The leaders of the 19 NATO countries gather today in the Mellon Auditorium between the White House and the Capitol to commemorate the signing here 50 years ago of the treaty which set up the North Atlantic alliance to defend western Europe against the Soviet Union and communism.

Today, there is no Soviet Union, while communism has all but disappeared. But there is Slobodan Milosevic and Kosovo and this means the NATO summit, in the words of President Clinton's national security adviser, Sandy Berger, will be "a working meeting of a military alliance in the middle of a conflict".

There will be no "triumphalism". The US air force fly-past has been dropped. The black tie banquets have been downgraded so that the contrast with the suffering refugees will not be too marked.

A special session today on Kosovo will assess the impact of the four weeks of air strikes, the plight of the refugees and what happens next. An oil embargo on Yugoslavia and the contentious question of a possible NATO ground force invasion of Kosovo will also be up for discussion.

President Clinton also wants the leaders to plan for a post-Milosevic era in south-east Europe when economic aid and security arrangements will give the region a stability it has lacked since the collapse of the Ottoman Empire. An economic package modelled on the Marshall aid plan of 52 years ago and involving the US, the World Bank and the EU will be discussed.

Kosovo and its sufferings were not on the original agenda for the summit. This was to celebrate NATO's 50 years, which have seen the ending of the Cold War without the alliance having to fire a shot, to welcome the three new members, Poland, Hungary and the Czech Republic, and to agree on a blueprint for the future.

This "new strategic concept" will allow new missions for NATO outside its own territory and against new kinds of threats such as terrorism and drugs. The wording will be vague because the US has ideas on a "global role" for NATO which make the European partners uneasy, such as policing the Middle East, the Gulf and North Africa.

The delicate subject of whether UN Security Council approval would be needed for new missions may also be brushed over as the US and Britain point out that the NATO air strikes would have been vetoed by Russia and probably China if UN sanction had been sought.

The summit will also bring together the 19 NATO members and the 25 members of the Partnership for Peace which was designed to open links between the former Warsaw Pact members and NATO but has since attracted the European neutrals such as Austria, Switzerland, Sweden and Finland but not Ireland. The Government has now indicated its intention of joining PfP without the need for a referendum which Fianna Fail originally said it would hold.

A big question mark hangs over Russia, which is a member of PfP but has its own special link with NATO through a Permanent Joint Council. Russia objects strongly to the NATO air strikes on Yugoslavia and has been reluctant to be part of the Washington summit.

Further enlargement of NATO will be approved in principle. Because of Russian sensitivities, it is not the time to designate the next members, although the three Baltic states and Romania, Bulgaria and Slovenia are pressing their claims.

Other NATO documents will be approved. One will deal with the threat to NATO from weapons of mass destruction which include chemical and biological as well as nuclear ones.

There will be an updating of the tricky relationship between the US-led NATO and an embryonic "European Pillar" whereby the Europeans can sometimes handle their own security and defence as the Western European Union but under a watchful American eye.

When the Cold War ended and the Soviet empire dissolved, the question of NATO's own future was at stake. Set up to ensure "collective defence" under Article V of the Washington Treaty against any attack on the territory of its members, was the alliance still needed when there was no longer a Soviet threat?

NATO said yes, as Russia still had a huge nuclear arsenal even if it had no inclination to use it. Under American prodding, NATO risked Russian distrust and opened up to new members. It established PfP as a half-way house for prospective members and as a way of co-operating in "crisis management" situations for other countries, including Russia, which did not seek to join NATO.

The violent break-up of Yugoslavia was the first test of NATO's new mission of ensuring stability all over Europe. After much hesitation, NATO took decisive action in Bosnia's ethnic conflict in 1995 before the alliance's credibility was totally undermined. As Robert Hunter, a former US ambassador to NATO, puts it in the current Foreign Affairs: "In a very real sense, a Bosnia that had almost destroyed the alliance proved its salvation."

This led to the Dayton agreement involving Bosnia, Croatia and Serbia, which had President Milosevic's blessing. But now NATO has been drawn into armed conflict with Mr Milosevic over Kosovo.

The alliance's credibility is again on the line in the Balkans. It cannot afford to fail. That will be the message coming from the Washington summit.

Senator Joe Biden, the ranking Democrat on the Senate Foreign Affairs Committee, says: "If we do not achieve our goals in Kosovo, NATO is finished as an alliance."

The US Secretary of State, Ms Madeleine Albright, insisted this week that "our alliance consensus is rock solid and NATO operations will continue until this crisis ends on just and durable terms."