JUST when they thought everything was going smoothly she emerged from the past again. The economy is looking up, MPs were returning from the Easter recess but then the ghost of prime ministers past loomed up, and at her most outspoken condemned the British, the European and the whole Western approach to the Bosnian conflict. Margaret Thatcher's own bombshell came on BBC television news at 6pm on Tuesday - it could not have been timed more perfectly - and tore right through the heart of government in the strongest language we have ever seen her use. She had, no doubt, consulted wisely, especially in the US where her call to arm the Bosnian Muslims has reached to the heart. She blamed the European Community for its prevarication over the past year - her old bete noir showing itself again. But much, much more significant was the vitriol for her own Government, for the men who, by and large, she appointed to positions of high office. The Prime Minister John Major was particularly stung by Lady Thatcher's suggestion that Britain needed a Prime Minister like Churchill with a ''lion heart'' - the inference being that he had anything but that. Once again the words of Margaret Thatcher are dominating Westminster. There are many in the Conservative party who now detest what she stands for, but there is no denying the popular mood in the country which looks back on her period in office of being at least a time of certainties rather than vacillation and one government slip after another. There are even suggestions that her time might come again if John Major continues to founder over Maastricht. After all who is there around in the Commons to replace him? Lady Thatcher demonstrated her resolve later on US television saying: ''We cannot just let things go on like this, it is evil. If these governments are not moved by the position of ethnic cleansing in Europe, two million refugees, mass graves being found in Croatia, then they should be. ''All that is required for evil to triumph is that good men do nothing. Humanitarian aid is not enough.'' She hit back at those like Defence Secretary Malcolm Rifkind who have accused her of talking ''emotional nonsense'' saying: ''I hope they do feel emotional about what's happening. And when they have felt that, a little bit of resolve on their part would not come amiss.'' The Government's position here has always been that there is no popular sympathy for intervening in Bosnia beyond humanitarian relief. But that mood would appear to be shifting rapidly following the horrors of Srebrenica. A poll for the Daily Telegraph on Thursday - carried out even before Lady Thatcher's intervention - showed 61 per cent of Britons believe it would be desirable to send an international force to Bosnia to try to bring about a peace settlement. Margaret Thatcher has once again struck the right note when so many had written her off. She has dismissed the tired rationalisations of Douglas Hurd and Malclolm Rifkind that nothing can be done apart from sanctions, and essentially accused them of lacking courage. She accused the Foreign Secretary, she herself appointed, of issuing a ''terrible and disgraceful phrase'' when he said arming the Bosnian Muslims would create a ''level killing field''. This latest attack matters more than her earlier criticism of the Maastricht Treaty, because it represents a forcible intervention in a debate none of us can run away from. Maastricht does not involve genocide, ethnic cleansing, or mass rape. Paddy Ashdown, the Liberal Democrat leader, has been expressing similar views for some months, but Lady Thatcher still retains the ability to capture the public's imagination. Westminster has become polarised - an issue the Government thought it had under parliamentary control is now running free. The arguments against increasing intervention are well known - you can't stop at air strikes, the food convoys would be ended, Russia would start arming its allies the Serbs and British and other troops would be bogged down for years in a protracted campaign which would make Northern Ireland look like an exercise in logic. The pro and anti-interventionists both argue that to act or not act could lead to a full Balkan war dragging in Greece, Turkey and Russia. David Howell, the Conservative chairman of the Foreign Affairs select committee, would like air strikes to be considered. He feels, and others agree, that it is all very well saying we don't want to get involved any deeper, but if the present policies continue, we may just end up appeasing the Serbs more and more as they spread their horror, not just across Bosnia but through the enclave of Kosovo, Montenegro and further south. But the equally influential chairman of the Commons Defence select committee Sir Nicholas Bonsor has welcomed Government caution. And Winston Churchill MP, whose grandfather was dragged by Lady Thatcher into the debate, welcomed her stance. The inevitable consequence of the present policy was that ethnic cleansing and genocide would continue. Sir Charles Powell, her former private secretary, believes her intervention will encourage a rethink on Bosnia. He may well be right and it is likely she consulted him before speaking. The final significance of her comments may lie not in their detail as in the challenge they represent to Western governments to come up with an alternative. If she has been scathing about her own government she may be hoping for sterner stuff from Washington. Away from Westminster, Margaret Thatcher knows that her real audience must be the United States, where she has arguably a greater standing than in Britain.