Nato's 50th birthday was to have been a proclamation of its relevance for the post-Cold War world. But ceremonial and celebration has given way to a down-to-earth working summit.

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This is a critical moment for the Atlantic Alliance. In many ways its future depends upon the outcome of the conflict in Kosovo. And this summit will be dominated by little else.

Nato has long insisted that it still has a key role in European security despite the ending of the Cold War.

Nato's old mission - the defence of its own members - is not in question, though the absence of any real threat to the Alliance makes territorial defence largely a residual, albeit an important mission. Nato's new task - the spreading of a blanket of security outwards from its own borders - has already been tested in Bosnia, where a Nato-led military force has brought a certain stability to the country.

Nato's greatest challenge

The summit will focus on how far to go in Kosovo

But Nato's new mission faces its greatest test in Kosovo, it's first war in its 50-year history.

The Kosovo crisis has demonstrated some of the Alliance's greatest strengths and also some of its weaknesses. Its military performance - within the restricted rules of engagement that have been set - has been professional and impressive. Its diplomatic performance has been less positive.

Nato in some senses stumbled into this crisis. In political terms the Alliance finds it difficult to present anything other than the lowest common denominator of it 19 members. Kosovo has once again demonstrated the need for US military clout and diplomatic muscle to be linked to that of the Europeans.

The BBC's David Shukman looks at the last 50 years of the alliance

But it has also demonstrated something of the re-balancing that has taken place within NATO. The European voice is stronger. France, Britain and Germany have all made credible military contributions.

Indeed for the French and the Germans the Kosovo crisis has contributed to making them more normal members of the Nato club. Gone is Germany's reluctance to use force and the air campaign has seen French aircraft integrated into the Nato planning in a way that would have been unthinkable only a few years ago.

Kosovo has also provided answers to some of the great questions facing Nato, not least the debate about the circumstances in which it might use military force. Nato pressed ahead in Kosovo without a specific UN mandate and ignored Russia's calls for restraint. But Washington's broader efforts to link the Alliance with its global security concerns may have been hampered by the crisis. Nato has enough on its plate as a Euro-Atlantic organisation without casting its net much wider.

What next?

General Wesley Clark, Nato's supreme commander

This summit will see the unveiling of the Alliance's new strategic concept - its mission statement for the future. It will also see lofty declarations on Kosovo, though behind the scenes real debate will take place not just on a diplomatic end-game but also on the need to begin consideration of some sort of ground campaign - if an when - the air campaign achieves its goals.

Nato expansion is the great issue left off this summit agenda. It has begun to expand its membership eastward to take in Poland, Hungary and the Czech Republic.

Alliance spokesmen stress that it is really the only credible military organisation of its kind. Nato had decided that three new members were enough for now, though its door would remain open and there will be much talk of enhanced and tailored partnership programmes for aspiring members.