THE United States should join Germany and others in recognizing Slovenia and Croatia as independent states.First, America's principles support national self-determination and the right of any people to part with their government when it becomes "destructive" to the aims for which it was established. American presidents have often waffled on this principle, preferring not to offend imperial regimes in Paris, Djakarta, and Moscow. But there is no realpolitik or moral reason to worry about the sensitivities of the Serbian regime in Belgrade. Second, Slovenes and Croats have long seen themselves as distinct nations. For years they have resented Serbian domination. Any people believing itself a nation is one and has a right to shape its destiny. Third, whether or not Slovenia and Croatia are viable economically is for them to decide. Other small nations - Singapore, Switzerland - do quite well, even though they must trade for many ingredients of modern life. Of ex-communist societies, Slovenia and Croatia have perhaps the best prospects for trade with the West. Absent Serbian blockades, their ports give access to the seas; a sympathetic Austria lies a short distance away by rail and road, as does Italy. Meanwhile, independence need not prevent t heir trading with other ex-communist states or the third world. Fourth, violence and sullen poverty are more likely within the Yugoslav Federation than outside it. A Yugoslav union or federation was attempted between the two world wars and since 1945 with the same result: domination by the largest nationality, the Serbs. Independence would undercut the prospects of violence in the long run and even in the near term. Many analysts warn not to recognize the two breakaway republics while peace efforts continue, because Serbia may counter with even greater violence. But multiple "cease-fires" and ongoing peace talks have done little to restrain Belgrade's generals from trying to break Croatia. The longer that Croatia's recognition is delayed, the more time the Serbian forces have to expand the area under their control. As long as the situation remains undefined, Belgrade will use its superior military power to expand its turf. When Ljubljana and Zagreb win recognition, Belgrade will not so readily behave as though might makes right. How should the US recognize these states? It should do so conditionally - contingent upon Croatia's dealing constructively with the Serbian enclaves on its territory. This could take three forms: (1) plebiscites giving disputed regions a choice to stay with Croatia or join Serbia; (2) establishment of special arrangements to ensure full rights to any minority enclaves voting to remain in Croatia; (3) a right to resettlement in Serbia. Each solution presents major complications. Some Serbian enclaves in Croatia are remote from Serbia. To link them by corridor to Serbia would cost Croatia a good deal of territory. Whatever happens in Croatia will also affect Serbian-populated regions in neighboring Bosnia-Herzegovina. If Serbs choose to remain within Croatia, legal guarantees may not suffice to ensure their safety. The third approach - exodus - would be expensive. Still, this expense is just one of many bills that will fall due when the fighting stops and recovery begins. Since Germany is already burdened with assimilating the former East Germany, Austria should lead in rebuilding Slovenia and Croatia and underwriting resettlement in Serbia. Vienna once ruled these parts as part of the Austro-Hungarian Empire, and Austria owes the world something for the prosperity it has known since 1955. Washington and the European Community might also condition their acceptance of Slovenia's and Croatia's independence on those republics' acceptance of permanent neutrality and far-reaching arms limitations, as Austria did in its 1955 Constitution and State Treaty. These documents provide a good model for the nations emerging from the ruins of communism. Serbia should also be pressed to accept such limitations, perhaps in exchange for economic aid. By now all Balkan nations should be alert to the advantag es of peace and the costs of war. Washington should deal constructively with Slovenia and Croatia, not being the last to hop onto the trolley. It should set precedents to guide others out of the chaos brought on by the implosion of the former communist bloc.