One of the reasons that Nato embarked on military action against the Serbs 50 days ago was to preserve its own credibility.

To do nothing after issuing so many threats, it was argued, would be disastrous for its ambition to be the guarantor of civilised stability throughout Europe.

David Shukman: "No confidence at how long the campaign will last"

But the campaign of air strikes has failed so far to bend President Milosevic to Nato's will.

The new Strategic Concept adopted by the Nato summit in Washington last month says it will strive to secure a just and lasting peaceful order in Europe.

That is an aim that goes beyond simply deterring an attack on any of its members. But it is also a more complicated and controversial concept.

Nato's role extended

Since the fall of communism and the end of the Cold War, Nato has seen its role as extending security into eastern Europe.

The three former communist states to join the Alliance so far - the Czech Republic, Hungary and Poland - looked forward to the safe haven of the world's most powerful military organisation.

Instead they found themselves on the perilous edge of an undeclared war.

The ethnic violence being perpetrated in Kosovo was intolerable to western democracies - it highlighted the Balkans as the prime blot on the civilised landscape of Europe.

The big powers resolved to get involved earlier than they had done in Bosnia, and started making demands on Belgrade and setting deadlines in the early part of 1998.

Credibility problems

As the months passed and no action was taken to enforce those demands, more and more questions were asked about whether Nato was serious.

Preparations were made for a large-scale air campaign, though many doubted publicly whether it would be effective.

In the end, after the failure of the peace talks, the doubts were ignored. Nato simply had to do something: its credibility was at stake.

Serb propaganda compares Nato to Nazi Germany

After 50 days of air strikes, the doubts have grown and the criticism has mounted. The bombing has clearly done great damage to the Yugoslav military and other infrastructure.

But it has not been able to stop the killings and the mass expulsion of the Albanian population of Kosovo.

Most important, it has not yet forced any change of policy or real concessions by Mr Milosevic; in other words, it has had little or no political effect.

To try to increase the pressure on Belgrade, Nato has steadily widened the target list away from things which are indisputably and exclusively military.

Civilian casualties have been limited, but they have inevitably increased. The attention of the media, denied first-hand reporting of atrocities inside Kosovo, has often focused on the mistakes of the bombing.

A 'clean' war

Every strike that kills civilians has tended to undermine the concept of a just war, undertaken not out of self-interest but to defend an oppressed and vulnerable people.

Such incidents are made more damaging by the perception that Nato's military are themselves running relatively little risk. It is not heroic to drop bombs from 15,000ft.

In an effort to minimise casualties to their own troops, Nato governments announced from the start that they would not fight a ground war.

This has been widely criticised as a big mistake, at least psychologically, since it removed the most potent threat to the Serb military operation in Kosovo and gave Mr Milosevic a freer hand.

Ground forces

Civilians have also been caught by the bombs

A few weeks ago some Western governments, notably Britain, were hinting that there might be a ground operation, provided that Serb forces in Kosovo had been so damaged by the bombing as to be incapable of offering serious resistance.

At that stage Western public opinion appeared to be more in favour of using ground troops than their political leaders.

But now that option is not being talked about at all - it appears to be off the agenda.

There could be no ground war without full American participation, and President Clinton has shown no inclination to give a lead in that direction.

Paradoxically, events up to now have demonstrated the weakness of the United States, the greatest military power in the world. Advanced technology has only limited value without the will to run real risks on the ground.

So the only military strategy on offer is to intensify the air strikes. The dangers of that were demonstrated by the astonishing blunder of bombing the Chinese Embassy in Belgrade.

This also had the effect of complicating the effort to secure a Security Council resolution setting out the framework for a settlement - including the deployment of an international military force in Kosovo.

Back to the UN

In any event, some see the recourse to the United Nations as a setback. Nato took the view that it had the right to take military action in Europe without specific Security Council authorisation.

The United States has sometimes been openly contemptuous of the UN. A settlement backed by Russia and China would almost by definition be unsatisfactory to the West.

The problem for Nato is that it has set out its aims in Kosovo without any ambiguity.

Western politicians insist that they will reverse ethnic cleansing and ensure the safe return of all refugees to their homes. But how that is to be achieved becomes less and less clear.