EUROPEAN Community summits have an uncanny way of being disrupted by international crises.Predictions that the past weekend's summit here would be an uneventful stock-taking of the EC's current negotiations for greater political and economic integration were shattered by the thunder of civil war in Yugoslavia. With bombing and deaths mounting at their doorstep, leaders of the EC's 12 member countries acted quickly Friday with a mediation mission that initially appeared to garner results. By yesterday, the cooling-off period the EC thought it had accomplished appeared in doubt, but leaders insisted their intervention marked an important turning point for post-cold war Europe. "We have demonstrated that at the political level we already have a rapid reaction force," said Italian Foreign Minister Gianni de Michelis, returning Saturday from emergency talks in Yugoslavia. But observers, including some US officials, said the EC should have used its leverage sooner with Yugoslavia and its breakaway republics to head off the resort to violence. With the threat of a moratorium on economic assistance in its pocket, the EC delegation received promises for an immediate cease-fire from Yugoslav federal authorities, and assurances from Slovenian and Croatian leaders that independence measures would be suspended for a three-month negotiating period. But yesterday, the assurances appeared either broken or threatened, which raised the prospect that the EC might act this week to freeze economic aid. THE last EC summit in April was dominated by emergency action to assure safe havens for Kurds inside Iraq. This pattern of summit agendas being overtaken by international events has had at least one positive effect: It has allowed the EC to throw off some doubts raised by its inaction in the Gulf crisis and to demonstrate that it is capable of playing an influential role on the international scene, especially in Europe. "A certain European credibility is beginning to see daylight," said Luxembourg's president, Jacques Santer, whose country yesterday completed its six-month presidency of the Community. Referring to the ongoing negotiations for treaty reforms to enhance common EC political action, he added, "We must now get down to institutionalizing this political dimension." If anything, the outbreak of war at its door was a sobering reminder to the Community that it will not be able to build a prosperous, democratic, and stable union within the borders of its 12 members if just outside the wall there is instability, violence, and economic collapse. Britain's Prime Minister John Major likely had this in mind when he said at a post-summit conference Saturday, "We have to be careful that in deepening [the EC] we don't throw down such a girdle that we make widening [to neighboring countries] more difficult at a future point." Mindful that the emergence of Eastern Europe's nationalities represents a short-fused powder keg, EC leaders attempted to balance their support for Yugoslavia's unity with recognition of the basic right to self-determination. Germany's Chancellor Helmut Kohl, whose country is home to a large and predominantly Croatian Yugoslav community, was the most outspoken backer of self-determination and advocated suspending all aid to Yugoslavia. But Mr. Major and French President Francois Mitterrand tipped the balance for Yugoslavia's "territorial integrity." With backing from other European states, the EC also triggered the recently formulated emergency intervention mechanism of the Conference for Security and Cooperation in Europe (CSCE). The procedure is likely to lead to a CSCE committee meeting by mid-week in Prague. Yugoslavia, a CSCE member, accepted the intervention. But, referring to the conflict's internal nature, one EC official said, "The EC can offer assistance in negotiations, but if we're not wanted we can't impose anything." Despite the focus on Yugoslavia, EC leaders did achieve the expected progress review of the first six months of negotiations in two intergovernmental conferences on economic and monetary union and political integration. LEADERS agreed that the two conferences should be wrapped up at the EC's December summit in Maastricht, Holland. That will give national legislatures time to ratify amendments to the EC Constitution by the time the much-anticipated single market takes full effect at the end of 1992. Deliberations on economic and monetary union are the most advanced. A long and heated debate arose over Britain's insistence that the summit's final communique include a reference to its objection to a timetable for creating a European central bank. A Dutch official, whose country takes over the EC presidencey today, called inclusion of the reference "a psychological and political setback." EC Commission President Jacques Delors said, however, that the controversy "shouldn't mask the real progress" already made on economic reforms. Saying he is less optimistic about the political integration negotiations, Mr. Delors cited six main "points of friction a common defense and security policy, changes in the decision-making process, extension of Community jurisdiction to new areas, increasing majority voting, true legislative powers for the European Parliament, and strengthening of the Community's "social dimension."