IT is one of the accidental symmetries of history that ethnic tensions in southern Europe have been the alpha and omega of the 20th century.As the century began, nationalism among the splintered ethnic and national communities of the Balkans drew the great powers of Europe into crisis and eventually into the carnage of World War I. As the century ends, a reawakened nationalism among many of the same ethnic communities threatens Europe again. Few experts believe the result this time will be another major war. But as Yugoslavia unravels and as captive nationalities in several other countries clamor for autonomy or independence, ethnic strife could prove the biggest obstacle to the creation of new security arrangements in post-cold-war Europe. Warring Serbs and Croats have broken 12 cease-fires in four months, so European Community ministers will meet today to decide whether to suspend a peace conference underway at The Hague and impose economic sanctions against Serbia. "If the EC can't find a solution, it will limit the chances of developing a common security and foreign policy," says the Rand Corporation's Stephen Larrabee. "It could retard the whole process of European integration." The crisis has exposed EC divisions over whether to recognize the breakaway republics of Croatia and Slovenia. The absence of a military force to back up its peacemaking efforts has also demonstrated the EC's limitations in coping with ethnic conflict. Failing to solve this new Balkan crisis could set back efforts to translate next year's economic union into closer political ties. "If the EC fails to get its act together, they will have to admit to themselves that they can't have a political union," says the Brookings Institution's Wolfgang Reinicke. But he adds: "The Balkan crisis has been useful in propelling the EC into action and clarifying differences." If the EC does find a way to bring the conflict in Yugoslavia to an end, the lessons could prove transferable. "The implications would be important for setting a framework for the resolution of conflicts yet to come in post-Soviet regimes," says Stephen Burg, a professor of politics at Brandeis University. The Yugoslav crisis has effectively ended an experiment in conglomerate statehood arranged by the victorious powers at the end of World War I. Meanwhile, other national and ethnic rivalries are smoldering just below the surface. Some, like strife between Czechs and Slovaks, exist within national borders. Others, like tensions created by the presence of a huge Hungarian minority in Romania, cross national boundaries. Beyond the Balkans, ethnic and national conflicts lurk in the ruins of the Soviet Union. The result, Dr. Larrabee notes, is that the center of crisis in Europe has shifted south and east, away from the lines in central Europe that for most of the century have divided opposing French- and German-anchored alliance systems and, during the cold war, the armies of NATO and the Warsaw Pact. As they compare the pattern of ethnic disturbances that have framed the 20th century, diplomatic analysts say there are instructive differences as well as obvious similarities. For example, while it was the overattention of the European powers that exacerbated the nationalities problem before World War I, it is now their inattention that has allowed ethnic strife to flourish. Absorbed in the current process of economic integration, the EC nations have been drawn into the Balkans only reluctantly. "Before World War I, Western Europe had military and strategic interests in the region," says Ernest Petric, who represents the Slovenian republic in Washington. "Now its main interest in the region is stability." The benign role of the European powers and the end of the cold war have produced another paradox: Once seen as a catalyst of regional tensions, the great powers are now viewed as the only hope of defusing them. "For hundreds of years the Balkan people have been trying to get the superpowers out of the Balkans," Larrabee says. "Now they would like greater involvement by the superpowers, especially the US and democratic Russia, to regulate these conflicts." Experts also note the irony that tensions in southern Europe have been inflamed by the very process of democratization urged by the West during long years of communist rule. As ethnic groups find their voices in the politics of post-cold-war Europe, they are increasingly raised in favor of self-determination that could presage a dangerous period of destabilization. Forty-five years of Communist rule muted ethnic tensions in the polyglot states carved out of the former Ottoman and Austro-Hungarian empires following World War I. But as soon as communism collapsed, competing ethnic and national groups picked up where they left off. The result is that the unsolved problems of the past have come back to haunt the architects of the "new" Europe. Unlike the nationalism of the past, which was essentially a mass, peasant-based, anti-foreign movement, the movements sweeping southern Europe today have deeper economic roots. "Nationalism has become the expression of elites with a vested interest in having their own economy, their own economic system, their own markets," explains Dr. Burg. "If the people leading national movements also have these interests, there is still some hope for stemming violence," says Burg. "You may find material interests driving them together again." But as Burg acknowledges, such optimism runs hard against the animosities that, especially in Yugoslavia, pose alternate risks to stability in the region. On one hand, Croatian and Slovenian separatist movements threaten to unleash an uncontrollable process of fragmentation. On the other hand, the ambition of Serbian nationalists forcibly to gather Croats, Albanians, Macedonians, and Muslim Slavs into a "Greater Serbia" threatens to exacerbate ethnic hatreds, create refugee problems, and trigger cross-border incidents. "If you allow Serbia to pursue its conquest, you establish the principle that a minority can be terrorized by a majority," says Johns Hopkins University geographer Jack Fisher. "If you don't, you open the door to endless ethnic conflicts. It sets a bad precedent either way."