FIFTY YEARS AGO, the world codified its revulsion at the Nazi annihilation of 6 million Jews in a treaty against genocide, obliging signatories to stop the most egregious of war crimes and punish the perpetrators.

Then, the treaty was all but ignored. Like an old tome in the dusty recesses of a library, it languished as more than 1 million people were slaughtered in Rwanda and Bosnia-Herzegovina between 1992 and 1995 because of their religion or ethnicity.But the trial of a Bosnian Serb now under way in The Hague is expected to give new meaning and legal weight to the 1948 United Nations Convention on the Prevention and Punishment of the Crime of Genocide.

The case of Milan Kovacevic marks the first time an international criminal court is being asked to interpret the treaty. In doing so, the U.N. War Crimes Tribunal on Former Yugoslavia will decide if acts other than wholesale extermination - such as ethnic cleansing, mass rape and incarceration in inhumane conditions - constitute genocide.

A broader definition of genocide, experts say, could make it harder in the future for the United States and other signatories to evade their treaty obligations to intervene in conflicts in which people are targeted "in whole or in part" for their faith, nationality, ethnicity or race.

Already, a U.N. tribunal has charged Rwandans for war crimes under the U.N. treaty (and others have been convicted under Rwanda's war-crimes statutes). But the Bosnia case is the first based on the U.N. treaty to come to trial. And whereas the charges in Rwanda are straightforward, involving mass killings, experts say the case against Bosnian Serb leaders will hinge on a broader definition of genocide.

The case could "force all governments . . . to be honest . . . about when genocide is occurring," says Paul Williams, a former State Department legal expert now at the Carnegie Endowment for International Peace, a Washington-based research institute.

Professor Francis Boyle, an expert on human-rights law at the University of Illinois in Champaign-Urbana, agrees. The Kova-cevic case, he says, could make the convention "a living, breathing body of law that will have an impact on the future course of international relations."

Experts point out that because of the broad definitions of the treaty, the Clinton administration was able to avoid declaring that genocide was under way in Bosnia, averting the obligation to intervene in a conflict it strenuously sought to stay out of.

"If former Secretary of State Warren Christopher would have said this is genocide, we would have had to act," says Williams, who then worked at the State Department. "But this administration did not want to act. So, the legal office was asked to perform legal gymnastics to avoid calling this genocide."

Experts say that, regrettably, the need to refine the application of the treaty has become critical in today's geopolitical climate.

Because ethnic and religious conflicts are likely to persist, even intensify, there is a strong possibility the world could witness new attempts by national, ethnic or religious groups to wipe out their rivals. More refined definitions of actions that constitute genocide might deter such conduct, experts say.

Furthermore, it would be enormously useful for a new international criminal court - talks on which are to be wrapped up Friday by 161 nations in Rome - to have precedents on which to adjudicate future genocide cases, these experts say.

Genocide is the only crime over which the Rome conferees all agree the new international criminal court should have automatic jurisdiction.

Since it entered into force in 1951, the convention has never been used for a conviction by an international criminal court. Instead, some states have drafted anti-genocide laws based on the treaty and used those to prosecuted war-crimes suspects.

The Tanzania-based U.N. tribunal is to try on genocide charges dozens of Hutus allegedly involved in the slaughter of an estimated 800,000 Tutsis in the 1994 Rwandan civil war. Experts say that while Hutu officials clearly oversaw mass murder, proving genocide charges against Bosnian Serb leaders will be more difficult.

Prosecutors in The Hague contend the Bosnian Serbs' campaign consisted of "ethnic cleansing" involving killings, mass rapes, torture and the evictions of some 700,000 Muslims and Croats. Kova-cevic is charged with complicity in genocide for allegedly participating in that campaign as a member of the "Crisis Staff" of civilians, police, and military officers who directed the seizure of the northern town of Prijedor in April 1992.

As the No. 2 civilian official, prosecutors say, he helped set up and run three prison camps in which thousands of Croat and Muslims were incarcerated.

Prosecutors contend Kovacevic, who pleaded not guilty when the trial began July 6, shares responsibility for executions, beatings and sexual abuse of camp inmates.

By sustaining those charges, the tribunal would establish as violations of the genocide convention the acts that occurred in the creation and administration of the camps, experts say. A conviction would also recognize the "command responsibility" of Bosnian Serb civilian officials in genocidal acts committed by military and police units, experts say. Such a finding could strengthen genocide cases against war-time Bosnian Serb political leader Radovan Karadzic, the most-wanted accused war criminal.