PERSPECTIVE ON DOMESTIC TERRORISM : How Much Security Is Enough? : America’s structure and values and the nature of potential threats make it impossible to totally safeguard the nation.

Fearing that the anniversary of the Oklahoma City bombing might inspire new terrorist attacks, security has been increased at federal buildings nationwide. This is only the latest in a series of alarms that have led to increased security at airports, courthouses, landmark properties, public transportation systems and other potential targets.

The anniversary of the World Trade Center bombing, the date of the verdict and the day of sentencing of Sheik Omar Abdel Rahman and his coconspirators for their role in planning terrorist attacks in New York, the Unabomber’s threat to the Los Angeles International Airport and subsequent alerts at other U.S. airports all have had security planners holding their breath.

Terrorism works this way. Terrorists choreograph their violence to provoke fear and alarm. Frightened people demand security, but how much security can people realistically expect? How much can be provided? How much is enough? What makes these questions so difficult to answer is the absence of an agreed-upon assessment of the terrorist threat.

Terrorism differs from armed conflict and ordinary crime. It is much more a matter of perceptions than of easily quantifiable risks or losses. This is especially true in the United States. A terrorist threat clearly exists as demonstrated by the World Trade Center and Oklahoma City bombings. There also have been a number of failed bombings and discovered conspiracies. Had all of these succeeded, the last three years would have seen another four or five large-scale bombings with potentially hundreds of casualties.

The terrorist threat remains high today. It is real but murky and hard to quantify. We have no way of estimating the likelihood of another attack and therefore of calculating how much to invest in prevention.

Statistically, the threat terrorists pose to human life is minuscule compared with ordinary crime. The lives lost in the Oklahoma bombing clearly were a tragedy of major proportions, but given the country’s appalling murder rate, we suffer the equivalent of an Oklahoma City every 3.5 days. Perceptions of terrorism, however, are not determined by statistics but rather by dramatic incidents that make the threat appear greater than it actually is.

Terrorists also have a real advantage over security planners. Terrorists select targets on the basis of their symbolic suitability first, and second, their vulnerability. Suitability can be broadly interpreted. Anger at the government or its policies can be demonstrated at thousands of venues. As security has been increased around the traditionally preferred targets of terrorism--commercial aviation, government buildings, diplomatic facilities--terrorists simply have shifted their sights to other, softer targets.

They can attack virtually anything, anywhere, anytime. Government and private security together cannot protect everything, everywhere, all the time. It also means that the costs of security are not driven by the magnitude of the threat but rather by the magnitude of targets to be protected.

As the motives for terrorism have changed from ideological conflict to ethnic hatreds and religious fanaticism, terrorists have become more willing to carry out large-scale indiscriminate acts. The car or truck bomb packed with hundreds or thousands of pounds of explosives exemplifies this trend and complicates security measures. A massive truck bomb makes its point wherever it is detonated. Shutting down subgrade parking and surrounding buildings with concrete barriers makes sense for the building’s operators but doesn’t prevent the attack or necessarily reduce casualties.

The costs of security are not just financial. Federal buildings are not garrisons. Ours is a government of the people, by the people and for the people. An attack on a federal building is an attack on the American people, and those were ordinary Americans who were killed in the explosion. Security measures can separate people from their government. They should not be allowed to do so.

If terrorists want to “celebrate” the first anniversary of the Oklahoma City bombing, security cannot stop them. Increased security, however, may dissuade the less determined adversary from carrying out an attack at his preferred site. Security also helps deal with the inevitable hoaxes that anniversaries inspire; it is easier to assess the credibility of threats if security measures have been put in place. Visible security measures cannot comfort a nation living in fear (and may only remind people of the threat), but they can reassure those behind the barriers. Finally and most cynically, increasing security provides some insurance against future accusations--inevitable in our litigious society if something goes wrong--that nothing was done.