Has anyone noticed how nervy governments are increasingly trying to head off danger at the pass? Alan Dershowitz has

There have been several apparently unrelated stories in the media over recent weeks that reflect a growing but largely unnoticed trend in the ways governments seek to confront dangerous conduct. This trend is towards the use of pre-emption rather than (or in addition to) deterrence as a primary mechanism of social control.

The first story concerned the possibility of a pre-emptive attack on Iranian nuclear facilities; although diplomatic efforts are under way to prevent Iran from developing nuclear weapons, the preventive military option has not been taken off the table. The second was about how Israel continues to employ targeted killings, most recently against the chief bombmaker for Islamic Jihad, in an effort to prevent rocket attacks against Israeli civilians. Such killings are part of an overall strategy - being emulated by the US - to get the terrorists before they can strike.

The third dealt with the violent reaction of Muslims to the publication of cartoons deemed offensive to the Prophet Muhammad; some have suggested the need for prior restraint of the publication of offensive material thought likely to provoke this kind of violence. The fourth described efforts by several American states to enact legislation that would allow for the preventive detention of dangerous sex offenders after completion of their prison terms. The fifth was about efforts by various governments to try to prevent a devastating outbreak of bird flu through vaccines, antiviral drugs and quarantine programmes; such preventive efforts also relate to potential germ and chemical weapons attacks. The sixth concerned the hearings being conducted by the US Congress to determine whether domestic eavesdropping by the National Security Council is justified by the need to obtain intelligence and whether newspapers should be prevented - enjoined - from disclosing classified information about surveillance of potential terrorists. Finally, there was the proposal for UK national identity cards - either voluntary or mandatory - going through Parliament as part of an overall preventive strategy against terrorism and almost certain to cross the ocean to the US.

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Though these stories describe very different types of government actions, they share a common characteristic. In each case, a government is acting in anticipation of a feared harm rather than waiting until the harm occurs.

This shift from deterrence to pre-emption has enormous implications for civil liberties, human rights, criminal justice, national security and foreign policy - implications that are not being sufficiently considered as a whole.

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Although the seeds of this change were planted long ago and have blossomed gradually over the years, it was the terrorist attack against the US on September 11, 2001, that expedited and, in the minds of many, legitimated this important development. Following that attack, the former Attorney General John Ashcroft described the "number one priority" of the Justice Department as "prevention". The prevention of future crimes, especially terrorism, is now regarded as even "more important than prosecution" for past crimes, according to the current Justice Department. In his confirmation hearings of January 5, 2005, Attorney-General Alberto Gonzales reiterated that the Administration's "top priority is to prevent terror attacks". The tactics that have been employed as part of this preventive approach include tighter border controls, profiling, preventive detention, the gathering of preventive intelligence through rough interrogation and more expansive surveillance, targeting of potential terrorists for assassination, pre-emptive attacks on terrorist bases and full-scale preventive war. We are doing all this and more without a firm basis in law, jurisprudence or morality, though there are certainly historical precedents - many quite questionable - for preventive actions.

The need to employ pre-emptive tactics is understandable, if not always justifiable, in the post-9/11 world. Among the most frightening sources of danger today are religious zealots whose actions are motivated as much by "otherworldly" costs and benefits as by the sorts of punishments and rewards that we are capable of threatening or offering. The paradigm is the suicide terrorist, such as the ones who carried out the 9/11 attacks and who continue to threaten Europe today. We have no morally acceptable way of deterring those willing to die for their cause who are promised rewards in the world to come. Recall the serene looks on the faces of the suicide terrorists as they were videotaped in the final hours of their lives passing through airport security. It is not that they are incapable of making "rational" cost-benefit calculations. But these calculations involve benefits that we cannot confer (eternity in Paradise) and costs (death) that to them are outweighed by the expected benefits. We are, of course, physically capable of deterring even those who welcome death - we could threaten to punish their living relatives, as Hitler and Stalin did. But the punishment of innocent kin - what the Nazis called Sippenhaft - is incompatible with democratic moral values. Accordingly, if suicide terrorists cannot be deterred, it is logical to try to prevent them from carrying out future terrorist attacks. But this approach carries with it not only real advantages, but also serious disadvantages.

One of the great difficulties of evaluating the comparative advantages and disadvantages of deterrence versus pre-emption is that once we have taken pre-emptive action, it is almost never possible to know whether deterrence would have worked as well or better. Moreover, at the time the decision has to be made - whether to wait and see if deterrence will work or whether to act pre-emptively - the available information will likely be probabilistic and uncertain. It is also difficult to know with precision the nature and degree of the harm that may have been prevented. For example, if a pre-emptive attack on the German war machine during the 1930s had succeeded in preventing the Second World War, we would never know the enormity of the evil it prevented. All that history would remember is an unprovoked aggression by England.

The conundrum, writ large, may involve war and peace. Writ small it may involve a decision on whether to preventively incarcerate an individual who is thought likely to kill, rape, assault or engage in an act of terrorism. At an intermediate level, it may involve the decision to quarantine dozens, or hundreds, of people, some of whom may be carrying a transmittable virus such as severe acute respiratory syndrome. At yet another level, it may raise the question of whether to impose prior restraint on a magazine, newspaper or television network that plans to publish information that may pose a threat to the safety of our troops or our spies, or that may be about to run a cartoon that risks offending people inclined towards violence. Since the advent of the web, which, unlike responsible media outlets, has no "publisher" who can be held accountable after the fact, there has been more consideration of before-the-fact censorship. If the Administration of US President George W. Bush had possessed the authority to impose prior restraint on The New York Times before it disclosed the existence of the National Security Council Surveillance Program, you can be sure that it would have tried to invoke that power.

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We must begin a discussion now about the circumstances in which various types of pre-emptive and preventive actions are and are not justified.

Plainly, some such actions are desirable: if an enemy - either a state or a terrorist group - is in the final stages of planning an imminent attack or invasion, a pre-emptive attack would appear justified (as was Israel's pre-emption against the Egyptian and Syrian air forces in 1967). Although a literal reading of the UN Charter would prohibit an armed attack except in response to an armed attack, no nation would forbear in the face of an imminent and certain aggression. Equally plain, some anticipatory actions are not desirable: the American decision to preventively detain more than 100,000 innocent Japanese Americans after the attack on Pearl Harbor in 1941 was immoral and foolish.

There are also some close calls: the US-led invasion of Iraq, ostensibly to prevent the use of weapons of mass destruction, continues to divide much of the world. Governments should not simply make ad hoc decisions regarding this array of powerful, but potentially dangerous, mechanisms of social control. We need a jurisprudence - a widely accepted morality - that strikes the appropriate balance between security and liberty. So, let the conversation begin.

Alan Dershowitz is a professor of law at Harvard University. His latest book, Preemption: A Knife That Cuts Both Ways , will be published in May, ?14.33.

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