Two weeks of bombing has not persuaded the Serb leaders to give in. How much is enough?
The man in charge of Nato's air war over Kosovo left no doubt of his intentions in interviews on Friday night. "We're going to intensify and tighten the pressure on Milosevic and his instruments of power,'' said General Wesley Clark, the Supreme Allied Commander in Europe.

And the duration of the campaign would depend on the Serbian leader. "It is to some extent a function of his threshold for pain and loss. It is a function of the coherence of his top leadership cadres," he said. "And it's a function of the ability of the Serb military and police forces to sustain the kind of punishment that is being inflicted and will be inflicted increasingly on them."

In other words, what Nato is seeking is a theoretical point beyond which the Serb leadership and military will not go: call it the pain threshold. Yet, despite two weeks of bombing, neither point has been reached. And the question that remains, after two weeks and tons of high explosive, is: how much is enough?

The campaign was never designed to destroy Serbia and bomb the country into total submission. That was ruled out by a desire not to destabilise the region, to leave Slobodan Milosevic a way out, and to limit the forces involved.

Initially, the aim was to persuade Belgrade to reverse its course and accept the Rambouillet agreement, with the threat of worse things ahead if it refused. Then it shifted towards damaging the ability of the military to carry out operations in Kosovo, and persuading military commanders that if they did not stop then their forces would be destroyed. Both assumed that force would lead to a shift of policy in Belgrade. The key was not just to do damage, but to get it right quantitatively and qualitatively - and that required very precise calibration of force to achieve a policy goal.

The economy has suffered. Petrol is hard to come by; the dinar has fallen by more than 10 per cent; salaries are not being paid. But as Professor Steve Hanke of Johns Hopkins University points out, "the Yugoslav economy was already deeply dysfunctional". Conventional economic activity had all but stopped; the smuggling of drugs, cigarettes and arms, stolen cars and other forms of illicit business had taken over. The currency had virtually ceased to mean anything, and most significant transactions took place in Deutschmarks.

In any case, the aim was not to destroy the economy and crush the population: Nato has always insisted that the Serb people are not the targets. The point of hitting economic targets was to cut off the military - its supply and logistics. The two main Yugoslav refineries have been so badly damaged that they are not operating, Nato says. "In total, the military and strategic reserve fuel storage capability has been reduced somewhere over 20 per cent of capacity," claims the Pentagon - perhaps more, given the damage to transhipment and pumping facilities.

Bridges between Kosovo and the rest of Yugoslavia have been cut; so have lines of communication that would allow oil to be brought from Vojvodina to be refined. "The road traffic ... is disrupted but not completely choked off," said Rear Admiral Thomas Wilson of the Joint Staff. "We'll never completely choke it off, and the idea is, of course, to reduce the daily through-put that they can get on these roads and force them to go into not only passes and roads where they may have a much slower pace of operations, but also where we might be able to engage them better.

"We believe that we can reduce by more than half on these routes the amount of through-put capacity going into Kosovo or potential for going into Kosovo on a daily basis. We're continuing to see strong evidence ... of supply problems and fuel problems plaguing the Serbian forces in a number of different areas and among a number of different units."

The weakest area has apparently been in hitting the Serb forces on the ground. Plenty of damage was done to fixed facilities such as headquarters offices, marshalling points and other buildings. But it is unclear that anybody was actually there when the bombs and missiles landed. Attacks on targets in the field were virtually ruled out by the weather. Until Monday, Nato was reluctant to say that it had hit a single military vehicle.

Even when the weather cleared up, the amount of damage done was apparently small. The Serb forces are acting in small groups; they are usually positioned among civilians;

they have hidden their forces and dispersed them; and the engines are often turned off when inactive, making infra-red targeting less effective than it was in the Gulf. By Friday, the weather had turned again.

There is no consensus on what damage has been done, nor on its significance. Some analysts think that the regime has suffered very badly, and that Nato is close to success. "It is working. The signs are all around," said James Gow, of the Woodrow Wilson International Center for Scholars in Washington.

He cites Belgrade's attempts to reach out to the Russians and Cyprus to broker deals, the proposal for an Easter ceasefire and the end of attempts to expel the Kosovo Albanians. The attacks on highly visible targets in big cities help to show people in Yugoslavia that Nato is getting through, he says. More will be required, but the air offensive is heading in the right direction.

Nato was underlining at the end of last week that Kosovo was now cut off from the rest of Serbia, that air defences were badly damaged, and that attacks were being pressed home against ground units. But the message that their goals are being achieved does not mean that they will stop: the aims, repeatedly stated, are that before the bombing will end Mr Milosevic must himself shift his policy, removing his forces, allowing refugees to return and accepting a new political deal for Kosovo. Whatever has been achieved, that still remains out of reach.

Anthony Cordesman, a military analyst at the Centre for Strategic and International Studies in Washington, believes that the campaign is fundamentally flawed. "The Serbs do not rely on fixed bases and massive depots - the targets Nato is bombing," he wrote last week. "The entire military doctrine of the former Yugoslavia was based on a belief of no forward defence against a massive Soviet air and land attack, followed by quick dispersal, defence in depth and fighting a war of attrition." If that is right, then it would be very tough to push the air attacks far enough to achieve the magic blow required.

After the Nato air attacks, the Yugoslav authorities had their own strategy, and it was not a symmetrical response. Their strategy has been to "fight the battle Serbia wants to fight while Nato fights the battle it wants to fight", said Mr Cordesman. In the process, they have created "new facts on the ground" to which the alliance has been slow to react. Nato rapidly established air superiority, it carried the conflict to the strategic centres of the regime and it targeted forces on the ground. But it was unable to shift the Serb position.

There may be many reasons why the campaign has not yet achieved its goal. Western diplomats say repeatedly that they always anticipated the strategy would take time, though they are starting to admit that the campaign has taken a number of turns that they did not anticipate. The weather badly hampered the ability of the Nato air forces to put aircraft into the sky. Once they arrived over the target, they could not always drop their ordinance, and when they could, they were often unable to use the precision-guided munitions that were judged so essential to the campaign.

Nato has also said - curiously - that the terrain does not lend itself to an air campaign. "It is hilly terrain. There are a lot of trees," said a Pentagon official last week. "It's not open terrain; there aren't clear lines of where the bad guys are and the friendlies are." Presumably that, at least, must have been factored into their planning.

But the calculations which drove the campaign relied crucially on the political behaviour of the Yugoslav regime. If there was a gamble that Belgrade would crumple quickly under pressure, it was wrong. If it was believed that anger would rise towards the government as the bombs fell, that has backfired badly. And while the bombing of Serb forces in the field may ultimately keep them from further action, they have already done much harm.

In the end, if Operation Allied Force is not achieving its goals, then the reasons lie in a miscalculation of the pressure point as much as in the pressure applied.