Serge Brammertz, the tribunal’s chief prosecutor, told the Security Council last week that despite the large body of evidence proven in “case after case,” the denials and the refusal to accept facts, even by government officials, were “loud and clear.”

“Genocide is denied. Ethnic cleansing is denied,” he said.

“When irresponsible officials use division, discrimination and hate to secure power, conflict and atrocities can gain a logic of their own,” Mr. Brammertz said. “That was true two decades ago when genocide and ethnic cleansing began, and it remains true today.”

At the opening of the trial on Tuesday, Douglas Stringer, a prosecutor, portrayed the two former secret police chiefs as close to Mr. Milosevic, who had himself gained control of the institutions and agencies of the federal government of what was then Yugoslavia.

Mr. Milosevic entrusted them with all the critical aspects of secret police activities leading up to and during the wars, Mr. Stringer said.

The men set up clandestine training camps for paramilitary fighters and acted as chief organizers, paymasters and suppliers for those units, he said. The paramilitaries, some of whom were convicts, became notorious for their brutality and, according to Mr. Stringer, “looted on an industrial scale.”

Far from spontaneous, the prosecutor said, the Serbian state security at first placed their operatives in positions in Bosnia and Croatia that were scheduled for “ethnic cleansing.” He said these operatives were known as “doublehatters,” at once linked to the Belgrade government and also key players locally who relayed orders to the paramilitaries. All the activities “were covert to conceal the hand of Milosevic,” Mr. Stringer said.

The fate of Mr. Stanisic and Mr. Simatovic will be crucial in legally determining the role of the Serbian state in the wars in Bosnia and Croatia that killed more than 130,000 people. After two decades of trials at the tribunal in The Hague, no officials of the Belgrade wartime government are serving sentences, only Bosnians and Croats.