Their position was set out in a joint statement issued at the end of a three-hour meeting on the situation in Kosovo, during which the 19 leaders were briefed on the state of the Nato operation by the Supreme Allied Commander, Europe, General Wesley Clark. Adopting an optimistic tone, he said: "We are winning. He is losing, and he knows it." This working meeting had replaced much of the ceremonial that was originally planned for Nato's `birthday' summit.
The alliance also rebuffed an attempted peace initiative, hammered out in Belgrade by Viktor Chernomyrdin, the Russian envoy, while remaining careful not to reject Russian mediation altogether. A proposal by Mr Chernomyrdin that he fly to Washington today to present the initiative in person, which was seen by President Clinton and the British Prime Minister, Tony Blair, as a ploy to gatecrash the alliance summit and weaken Nato unity, was diplomatically allowed to lapse.
The Russian overture did not meet two of the West's key demands: the removal of Serbian forces from Kosovo and the emplacement of an armed international peacekeeping force, Nato leaders said. The initiative would have ruled out Nato forces by insisting that no troops from "aggressor nations" would be allowed into Kosovo and required that any forces entering Kosovo be unarmed.
But Nato leaders sought to avoid the impression of a humiliating snub to the Russian envoy. The Nato spokesman, Jamie Shea, said the Chernomyrdin/Milosevic plan seemed to "fall well short" of Nato's demands but would be a good sign if it showed Mr Milosevic was struggling to find a way out. "The next time he gets in touch it has to be a much more substantial offer," he said.
Among the new element in Nato's position, as set out in the joint statement, was endorsement for the European Union's declaration of an oil embargo and an undertaking to explore the imposition of a naval blockade.
Such a blockade, favoured by the United States and Britain, was to be discussed further by alliance defence ministers. Several countries, including France and Italy, expressed concern at the damage that such a blockade could cause to Montenegro. While legally a constituent republic of Yugoslavia, Montenegro has a democratically elected government and has proclaimed its neutrality in the current conflict. This concern was recognised in the statement, which said that the consequences for Montenegro were to be taken into account.
Nato could decide to delay enforcing a blockade until the effects of the embargo became apparent. Companies from several Nato countries, including Britain, France and Italy, are documented as having supplied fuel to Yugoslavia's state oil company even within the last 10 days.
The statement passed over the contentious issue of ground troops, even though the failure to extract significant concessions from Mr Milosevic so far, has increased the political pressure for an armed invasion of Kosovo, especially in Britain and the United States. In a nod to France, Nato leaders also held out the possibility of a role for the United Nations, saying that a suspension of air strikes "could" follow a UN Security Council resolution "which we will seek".
The statement also hinted at a compromise over the future of Kosovo once - as Nato insists - Mr Milosevic meets alliance demands. There should be, it said, 'an international provisional administration... under which its people can enjoy substantial autonomy within the Federal Republic of Yugoslavia.' Such an arrangement would satisfy US reluctance to recognise a fully independent Kosovo - which it sees as presaging more redrawing of boundaries and further 'Balkanising the Balkans', in Mr Clinton's phrase. Autonomy 'under the FRY', however, would go some way to satisfying Kosovar Albanian requirements to be freed from Serbian suzerainty. Kosovo is currently a province of Serbia, rather than an autonomous republic of Yugoslavia - like, for instance, Montenegro.
The mood of the day, however, was sombre, and the gravity of the current situation was underlined by the Nato Secretary General, Javier Solana, at the opening session. "As we meet in Washington today," he intoned solemnly, "Europe is confronting a very serious crisis. Images of hundreds of thousands of deported people, burnt homes and destroyed villages recall images we hoped never to see again."
President Clinton for his part stressed the moral crusade on which Nato believes it has embarked, saying: "We are in Kosovo because we want to replace ethnic cleansing with tolerance and decency; violence with security; disintegration with restoration; isolation with integration into the rest of the region and the continent."
Tony Blair said: "Reversing the hideous policy of ethnic cleansing is the best anniversary memorial NATO could have." The only dissonant note was struck by the Italian Foreign Minister, Lamberto Dini, who publicly condemned the overnight missile strike on the Serbian broadcasting building, saying that it was "not in the plans".
While most of the pomp and circumstance of the planned anniversary celebrations had been progressively abandoned over the past month as the conflict in Kosovo escalated, one formal ceremony remained. Military fanfares and a forest of flags greeted the leaders in the Mellon auditorium in the afternoon, where they signed a Washington Declaration and commemorated the signing of the treaty that created Nato on 4 April, 1949, at the height of the cold war.
This was a rare lightening of the prevailing mood, however. A better reflection was the weather in Washington, which deteriorated through the day from almost bright to dismal, with a violent thunderstorm on the way. The favourite cliche among American pundits was that a birthday party had become a council of war. For the truly pessimistic, a wedding had become a wake for an alliance whose time and credibility were exhausted.
Nato, which had prided itself on never having engaged its troops in any conflict on the continent of Europe, and on keeping the peace through the deterrent force of military strength, was marking its 50th anniversary at war. A generation of leaders, many of whom cut their political teeth in the anti-war movements of the Sixties, had ordered troops into the first serious armed conflict in Europe since the Second World War.
When Bill Clinton, Tony Blair and the other allied leaders assembled at the Ronald Reagan International Trade Center in Washington, their serious expressions and dark ties spoke volumes. Even the US Secretary of State, Madeleine Albright - who usually sports outsize hats, outsize brooches and telegenic primary colours - was in charcoal grey.
A security cordon in operation for almost a mile around kept away all but delegates and journalists. Until Sunday, this is a secure area in a capital that is talking war.
t President Milosevic must:
i) Ensure a verifiable stop to all military action and the immediate ending of violence and repression in Kosovo;
ii) Withdraw from Kosovo his military, police and paramilitary forces;
iii) Agree to the stationing in Kosovo of an international military presence;
iv) Agree to the unconditional and safe return of all refugees and displaced persons, and unhindered access to them by humanitarian air organisations;
v) Provide credible assurance of his willingness to work for the establishment of a political framework agreement based on the Rambouillet accords.
t As long as Belgrade fails to meet the demands of the international community, air operations against the Yugoslav war machine will continue.
t Allied governments are putting in place additional measures. These include intensified implementation of economic sanctions and an oil embargo.
t Nato is prepared to suspend its air strikes once Belgrade has accepted the above conditions and begun to withdraw its forces from Kosovo.