Justifying Nato intervention on a sovereign state for actions within its borders, President Clinton said that if people had heeded Winston Churchill's warnings sooner, the world might have stood up sooner to Adolf Hitler.
Preparing the American public for full-scale intervention in Kosovo, including the possibility of casualties, Mr Clinton issued this threat to the Yugoslav President, Slobodan Milosevic: "If he will not make peace, we are willing to limit his ability to make war ... We will limit his ability to win a military victory and engage in ethnic cleansing and slaughter innocent people. Like any other military action, there are risks in it." But he warned: "The dangers of acting must be weighed against the dangers of inaction."
Mr Clinton was addressing an audience of civil servants in Washington in a speech that had been planned to deal with domestic affairs, but was hastily rewritten to incorporate a comprehensive explanation and defence of his administration's policy in the Balkans. Couched at times in the language of an elementary textbook, the speech was an attempt by Mr Clinton to counter criticism that he had not convinced Americans of the need for intervention in Kosovo, and it was broadcast live by all the main US cable news channels.
Shortly beforehand, US television had relayed from London, Tony Blair's statement in the Commons and the lively exchange that followed - a transatlantic televisual demonstration of alliance solidarity as the last preparations were completed for action.
While a test of leadership for both Mr Blair and Mr Clinton, the political calculations were different for each, and the precise objectives from military intervention remained to many unclear.
For Mr Blair, a prime concern was to refute claims that he was once again acting as President Clinton's poodle by deploying the Royal Air Force alongside US jets. The emphasis was on the joint Nato action, which brings together a dozen Nato countries and 400 planes.
He also stressed the cohesion of the alliance. "To walk away now would not merely destroy Nato's credibility, more importantly, it would be a breach of faith with thousands of innocent civilians, whose only desire is to live in peace and who took us at our word," he told MPs in a passionate defence of the imminent action.
Mr Clinton, too, presented the credibility of Nato and US warnings to Mr Milosevic as the overriding factor, with the accompanying fear that threats that proved empty towards Belgrade would undermine US credibility elsewhere in the world. Like Mr Blair, he dwelt in his speech on the violence and atrocities committed by Serbs against Kosovars, and drew a parallel with Bosnia to illustrate the penalties of intervening too late.
But he also extended his argument into a defence of the transatlantic alliance, and how it helped America - speaking of the need to safeguard "a Europe, safe, secure, united, a good partner for trade, and someone who will share our burden in solving the problems of the world".
The chief risk for both leaders is that Nato forces could become bogged down in the Balkans. Mr Blair was adamant that any attacks would be limited to air strikes, adding that the purpose would be "to reduce Milosevic's ability to suppress his people". Mr Clinton left such technicalities to the Pentagon, perhaps for fear of fuelling already strong opposition in Congress to the idea of military intervention. Although the White House pep talk won the support of a majority of Republicans, a significant number remained unconvinced.
The Pentagon spokesman, Kenneth Bacon, insisted that there was a strategy: "We have plans for a swift and severe air campaign." he told reporters. "We believe this will be painful for the Serbs. We hope that relatively quickly ... the Serbs will realise they have made a mistake." His inference was that the purpose of any attacks was to push Mr Milosevic into agreeing the terms for Kosovo autonomy that they rejected at last week's Paris talks. But this was never set out definitively, either by Pentagon officials or by the White House, and if Nato forces become bogged down, Mr Clinton will have hell to pay with Congress.
The dramatic mid-Atlantic turnaround yesterday by the Russian Prime Minister, Yevgeny Primakov, who had been on his way to the US, was a graphic illustration of the diplomatic risks involved. Apart from the damage to relations between the two countries, Mr Primakov's standing in Moscow could also be affected - destabilising the political situation there still further.