THE Yugoslav Army is leaving Slovenia under the EC-brokered cease-fire, and life is returning to normal there.The same cannot be said for the rest of Yugoslavia. In the Balkans some undefinable line has been crossed - something has been loosed in the minds of the people. In the past, appeals to reasonableness and self-interest among differing republics carried weight. Now, with blood spilled daily in the convulsing republic of Croatia - as Croats battle ethnic Serbs and the Serbian-leaning federal Army - it seems, in the short term, that passion, not reason, may rule the day. Each republic, internally, has become a hothouse of conflict - as the crisis in Yugoslavia reaches a new level. Public comments in Belgrade Monday by a leader of a right-wing political party that Serbs should "slaughter" the proposed European Community observers in Croatia, make the point. Antinationalist factions in Serbia, such as the mothers of soldiers and students against war and proto-fascism, need support. The key to at least temporary social and economic sanity in Yugoslavia appears now to rest with a brokered cease-fire in Croatia. Three EC foreign ministers, headed by Dutch delegate Hans van den Broek, will visit Croatia this weekend to act as brokers, as they did earlier in Slovenia. Strategically the trip gives the EC a new political (even quasi-security) role - countering its weak image during the Gulf war. Yet Croatia won't be as easy to deal with as Slovenia. Political power in Yugoslavia increasingly gravitates around Serbia's Slobodan Milosevic and the Serbian-led federal Army. They've made Croatia the battle line. Croatian president Franjo Tudjman is in desperate straits. He has few options. Earlier this week he refused to travel to Belgrade to meet with the collective presidency of Yugoslavia and the presidents of the six republics, saying internal Serbian-led "terrorism" made it impossible for him to leave. Mr. Tudjman is probably prepared to accept anything the EC delegation offers. He doesn't want his republic to exist as occupied territory. Yugoslav federal Prime Minister Anton Markovic has lost much authority but is the only real link between the EC and Yugoslavia. There is reason for some hope in the EC mission, for Serbia doesn't have diplomatic alternatives to the EC talks. Serbia, despite its bluff, is not yet ready to alienate itself completely. Besides, it badly needs EC funds. What remains outside the purview of EC talks is the deepening ethnic fissure in the Balkans.