UNITED Nations efforts to build a Pax Universalis - President Bush's phrase - need to take firm root in a soil of sympathetic world understanding, free of illusion and exaggerated expectations. To that end, the global audience needs to know realistically what it is reasonable to expect from the UN in the building of a new world order.The post-cold-war UN can do much that it could not do before. Freed of the distortions of great-power rivalry and the small-power maneuvers that rivalry inspired, the world organization has been taking on challenges in Cambodia, Afghanistan, and even El Salvador which had been far beyond its previous capacity. But except when, as in the case of Iraq, a major government or coalition is prepared to use military or economic power under a UN umbrella, there are only three directions in which a heterogeneous assembly of governments can make use of its greater freedom of action: * It can intensify and focus moral suasion and pressure on governments perceived to be wrongdoers. * It can provide convenient forums for negotiation of differences within a framework of principle. * It can introduce a physical buffer between adversaries willing to accept external restraint - or not willing to incur the onus of rejecting it. This is valuable. It is not to be belittled. But it is miles short of the Pax Universalis Mr. Bush envisages. There is an uncomfortable inference that what the president really wants of the UN is a blessing on future American activity on the global scene, a kind of Pax Americana established ostensibly on behalf of the UN. Even if the United States were always wise and benevolent, such equating of American interest with the will of the world community would be impracticable. Iraq is a very special case. Widely variant strands of national interest converged. The Soviet Union had an impelling incentive to cooperate; it needed Western aid. The wrongdoer had virtually no external sympathy or support. Even given the changes which have transformed the world balance of power in recent years, these are circumstances that are unlikely to be often repeated. Only once before - in Korea, in 1950, when the US also was dominant - has Washington been able to persuade the UN to hoist its flag over an American military enterprise. And only once before - in the then-Belgian Congo, in 1960 - has anyone else done so. Every crisis is different. Consider Yugoslavia. There, an aggressor (Serbia), pleading self-determination of minorities and riding a wave of virulent nationalism, invaded a much weaker province (Croatia). There was no great power whose national interests justified military intervention. European Community attempts to mediate and persuade were frustrated at every turn. One major Western diplomat, pressed to suggest remedial steps, said privately: "I guess we'll just have to let them fight it out." The UN can perhaps do better than that, but no one is suggesting a Desert Storm, and economic pressures are very hard to focus in the circumstances. Sanctions might also be counterproductive, intensifying the nationalisms that are at the root of the problem. Consider the Middle East. More than 40 years of UN diplomacy has not brought the parties together. The US, which is trying to do that, could use some effective UN suasion and pressure. But even carefully crafted UN pressure lacks the persuasiveness of perceived objectivity. The present government of Israel feels free to reject even the basis for negotiation - a swap of land for peace - to which virtually the entire world community (including previous Israeli governments) agreed. UN peacemakers are having better success in Cambodia. The challenge has been to disengage Vietnamese influence without substituting something even worse: the return of a murderous predecessor regime with ties to China. The effort seems to be working, in part because Moscow (and perhaps Hanoi) have had more reason to cultivate improved relations with the US than they have had to contain China. In Afghanistan, a UN presence is helping phase out a conflict which is anachronism in the post-cold-war era. In El Salvador, the office and person of the UN Secretary-General are being used to the same end. The bottom line appears to be that the UN can make major contributions when conditions are ripe, but needs major power help at other times. It is no reflection on the UN that this falls short of a Pax Universalis. The UN was denigrated for much too long; now, it is not a favor to the world body to oversell it.