GREEK mythology says that Hope alone remained in Pandora's box when its untimely opening released desires destined to play havoc among mankind. Now Yugoslavia is opening a veritable Pandora's box in Balkan politics.The crisis in Yugoslavia has revived this region's reputation as Europe's powder keg. So far, the conflict is confined to Yugoslavia. But four decades of stability under communism are threatened throughout the region. Neighboring Albanians worry that Serbian nationalism may not only break up Yugoslavia, but also threaten Albanians living in Serbia's southern Kosovo region. Bulgaria and Romania have also realized that the ethnic violence in Yugoslavia has destabilized the rest of southeastern Europe, frightening away badly needed foreign investment. Former Communist leader Josep Broz Tito's dream of unity among Yugoslavia's various nationalities, and the Balkan people as a whole, is dead. Albania, however, is fortunate: The small Adriatic country is virtually homogenous. But its neighbors' histories abound with local wars over territory and differing cultures and religions. Now these disputes are reemerging. Nationalist sentiment in Yugoslavia led to declarations of independence in Slovenia and Croatia in June, and a strong vote in favor of independence in Macedonia on Sunday. Croatia's declaration set off fighting between Croats and the republic's minority Serbs who want to remain within Yugoslavia. Neighboring Serbia is backing Croatia's Serbs. The strife could easily ignite violence among the ethnic Albanian majority in Serbia's Kosovo region. In northern Serbia's Vojvodina region, 400,000 ethnic Hungarians already have been caught in the cross-fire between Serbs and Croats. Under Tito, both Kosovo and Vojvodina were self-governing provinces. They had all the status of republics except the name. Now both are part of Serbia under a nationalist, Communist leadership which ousted moderates two years ago. Last year's rewritten Constitution also asserts Serbia's right to claim territory in other republics under the pretext of protecting local Serb communities. Macedonia became a Yugoslav republic after World War II. However, Greece and Bulgaria, which also each contain parts of the ancient kingdom, refuse to acknowledge Macedonia either as a nation or a separate republic. Out of Yugoslav's crisis new alliances are forming. Hungary supports Croatia's secession and is counting on an independent Croatia to be a counterweight to further Serbian pressures on Hungarians in Vojvodina. Romania, too, believes good relations with a powerful, nationalist Serbian government would be useful if differences it has with Hungary over Transylvania worsen. Serbia, with its large Hungarian population in Vojvodina, could be an ally against Hungary. Budapest ignored nationalist pressures and confined itself to efforts to persuade Former Romanian Communist leader Nicholae Ceausescu to treat Hungarians in his country better. Pressure is building for Budapest to push Romania harder on the issue. Bulgaria's 1 million ethnic Turks became restless in the 1980s when the Communists began a crude name-changing campaign to assimilate the Muslim Turks. Several hundred thousand fled to Turkey. Most found no future there, and returned to Bulgaria only to realize its new rulers were almost as reluctant as their predecessors to legalize rights for minorities. The tension has all the makings of a separatist movement. Strife in Yugoslavia may destroy hope that the lid be may be put back on Pandora's box before more damage befalls the region.