PRESIDENT CLINTON joined other world leaders in Sarajevo last week to promise to work for stability and prosperity in the former Yugoslavia and among its neighbors. Most of the talk was of economic reconstruction, which is crucial, but stability remains, for the moment, a military challenge as well. There can be no success in the Balkans until the Dayton peace plan is enforced, which means arresting top war criminals who have been permitted to go free and repatriating refugees who have not been permitted to go home.

In Kosovo, the international community has yet to make good on its pledges to beef up its security force and deploy a civilian police force. The bomb that damaged a Serbian cathedral in Pristina Sunday is a reminder of the urgent need to fulfill those pledges.

It remains true also that there can be no stability in the Balkans as long as Slobodan Milosevic remains in power. The authoritarian leader, first of Serbia, now of what remains of Yugoslavia, bears primary responsibility for a decade of war in his region. His promotion of Serb nationalism has cost Serbia dearly, but it has kept Mr. Milosevic in power. To preserve his rule now, he is sure to seek out new enemies and cause new bloodshed, if he is not checked. The West must do more to assist the forces of democratization inside Serbia. Western governments also must make clear not only that Serbia will receive no reconstruction aid as long as Mr. Milosevic holds sway, as Mr. Clinton reiterated Friday -- but also that Serbia will receive substantial help once it signals a willingness to democratize and live in peace with its neighbors.

For such a pledge to be credible to Serbs, the governments represented in Sarajevo Friday have to move quickly to provide such aid to nations, and parts of nations, that already qualify. Deserving recipients include Kosovo and Montenegro, which remain for the moment within Yugoslavia but outside Mr. Milosevic's control, and nearby countries that assisted in the Kosovo war effort, often at considerable political and economic cost: Albania, Macedonia, Bulgaria, Romania, Hungary.

Mr. Clinton came to the conference armed with some concrete promises, including investment guarantees and tariff reductions. The Europeans, who have promised to take the lead in Balkan reconstruction, offered no such specifics. If the stability pact is to have any meaning, Europe will have to ante up, and soon. That means providing aid, but also opening its own institutions so that southeastern Europe can hope to join in the prosperity that the Marshall Plan helped bring to Western Europe a half-century ago. And Europe must open up to the southeast without neglecting its obligations to democratizing countries in central and northern Europe, such as Poland and Lithuania, which already have waited too long for the institutional support they deserve and need. These are the central tests for a continent eager to play a leading role in the world.