Paris -- The ''South Slavs'' should never have been put together. To do so may have seemed a good idea at the time, but today it seems otherwise although those most concerned have yet to confront the alternatives, which could prove much worse. But when people are determined no longer to live together the problem becomes how to divide them with as little bloodshed as possible.
The two sides in Yugoslavia belong to different politico-cultural worlds, Muslim and Orthodox in the south and east, where the Byzantine and Ottoman empires dominated until the 19th Century, and Roman Catholic in the northwest, where the Slovenian people live in close proximity with Alpine German-speakers and Italians, and the Croats with Hungarians.
From the early 19th Century rival ideas existed on uniting the South Slavs. The Croatian view was that there should be a Croatian nation extending from the Alps to the Black Sea (which meant Bulgaria's inclusion as well). Serbs preferred a greater Serbia of not dissimilar dimensions.
The irenic ''Yugoslav'' idea was also put forward in the 19th Century. It said these were different but fraternal nations which ought to cooperate with one another and with their smaller Slavic neighbors. To everyone's surprise the Yugoslav idea won out in 1918.
It did so because the western Allies liked it, and because both Croats and Serbs feared the disorder left behind by war and the Croats feared Italian aggression. Serbia had been independent before the war (contributing crucially to setting it off), and possessed the administrative apparatus to run a united South Slav state. Croats and Slovenes accepted unqualified union with Serbia. This turned into a Serb-dominated dictatorship in 1929, which collapsed in 1941 under the shock of Axis invasion.
After that a Croatian fascist state allied to the Axis was created, and Serbia was partitioned.
The partisan and civil war of the 1940s was won by the Yugoslav Communists under Tito, a Croat, because the Allied powers favored him with aid and also because he mobilized a common patriotism to confront a peculiarly brutal occupation.
He was able to maintain unity after the war thanks to the federal structure he created with widely dispersed power except for his own, to his prestige as war leader and to the Soviet threat. It was possible then to believe that a genuinely multinational Yugoslavia could survive Tito's death.
This no longer is plausible. The immediate reason is the revival of Serbian expansionism under the deplorable Serbian Communist demagogue, Slobodan Milosevic. The underlying disintegrative forces are not only ethnic and historical but the economic and cultural discrepancies which persist among the component Yugoslav nations.
Slovenia and Croatia have installed democratic institutions and want independence and attachment to the European Community. Serbia wants an authoritarian federal Yugoslavia which it can dominate. The army seems to agree with that; 70 percent of its officers are Serbs. Montenegro's leaders also apparently agree. Where Bosnia-Herzegovina and Macedonia would stand in a crisis remains unclear.
Serbia has already virtually annexed ethnic-Albanian Kosovo, which Tito had made autonomous. While Croatia has renounced territorial claims on Serbia based on Croatian minorities there, Serbia has not done the same with respect to the Serbian 11 percent in Croatia. Fighting was only narrowly avoided last October when certain Serb communities in Croatia declared their autonomy and attachment to Serbia.
However, Mr. Milosevic's leadership is not uncontested in Serbia. There were disorders in March protesting his policies. Serbs sympathetic to the federal prime minister, Ante Markovic (whose economic policies have controlled inflation and promoted exports), have repudiated the idea of ''a Serbia isolated from Europe and the world by the actions of its president, and now isolating itself from Yugoslavia itself.''
There is a reasonable solution, of course. But reasonable solutions have a very low success-incidence where ethnic passions are in play. The reasonable solution, which the American and European Community governments support, is a loosened democratic confederation of Yugoslav nations with a degree of centralized economic authority a central bank, a national currency. Power over taxation, security and external relations would be given to the confederal states.
The unreasonable solution, civil war, is quite possible. A second unreasonable solution is breakup which, if civil war did not distort the outcome, would leave Croatia and Slovenia independent, Serbia presumably dominating Macedonia and Montenegro, Bosnia who-knows-where, and possibly war between Serbia and Albania over Kosovo.
Yugoslavia's neighbors prefer the reasonable solution. The actual choice may lie between the two unreasonable ones. In that case breakup obviously is better than civil war.
But the European democracies and the United States should make plain what they think, and try to bring home to the Yugoslavs the consequences of the course the country is on. These seem little considered, as if someone else would pay the price for what the Yugoslavs choose to do to themselves.
The western powers should make clear to Belgrade that the West has not time, space or toleration for ethnic wars; that such wars will be blocked if possible, otherwise isolated, and eventually punished through the economic and political ostracizing of those responsible.
If the South Slavs cannot live together, they must at least go their separate ways in peace.
William Pfaff is a syndicated columnist.