SINCE assuming office, Bill Clinton has managed to finesse US policy on Bosnia. But the Serbs' renewed campaign of terror against half-starved, poorly-armed Muslims and Croats is now pushing him towards what would be the most difficult decision of his fledgling administration: committing US forces - whether in concert with NATO allies, whether on the ground as well as in the air - to the urgent task of halting the ''ethnic cleansing'' that has claimed tens of thousands of innocent lives. Porous embargoes, toothless UN resolutions and expressions of official outrage can no longer disguise the fact that the international response to Serbia's systematically murderous aggression has been all thunder and no lightning. If Mr Clinton and his West European allies have dithered thus far, however, it is not because the situation is ambiguous. Indeed, Serb strongman Slobodan Milosevic's strategy is crystal clear: to conquer and occupy former Yugoslavia, and to expel or kill all the non-Serbs in these newly-gained territories. He will pursue these goals until they are met or until outside forces stop him. No, the reasons for US inaction lie elsewhere. There are at least four. One has to do with the fate of Boris Yeltsin. The centrepiece of Mr Clinton's foreign policy is preserving Russia's fragile reforms, reforms that depend, in his view, on Mr Yeltsin remaining in power. Forcing the Russian leader to go along with a UN vote to authorise military action against his fellow Slavs in Serbia might bolster the position of the ex-communists and ultra-nationalists trying to topple him. At home, Mr Clinton realises that there is little popular support for sending American troops abroad unless US interests are directly at stake. A short-term humanitarian mission to Somalia is one thing, but interceding in a fully-fledged civil war is another. It is not heartlessness that prompts many Americans to say: ''If these people want to kill each other, that's not our business.'' Mr Clinton is also loath to divert too much energy or resources away from the domestic agenda that put him (and only barely, at that) into office. The coming months are crucial, as Congress debates his economic package, and prepares to debate what can only be an extremely controversial health care plan. Finally, there is the ghost of Vietnam, which - eerily - rose again this week in the form of a secret document discovered in Russian archives. If authentic, the document virtually proves that the North Vietnamese held twice as many American prisoners atthe end of the Vietnam War than they have previously admitted. The US is still traumatised by its inconclusive war against communism in Southeast Asia and remains reluctant to dispatch young Americans into a distant and open-ended conflict. Ironically, however, the roles are reversed when it comes to Bosnia. Those Americans, both in government and outside, who opposed US military involvement in Vietnam are more likely to favour intervention in Bosnia. Those who supported US foreign policy in the late 60s - including most of the military - are now the ones who are gun shy. And yet, all these bulkheads for inaction notwithstanding, Mr Clinton must now feel intense pressure to intercede on behalf of Bosnia's beleaguered Muslims and Croats. Serbia's campaign is so clearly criminal, so clearly aimed at displacing or annihilating entire ethnic populations, that looking the other way is no longer morally defensible. Furthermore, if other tyrants are to be discouraged from executing similar pogroms, then the international community must stand firm against such flagrant crimes against humanity. Failing to do so poses an additional danger. If Serbia's aggression is unchecked in Bosnia, it may well proceed from there to nearby Kosovo, where Serbia has already targeted that region's large Albanian population. Bad enough that the Bosnian horror might be repeated. But a similar tragedy in Kosovo would almost certainly expand the conflict to include the neighbouring countries of Albania, Greece and even Turkey. Mr Clinton's problems would suddenly be multiplied many times over. Even on the domestic front, he has an incentive to take decisive action. Having reversed strong, principled campaign positions on a number of issues - especially US policy on Haitian refugees - the President can ill-afford to do it again. Candidate Clinton lambasted President Bush for not taking a stronger stance against Serbia, but has done little different since coming into office himself. It may be the wrong reason to do something, but it is surely a factor in Mr Clinton's political calculus. The revelation this week that a team of experts sent to Bosnia by the President's Secretary of State recommended military intervention - and that their report was suppressed - will make it that much harder for Mr Clinton to continue with his present course of talking loudly and carrying a small stick. At the very least, the President will probably engineer the lifting of the crippling arms embargo against Bosnia's non-Serbs, and the strict enforcement of the UN ''no-fly'' zone. But that will not be enough. Serbia already has a massive superiority in firepower, and it is fighting on the ground, not in the air. If he still continues his genocidal war - as surely he will - Slobodan Milosevic may well become Bill Clinton's worst nightmare; his very own Saddam Hussein.