Davis, a former top security official in the Clinton State Department.

The absence–so far–of further al Qaeda attacks in the United States suggests the value of destroying its sanctuaries in Afghanistan and of police dragnets here and overseas. And yet, the administration rarely misses a chance to warn about more attacks; Vice President Dick Cheney says another is "almost certain." The feds' jumpiness has rubbed off on many Americans, who wonder what, if anything, they should do. White House officials are worried that the series of vague warnings issued to law enforcement agencies–and then leaked–is inducing "threat fatigue."

The drumbeat of warnings spotlights a basic question: Are we less vulnerable now than we were right after September 11? Opinions vary. "We're safer, but we're not safe," cautions Phil Anderson, who heads the homeland security project at the Center for Strategic and International Studies in Washington. "The bad guys learned a lot from 9/11, and we did too."

The attacks on New York and Washington had an ominous demonstration effect. By seizing symbols of American mobility–airliners–and smashing them into symbols of commercial and political power–the World Trade Center towers and the Pentagon–al Qaeda killed nearly 3,000 people, closed Wall Street and air transport, and shifted the focus of national government overnight. That is a heady lesson for terrorists. "We're in a more dangerous time now, post-September 11," worries Stephen Flynn, a former Coast Guard commander and security specialist at the Council on Foreign Relations. "Any adversary who didn't understand the profound disruption you can cause by engaging in these kinds of acts learned it on September 11."

U.S. officials have also been doing some learning, especially about vulnerabilities that previously got little attention. Bush's strategy calls for "red teams" of government experts to "try to think like the terrorists" and anticipate the next surprise, as a senior aide puts it. They will examine a number of areas, among them:

SHIPPING CONTAINERS. The American economy relies on an efficient, low-cost system of sending and receiving cargo in containers. Much of it passes through seaports, then travels by train or truck. Only 2 percent of the 600,000 containers arriving each day are inspected, and authorities fret about the nightmarish prospect of their concealing weapons of mass destruction–or terrorists themselves. Customs agents are using more-sophisticated computer models to decide which containers to open, and they have started a program to check containers at foreign ports in Canada, Singapore, and the Netherlands. But enhanced security could delay goods and raise costs. "We're in a race, and we're not going to fix this in time for the next event," predicts Flynn.

TRUCKS. The transport of hazardous materials by truck poses an underplayed threat. Crashing a truck with explosive material into a crowded downtown area or a chemical plant is the sort of simple, brute-force attack that keeps analysts up at night. Saboteurs might have little difficulty getting a trucker's license or hijacking a truck.

AVIATION. Security breaches still plague airport checkpoints as the federal government hires its own people. Screeners failed to find phony guns and bombs nearly one fourth of the time in a recent test of 32 airports, even missing weapons after searching undercover agents who had tripped metal detectors. Plans to scan all checked baggage for bombs by year's end are mired in logistical problems. Yet the larger vulnerability may be in general aviation. One scenario: A private jet packed with explosives becomes a do-it-yourself cruise missile that crashes into one of the nation's 103 nuclear reactors. With some 500,000 licensed pilots, 200,000 private planes, and 18,000 airfields in the United States, securing general aviation is a daunting challenge. Fifteen private planes were stolen in the United States last year. Baggage and passenger screening is virtually absent on jets for hire and "flex jets," where ownership is shared. "You and I can go charter a plane," says security consultant Tom O'Gara. "Chances are we won't be checked."

CONVENTIONAL ATTACKS. Often lost amid the administration's focus on chemical, biological, radiological, and nuclear weapons is the possibility of conventional attacks, such as synchronized shootings or suicide bombings in malls or supermarkets. FBI Director Robert Mueller considers such bombings "inevitable." Deaths would not approach the number on September 11, but the demoralization and economic pain of hitting ordinary places could be deep. Says Michael O'Hanlon of the Brookings Institution, "I don't think you can stop it."

Other vulnerabilities are under review, too. Just 12 bridges across the Mississippi River are said to carry half of the nation's fiber-optic traffic. Just five feedlots supply virtually all of the nation's beef. With so much at risk, the administration will have to make hard choices. Defenses will be concentrated on those potential targets whose destruction would cause mass casualties or other serious damage.

There are risks, too, in overreacting. Costs of terrorism are compounded when transportation networks and infrastructure, like the U.S. postal system, are shut down. Security burdens could weigh down the economy. "No. 1, I worry about uncontrolled spending," says Randy Larsen, director of the Anser Institute for Homeland Security, a nonprofit research group. "We can spend ourselves into bankruptcy. We just can't defend ourselves against every attack."

Civil liberties could also be imperiled if overzealous government agents sweep through the corners of U.S. society for terror suspects. One wild card is whether more Americans, like accused U.S.-born "dirty bomb" plotter Jose Padilla, act for al Qaeda. "If Americans are recruited, that creates a whole different security picture, and it's very troubling," says former Virginia Gov. James Gilmore, who heads a congressional advisory panel on terrorism.

In any case, Americans will have to live with the risks of a war where the homeland is a battlefield, too. "We cannot ever be 100 percent secure against 100 percent of the possibilities 100 percent of the time if we're to remain a free and open country," concludes Homeland Security Director Tom Ridge. But the first step toward better security has already been taken: dumping false assumptions of U.S. invulnerability. "The minute you confront reality, you're safer," says former New York Mayor Rudolph Giuliani. Safer, yes, but also edgier.