Lawrence Freedman: The enemy may be shadowy, but this war will follow the usual battle lines

'What is possible, although still demanding, is to frustrate, demoralise and beat them back'

The enemy may be shadowy but it is by no means imaginary and its belligerence is open. After a series of modest hits against the US and its assets abroad, the enemy has struck spectacularly and, now emboldened, will try again. In these circumstances restraint may be advisable but passivity is not.

Whatever happens now will depend largely on diplomatic efforts, drawing on the shared resolve and grief of many countries. But armed force, even if only threatened, must back this diplomacy. That, however, is far easier said than done, which is why during the past few days those seeking to describe an appropriate concept of operations have appeared to be floundering.

The enemy does not have deployed military capabilities, a capital city or even – despite the focus on Osama bin Laden – a supreme leader and hierarchical chain of command. Mr bin Laden's al-Qa'ida is linked to a network of groups – largely established by militants who learned their trade fighting the Soviet Union in Afghanistan, – spread around the world, harboured unwittingly in western countries as much as in countries blatantly hostile to the West.

The first step is to agree a realistic description of the objective. The eradication of terrorism as a global phenomenon does not meet this test, because not only is the definition contested in many instances but also the phenomenon's existence is bound up with numerous conflicts, many beyond immediate resolution. It may be the means employed that makes the enemy so obnoxious but it is the ends pursued that help in its identification.

The enemy is distinguished by its brand of militant Islam and a belief that the US, and the western world in general, must be punished for its evil ways. Eradicating this political force is also an unreal objective, for these beliefs are deeply and widely held, and the exaggerated rhetoric being employed in Washington risks creating expectations that cannot be fulfilled.

What is possible, although still demanding, is to frustrate, demoralise and beat back al-Qa'ida and its associated groups. This objective is realistic in that it acknowledges that victory will be a matter of degree and is unlikely to be decisive. It requires thinking in terms of a campaign capable of being sustained over time, rather than a crusade which will subside, along with international resolve, in the face of setbacks.

This campaign may have many unique qualities but in some respects it will be quite familiar. The core issues are those that have had to be addressed in countless civil conflicts over the past half century, in which insurgent groups have sought to undermine the foundations of an established state. Unlike traditional armies, guerrilla groups and terrorists do not expect to hold territory. They need time more than space, for it is their ability to endure while mounting regular attacks which enables them to grow while the enemy is drained of patience and credibility.

To succeed, they must be able to mount a campaign on their own; and this requires being able to launch regular attacks to upset the equilibrium of the target society. This, in turn, requires not only that the guerrillas or terrorists survive all attempts to hunt them down, but that they do so in such a way that allows them to sustain their own campaign. If al-Qa'ida's leaders and militants are not caught that might be deemed a strategic failure in the West; but if the need to run and hide prevents them undertaking new attacks then a success can properly be claimed.

If the enemies are to be defeated, they must be separated from those who harbour them, whether in local communities or bases concealed within the borders of another state. The debate over how this might be achieved came to be characterised during the Vietnam War by the competing slogans of "hearts and minds" versus "search and destroy". The instinctive US response to the outrages of 11 September appear to come into the second category. Somehow those responsible must be found and hit. Countries that provide a haven will have to choose: either abandon their villainous clients or suffer the consequences.

Those preferring a "hearts and minds" approach point to the need to remove legitimacy from the enemy by addressing the wellsprings of its support. If this support evaporates so will sources of new recruits and funds, and eventually sanctuary. The trouble with "search and destroy" is that the destruction tends to be more accomplished than the search. If raids fail to differentiate between the guilty, the half-committed and the innocent, the principal result will be to generate many new recruits and supporters.

The misgivings are justified, but if taken to their extreme they appear to suggest that nothing can be done without first solving the numerous and intractable problems of not only the Middle East but also Central Asia; and that any military action is bound to make the situation worse.

Unilateral action by the US, lashing out with punitive and ineffectual strikes, will certainly be counter-productive. We can point to past behaviour and some bellicose statements that might lead us to worry about Washington's good strategic sense, but to dwell on these ignores just how high the stakes have become for the Bush administration. A sustainable campaign cannot afford rash strikes which have no tangible impact, because the political support may not be available for a second try.

Moreover, a readiness to accept higher military risks should reduce the political risks, by allowing for more focussed and discriminate engagements. The excessive reliance upon air power that has marked recent operations by the US appears likely to be balanced by the employment of ground forces. If so, before much can be done, there will have to be substantial preparation in logistics, intelligence-gathering and training, as well as diplomacy, in order to secure bases and overflying rights. More than five months passed after the Iraqi invasion of Kuwait before the launch of Desert Storm.

If this analysis is correct, then we are about to witness a formidable and conspicuous assertion of American military power, which can have political effects before any decisions need to be taken on its employment. There are many places to search for the terrorist groups and they will not be easy to destroy, but their own preparations for war, which as we have learned must also be extensive and complicated, can be disrupted.

Meanwhile, those who have edged close to the enemy may start to edge away. Intensive but sensitive diplomacy can take advantage of this, and so gain critical intelligence and even access to some of the guilty parties. It is foolish to overstate Western power and what it can achieve, but it can be as foolish to understate it.

The writer is Professor of War Studies at King's College, London