THE war now raging between Serbs and Croats in Yugoslavia, the escalating violence inflicted on Turks and other foreigners by neo-Nazis in Germany, and the ongoing tensions between Czechs and Slovaks in Czechoslovakia are not mere vestiges of the past. The ethnic clashes that were endemic in the old Europe have returned and may doom the efforts to create a peaceful new Europe.A major poll of European attitudes conducted earlier this year by the Times Mirror Center for the People and the Press offers frightening evidence that the end of the cold war has liberated Europe to return to its fierce past. Hour-long interviews with 13,000 people across Europe revealed a depth of hostility toward ethnic minorities unknown in the United States, despite our much-publicized efforts to combat racism. In each of four countries in the East - Czechoslovakia, Poland, Hungary, and Bulgaria - 40 percent or more volunteered that they held unfavorable opinions toward the principal minority people of that nation. For instance, 41 percent of Poles said they dislike Ukrainians, and 49 percent of Czechoslovakians said they disliked Hungarians. The situation was little better in the five countries - Britain, France, Germany, Italy, and Spain - surveyed in Western Europe. In Germany, only 31 percent had anything nice to say about Poles. In France, less than half the population reacted favorably to North Africans. By contrast, only 13 percent of white Americans say they hold unfavorable views of blacks. The country with the greatest potential for ethnic clashes similar to those in Yugoslavia is Czechoslovakia. Czechs and Slovaks are at odds on fundamental issues and neither like nor trust each other. They agree only that the greatest threat to the country comes from each other. European politicians like to believe that the territorial wars that have torn the continent apart have been condemned to the scrap heap of history. Yet the past may be very much alive in the present. Many Europeans feel that parts of their country lie within a neighbor's borders. Nearly seven in 10 Hungarians, 60 percent of Poles, and 52 percent of Bulgarians feel this way. Half a century after Hitler's quest for Lebensraum sent Europe up in the flames of World War II, 39 percent of Germans still express interest in neighboring lands. For the most part, European leaders have responded by shaking their heads and insisting that the new Europe will not tolerate ethnic clashes. But in Yugoslavia, the European Community has yet to back its words with concrete action. Instead of sending tanks to enforce peace, it has sent emissaries to broker quick agreements. What will Europe do if the Yugoslavian experience is repeated in Czechoslovakia? Or if intolerance in Poland or Germany flares into violence? Embers of a Europe thought buried are still alive. Extinguishing the flames of hatred once and for all cannot be done by pretending they do not exist, or by giving up on dealing with its results. If a new Europe is finally going to emerge from the ashes of the old, European leaders will have to actively intervene to protect minorities. So far, based on the evidence in Yugoslavia, the future looks anything but bright.