Antonio Cassese, 74, an Italian jurist whose forceful leadership of two international tribunals helped propel forward the trials of accused war criminals in the former Yugoslavia and alleged assassins in Lebanon, died Oct. 22 at his home in Florence.

He had leukemia, said his wife, Sylvia Cassese.

Mr. Cassese was a prolific academic writer and professor for several decades at the University of Florence, where he established a reputation as a top scholar of international law.

In 1993, he was appointed as a judge on the International Criminal Tribunal for the Former Yugoslavia, based at the Hague. That body — a massive operation established by the United Nations in the wake of the Balkan wars of the 1990s — was the first international criminal tribunal since the ones that followed World War II. It had the power to hand down life sentences, but not to condemn defendants to death.

The tribunal represented a “turning point in international relations,” Mr. Cassese said. Unlike in the Nuremberg and Tokyo trials for Nazi and Japanese war criminals, Mr. Cassese once noted, the Yugoslav defendants were not tried by their wartime enemies, but by the international community.

“For the first time,” he said, “the community of states is rendering a justice that is not that of the victors, imposed at the very time when the air is still being rent by the clash of arms and cries of pain.”

Some critics consider the tribunal a slow-moving ivory tower of the law. Eighteen years after its establishment, it is still working through the complex cases that came from the violent conflict among Serbia, Croatia and Bosnia and Herzegovina. Mr. Cassese, who became the court’s first president, was one of its most vigorous supporters.

He was regarded as an agreeable and modest judge; his chief mode of transportation was his bicycle. But when it came to the court, he rose to impassioned defenses of the international justice system.

In 1996, a New York Times reporter overheard him explode in frustration about Serbian leaders’ failure to cooperate with the tribunal and the message their intransigence sent to other dictators.

“Go ahead! Kill, torture, maim! Commit acts of genocide!” he shouted in a court hallway. “You may enjoy impunity!”

He once threatened to resign if he didn’t receive greater cooperation from world powers in arresting major figures wanted on war crimes charges. They included Radovan Karadzic, who was accused of ordering the massacre of 8,000 Bosnian Muslims at Srebrenica, and Serbian leader Slobodan Milosevic. The court’s job was to try leaders, he said, not “small fry.”

Mr. Cassese completed two two-year terms as president. When he stepped down in 1997, he expressed disappointment in what the court had managed to accomplish. At least 77 defendants were indicted, two were sentenced, and two trials remained in process. Milosevic and Karadzic were arrested after Mr. Cassese left the court.

Mr. Cassese told the Wall Street Journal that his hopes for the tribunal had been the result of “terrible naivete” and “sheer ignorance,” and that he had taken “for granted that all countries would immediately arrest those accused by the tribunal.” He said that he viewed his tenure as a “significant success tempered by some failings and much frustration.”

Nonetheless, Mr. Cassese could point to the more than 200 victims who testified about atrocities in the former Yugoslavia to show the court’s contribution to the historical record.

In his chambers, he displayed photographs of civilian casualties of the Balkan wars. “They say all that need be said about why justice must be done,” he told the Sunday Times of London in 1996.

In 2009, Mr. Cassese became president of the Special Tribunal for Lebanon, a court established by the United Nations that year to put on trial the alleged assassins of former Lebanese prime minister Rafiq al-Hariri, who was killed in 2005. Mr. Cassese’s failing health forced him to resign from the tribunal about two weeks ago.

Antonio Cassese was born Jan. 1, 1937, in Atripalda, a town in southern Italy.

“I read law only because I was urged to do so by my father,” he once wrote in an essay. “I wanted to study philosophy of humanities. . . . My father’s advice was that I should choose a field that would ensure a secure professional future.”

He studied at the University of Pisa, one of the most prestigious universities in Italy. He wrote books on topics including the prosecution of Japanese war criminals during World War II, human rights and broad studies of international law.

Beginning in the late 1960s, he represented Italy at the United Nations and in Geneva on numerous commissions dedicated to human rights.

After his work on the Balkans tribunal, he was appointed to a commission examining ethnic cleansing in the Darfur region of Sudan and a special court, backed by the United Nations, for crimes in the West African country of Sierra Leone.

In 2009, he and former Nuremberg prosecutor Benjamin Ferencz received the Erasmus Prize, a prestigious Dutch award for contributions to European culture.

Survivors include his wife of 50 years, Sylvia Fano Cassese, and their two children, Teresa Cassese and Francesco Cassese, all of Florence; and two granddaughters.

Mr. Cassese was known as a learned judge — one who wove references to Hegel, Voltaire and Plato into the first report on the Lebanon commission. Writing in the Lebanese Daily Star in 2008, he cited Phaedrus’s fable of the wolf and the lamb to explain his view of great powers that improperly justify their use of armed violence.

First, he wrote, the wolf “scolds the lamb because he is muddying his drinking water (even though the wolf was upstream). Then he argues that last year the lamb had called him bad names (but the lamb was only six months old). The wolf then snarls that if it was not the lamb, it was his father; after that, he immediately moves into action.”