VACLAV HAVEL, president of the Czech and Slovak Republic, was spit on by angry Slovak nationalists last spring when he visited Slovakia. Last week he was pelted by eggs in the Slovak capital of Bratislava as he prepared to give a speech honoring the founding of Czechoslovakia 76 years ago.Mr. Havel is trying desperately to keep the two halves of his country together. He thinks it would be better for the Czech and Slovak republics to move into European and world politics and markets in the 1990s as a unified state. He is probably right. The Slovaks have no experience as either a political or economic entity. A Czech republic without the Slovaks is less formidable than the present united country. Yet history is not being cooperative in the old East bloc. Most leaders there, from Havel to Lech Walesa and Boris Yeltsin, are getting desperate - more desperate than is being reported. The transition to democracy from ingrained habits of centralized socialism is difficult enough. But a furious new drive for ethnic and regional independence makes the political and social situation even more uncertain and complicated. The vicious fight between Serbs and independence-minded Croats in Yugoslavia should war n the West how far matters can go. Add to this the truth that most Eastern European economies are failing badly. As seen in the Polish election last week, a strange mixture of public discontent and apathy fills the air. Certainly the desire for autonomy is understandable among long suppressed ethnic groups. Many groups have legitimate historical reasons to split - the Baltics and the Ukraine from the Soviet Union, for example. Yet the passions involved are making the process messy at best and blind and malevolent at worst. The impulse to split in Czechoslovakia, as elsewhere, is not so much democratic as nationalist and tribal. Little long-term thinking is being done by leaders in the Ukraine, or in Slovakia. Slovak politicians in Bratislava aren't interested in discussions of constitutions, federal laws, or, more important, of how much creating a new state and its institutions will cost. They just want out. Many are beginning to play a dangerous game of separatism through hate. To what end? A sad scenario presents itself: Nationalist leaders who get independence by playing on ethnic passions find that the cost of security, food, institutions, and so on are beyond them. They struggle and are replaced - by whom? Craftier nationalists? Ex-communist bureaucrats? Regional mafiosos? Internecine fighting is possible, with other European countries loath to dispel it. Leaders in East Europe, like Havel, are looking simultaneously at the brighter world of West Europe - and at a dark abyss. The US and Europe must do more. East Europe needs assurance and aid. The US can form free trade agreements with new democracies. The EC must widen; Europe's political structures must open. Now's the time - before more than eggs are thrown.