On Thursday March 16, in the first major restatement of its national security strategy for nearly four years, the White House identified Iran as the single greatest danger to US interests. In the same announcement, the US government reiterated its faith in a policy of “pre-emptively” attacking regimes that it considers to be hostile, the policy that had gone much of the way towards justifying its war in Iraq and which has been widely criticised ever since. The principle of deterrence that had reigned throughout the cold war, the new national security strategy document argued, was no longer adequate to deal with international terrorism.

The principle of military pre-emption is sometimes known as the Bush doctrine, and it originally dates from a previous White House strategy document written in December 2002. After Iraq it was thought by many to be a dead duck but it will now – especially in the light of the unfolding diplomatic crisis over Iran’s nuclear ambitions – be the focus of renewed controversy (further stoked by the report by Seymour Hersh of The New Yorker that the US government has “intensified planning for a possible major air attack” – denied by the administration). The result is to make Alan Dershowitz’s new book, in which he attempts to rescue the idea of pre-emption from the morass of Iraq and to examine its worth as a general principle, yet more pressing and more relevant. Dershowitz, a Harvard law professor and a so-called “liberal hawk”, specialises in swooping on ambiguities and anomalies which most other liberals would prefer to sweep under the carpet. Whereas many liberals have been tempted to justify allowing government new powers for the war on terror because, in times of extremity, we can afford to set aside existing rules, Dershowitz prefers an appeal to our intellectual honesty. Times have changed, he says, and it is up to our theories to catch up. “The shift from responding to past events to preventing future harms,” he argues, “is part of one of the most significant but unnoticed trends in the world today.” The democratic world is experiencing a fundamental shift in its approach to controlling harmful conduct, he says, one away from its traditional reliance on deterrent and reactive approaches and toward more preventive and proactive approaches.

Before September 11 2001, both Dershowitz and the new national security strategy document argue, the doctrine of deterrence was confident that massive US military power could by itself deter the most belligerent impulses of the nation’s enemies. After 9/11, western democracies have been pulled free of those moorings and are in frightening new territory populated by unpredictable and irrational terrorists. Dershowitz is keen to excavate some historical precedents in favour of pre-emptive action, but what he produces is a mixed bag and scarcely convincing. In early modern England, he points out, English justices of the peace routinely went beyond the cumbersome rituals of the criminal law to apply preventive punishments for people – usually the poor and the indigent – who had not yet committed any wrong. Only in the late 19th century did arrests on “mere suspicion” become entirely beyond the pale and impermissible. In the 20th century however, the idea of prevention or pre-emption has periodically resurfaced, usually in times of emergency. After Pearl Harbour, for example, the US threw into preventive detention thousands of Japanese-Americans, and by the early 1970s the British government was detaining suspected IRA men without trial in Northern Ireland.

Dershowitz realises that his list of precursors hardly sets a moral example, but he argues that the doctrine of pre-emption can be on the side of the angels too. In the six-day war, he notes, Israel attacked pre-emptively to destroy the Egyptian and Syrian air forces on the ground, and has been congratulated by military strategists ever since. Decisions to act pre-emptively, he agrees, are neither good nor evil – they are complex and depend on a dynamic assessment of risk factors, including the nature of the harm feared, the likelihood that the harm will occur in the absence of pre-emption and the costs involved in a failed pre-emption. Deterrence, of course, remains the default position in liberal democracies. Almost the whole of the criminal law, after all, is based around apprehending perpetrators after they have already committed a crime and punishing them accordingly, rather than pre-empting crimes that have yet to occur. Dershowitz acknowledges that America’s vast military power still counts for something in deterring belligerents, but he thinks that we should now complement it with a jurisprudence designed to pre-empt problems before they erupt. The deterrent approach is certainly no use in dealing with Iran’s militant theocratic leaders, he argues. “Both Israel and the United States should have the right under international law to protect their civilians and soldiers from a threatened nuclear holocaust, and that right must include – if that is the only realistic option – pre-emptive military action.” Pre-emption is justified, he offers, “when a threat is catastrophic and relatively certain, though non-imminent, and when the window for effective prevention is quickly closing”.

The idea that the threat must be catastrophic is crucial here, because it is that which lights the touch paper of the strategy for pre-emption. Everything turns on the gravity of the worst-case scenario, but in recent years worst-case scenarios have begun to look more plentiful than ever. In Worst Cases: Terror and Catastrophe in the Popular Imagination, a new book by the American sociologist Lee Clarke, they flow like water. “If an NEO (near earth object) the size of a football field slipped through the atmosphere,” Clarke notes, “it could turn Miami into a smoking ruin in a matter of seconds.” Against the measured assurances of the experts, who inform us that worst cases are unlikely to transpire, Clarke is a militant believer in the idea that we are better off safe than sorry and that most catastrophes bring silver linings from which we can learn. At the end of his book he seems to be positively willing the disasters to happen, so that we can get started on the work of learning our lessons. “I cannot bring myself to say we need an NEO to obliterate a few thousand people,” he says, returning to his NEO hypothesis. “But it’s hard to see how the risk is going to get beyond the ‘giggle factor’ without, at the very least, a close call.”

Clarke distinguishes worst-case thinking – what he calls “possibilism” – from the traditional approach of probabilistic thinking, which focuses on the likelihood of something happening. “We shouldn’t concentrate so much on probabilities that we forget the possibilities. Failing to keep a proper balance skews our vision; as a result, our ability to learn about danger, and safety, is stunted and at least hampered.” The wisdom of possibilism, he contends, is why we’re urged to wear seatbelts and not to smoke. Besides, he says, the way that modern society is organised – its increased interdependence, the concentration of its populations in cities – cranks up the potential for more, and more serious, catastrophes. Dovetailing with Dershowitz, Clarke ends up arguing for a policy of “pre-emptive resilience”. Trying to prevent potential catastrophes, he argues, might even help to awake a new kind of people power and a new social solidarity. Before September 11 2001, according to Clarke, the US intelligence agencies failed to anticipate the devastating worst case – it was, he claims, “a failure of imagination”.

How does this new philosophy of prevention work when it is applied to the traditional geopolitics of the relationships between powerful states? In a short pamphlet for the London think-tank the Foreign Policy Centre, Andrew Small wants to find out. At a time when non-traditional threats and internal conflicts such as global terrorism are gobbling up the lion’s share of foreign policy attention, Small wants to refocus our attention on potential battles between superpowers – in this case, between the US and China.

The pamphlet was written last year – a year which, according to Small, risked tipping US-China relations over into a state of open geopolitical rivalry. Small begins by floating the possibility of a worst-case scenario – the emergence of an aggressive, nationalistic, authoritarian China, hostile to US interests, and a US equally determined to block or frustrate China’s ambitions. The stakes are so high, he goes on, “that the case for ‘early intervention’ should be compelling, even for those who take a more upbeat view of prospects”. He racks his brains for ideas on how to prevent this possible emerging conflict, but comes up with very little. There is, he admits, “no pat solution or single mechanism through which a conflict prevention agenda can be addressed. But there must, at the very least, be a serious drive to use the many structures that already exist – bilateral visits, the newly established global issues dialogue, think-tank conferences and seminars, track-two channels, and other official and unofficial talks – to thrash out a new relationship framework that can stably encompass these issues.” It all sounds a little lame.

Small’s none-too-successful attempt to graft the logic of prevention on to US-China relations reveals both the opportunities and limitations of the doctrine of prevention. As both Dershowitz and Clarke know, it works best where there is a plausible threat of something terrible but rather random happening. It is not, however, limited in its application to the war on terror, and neither is it the invention of dastardly US neo-conservatives. The seeds of the politics of prevention, Dershowitz notes, were planted years before 9/11, which only “expedited and, in many ways, legitimated this important development”. He recognises, too, that the idea that some things need to be prevented rather than compensated or prosecuted after the event is one that has its roots in European social and environmental policy in the 1970s and 1980s, and which is now a staple of thinking among European governments; only after the shock of September 2001 did the US fasten on to it as a military doctrine called “pre-emption”.

A few days before the White House reiterated its faith in a doctrine of military prevention, for example, the British government launched a major and highly publicised new campaign against rape that focused on changing male sexual behaviour. The campaign, according to the Home Office minister Fiona McTaggart, was justified because prison terms are no longer an adequate deterrent in these kinds of sex crimes. “It seemed to me,” she told the press, “that we need some really active crime prevention here.” It might seem fanciful to make a connection between attempting to change male behaviour towards the opposite sex and plans to change regimes in the Middle East, but both have their origins in exactly the same theory – imagining the worst possible scenario and motivation and then putting in place strategies that try to prevent it before it can ever occur.

Clarke is right that it is important to take worst cases into consideration, but focusing on potential catastrophes also stokes distrust and suspicion, and can even precipitate the catastrophe that it attempts to avoid. The idea that many things are worth preventing before they occur is also a noble aim. A stitch in time, after all, saves nine. But all depends on how we go about it. Clarke, for example, wants us to try to avoid another September 11 by conjuring up all sorts of “what if” catastrophes and working to prevent them, but these kinds of preventive measures might well distract from the business of accumulating real on-the-ground intelligence about Islamic extremism. Done haphazardly, initiatives aimed at prevention or pre-emption can both spectacularly miss the point and ratchet up the rhetoric of potential disputes, bringing them to the boil at lightning speed. With the same logic, after all, and if they were so equipped, the Iranians might feel themselves justified in blowing us up first.