NEVER AGAIN! The determination not to repeat the mistakes of the past can be one of the most creative impulses in human affairs. It founded the British welfare state, Nato and the United Nations, among other valuable institutions. But it can also trammel the minds of policy-makers - and generals - and ensure that they are always fighting the last war.

Just because we failed the Bosnians it does not necessarily follow that we should do in Kosovo what we failed to do in its Balkan neighbour. But all the evidence is that the lessons of Bosnia do in fact apply. In the case of Bosnia, the prevailing mood of public opinion was "Why should we care?". At least, until Martin Bell added the phrase "ethnic cleansing" to our vocabulary. The same isolationist tendency was alive and in boorish voice on the Conservative benches of the House of Commons last week when the Prime Minister's outline of possible action in Kosovo was interrupted by the cry: "Why bother?" But we learnt why last time. It was not sentimentality which took fright at war on European soil, it was self-interest. A small "civil" war in the Balkans could have ignited neighbouring nationalisms, including that of Russia, and reopened ancient antagonisms, including that between Greece and Turkey. As it was, the West acted too late to save the lives, homes and peaceful democratic co-existence of thousands of Bosnians. At least it prevented the conflict from spreading. Or spreading far. One consequence of the carving-up of Bosnia was the destabilisation of Kosovo next door, which is nominally a province of Serbia, but has a separate identity among its ethnic Albanian people.

The situation in Kosovo does not exactly parallel that of Bosnia. But the threat comes from the same source: Serbian aggression. And, because of Bosnia, this is a phenomenon we know something about. It is a peculiar form of aggression, similar to Russian attitudes - which may explain part of the fellow-feeling between the two countries as much as history and pan-Slavism. It should be understood, but not appeased. It should be made clear to the Serbians that their long-term interests are best served by reaching a settlement with the people of Kosovo, and that further repression not only invites the use of force but makes it more likely that they will lose Kosovo in the long run.

That is why we support Tony Blair and Paddy Ashdown in their reversal of the post-war assumptions of British politics, presenting Labour and the Liberal Democrats as the war parties while the Tories revert to their even older tradition as the party of appeasement. Mr Ashdown gets the credit for being right about Bosnia first, although one of Mr Blair's first actions as Labour leader back in 1994 was to appoint the interventionist Robin Cook as foreign affairs spokesman. Since he became Prime Minister, Mr Blair has taken a robust line in respect of Iraq's obligations under international law, and shows signs of being as adamant about the rule of law closer to home. Meanwhile, Michael Howard's pathetic, all-purpose response for the Tory opposition was that the Government should ensure that "any intervention should be based on clear and realistic objectives". As if anyone would suggest it should not be.

But Mr Ashdown is right to keep up the pressure on Mr Blair, and not just because he wants to be Foreign Secretary. The Liberal Democrat leader drew attention to the "chilling" rerun of six years ago, when Britain, then, as now, president of the EU, called a conference to discuss Bosnia. Now Mr Blair has called another conference - but this one must be an adjunct to action. So far, Mr Blair and George Robertson, the Defence Secretary, have talked of "sending strong signals", "watching very carefully" and "examining military options". These may be the right words, but they are only words, and what the people of Kosovo need is the threat of force to defend them. They need an international presence in Kosovo to observe what the Serbian authorities are up to, backed up by the threat of punitive military action if it turns out to be no good.

The case law of the new world order is gradually dissolving the sharp distinction between the internal affairs of states, which are excluded from international action, and conflicts between states, which are not. That applies as much in Kosovo as it does in Eritrea or Iraq. After the ethnic cleansing, Srebrenica and the war crimes of the Serbian generals, western Europe should say never again. And mean it.