But how will the public react when the first British serviceman dies?
On Friday, Tony Blair made what was only the fifth prime ministerial broadcast to the nation this decade. Dressed in a sombre suit and regimental- style tie, he told the people that it was right for Britain to send forces to Kosovo because "barbarity cannot be allowed to defeat justice". It was a statesmanlike performance setting out what you could call the Government's "ethical war policy", in which conflict is justifiable because it prevents human rights abuses.

But this was the war, one close friend said last week, which the Prime Minister "really didn't want". He strongly resisted the idea when Paddy Ashdown urged him to press ahead with military action back in September. The risks of military action in Kosovo are greater than in launching air strikes against Iraq, the villain less clear cut than Saddam Hussein, the British interest less immediately visible than in the Falklands conflict, and it is closer to home than any of the other recent actions.

Downing Street is aware that public opinion is not firmly behind the raids against the Serbs and people will turn on the Government if any of "our boys" are killed. The strategists' decision to play their trump card and put Blair on the box was evidence of serious concern about these rumblings. A campaign is also under way in the tabloid press to turn Milosevic into a fully-fledged James Bond baddie. On Friday the Sun depicted the evil "Slobba" as a paranoid drunk who got through two bottles of spirits a day sitting in the dark.

But political opinion is as divided as that of the public. During the half- hour discussion on Kosovo which dominated Cabinet on Thursday, ministers were careful not to come out directly against British involvement in the action. But there were serious questions - "coded witterings" in the words of one insider - from several senior figures, including David Blunkett, the Education Secretary, who were concerned that the implications had not been fully worked through.

This is the second time that a Labour government has sent British troops into conflict (the first being recent air strikes against Iraq). Now that the Cold War is over, the traditional political alignments, with the left being pacifist and the right isolationist, have broken down. Lord Healey and Tony Benn, old Labour adversaries, have been united in opposing the Government's decision to join the Nato action. They are backed by the Tory right which is privately fiercely critical of William Hague for being too supportive of military action. Then there is the rag bag of people on left and right - from Paddy Ashdown to the anti-nuclear campaigner Mary Kaldor - who think it is foolish to launch air strikes without being prepared to contemplate sending in ground troops. We have a strange situation in which left-wingers find themselves advocating more, not less, military intervention.

What is happening in Yugoslavia is symptomatic of a fundamental rewriting of the rules of engagement to fit the post-Cold War world. Ministers say this action is legally acceptable, not because the West is under threat of invasion, or that a vulnerable nation is under attack, but because the international community has a duty to prevent an "impending humanitarian catastrophe" within a country. As the Government develops an "ethical war policy" to match its ethical foreign policy, Blair justified the Kosovo conflict by saying it was essential "to defend our fellow human beings". He told the nation: "Act or do nothing."

Ashdown says a "new doctrine" of conflict is being developed to allow "international intervention within the domestic jurisdiction".

This is particularly true of Nato, which is trying to redefine its role in the run-up to the 50th anniversary next month. It is no longer possible for the alliance to justify its existence against the common enemy of Russia. Instead, military leaders speak increasingly of an improved peace- keeping role and enhanced humanitarian intervention. The Kosovo conflict is the first test of this new identity. The Americans are also reasserting their determination to stay involved in Europe through Nato, despite the European Union's plans for its own military capability.

The Government may be rewriting the rules of war, but it is getting such opposition from politicians of all parties because it has not written the final chapter. The Prime Minister has made an argument of the legal justification for bombing the Serbs but he has failed to identify the political aim, to set out the endgame for the Kosovo region. Ministers make much of the fact that they do not want to "bomb Milosevic back to the negotiating table" but they have not said what they want to do apart from "degrade" his ability to oppress the Albanians. On the Today programme yesterday morning, Doug Henderson, the armed forces minister, was unable to define what this meant. At one level it means that the raids cannot fail - the Government will be able to claim victory by saying it has taken out Serbian military installations, and has put Slobodan, like Saddam, "back in his cage". At another level, however, it will be impossible to say if they have succeeded.