THE HAGUE — Radovan Karadzic, the former Bosnian Serb leader, was convicted of genocide, war crimes and crimes against humanity by a United Nations tribunal on Thursday for leading a campaign of terror against civilians in the deadliest conflict in Europe since World War II.
Mr. Karadzic, 70, was sentenced to 40 years in prison for his role in lethal ethnic cleansing operations, the siege of Sarajevo and the slaughter of 8,000 Muslim men and boys in Srebrenica in 1995, in proceedings that were likened to the Nuremberg trials of former Nazi leaders.
The trial here was the most important in the 23-year history of the United Nations International Criminal Tribunal for the Former Yugoslavia, and a defining test for the entire system of international justice, human rights advocates said.
“Twenty-one years after Karadzic was indicted, this verdict is a forceful manifestation of the international community’s implacable commitment to accountability,” the United Nations human rights chief, Zeid Ra’ad al-Hussein, said in a statement.
The conviction offered a note of closure to the bloodiest European conflict since
World War II, a civil war that tore apart the former Yugoslavia and left more than 100,000 people dead. The main Balkan combatants, Serbia, Bosnia, Kosovo and Croatia, are now themselves members or aspiring members of the European Union, an achievement for European unity at a time when the bloc faces severe strains over migration and economic stagnation.
While the tribunal had convicted many lesser figures of war crimes, it had never prosecuted as senior a figure as Mr. Karadzic. Slobodan Milosevic, the Serbian president whose extreme nationalism instigated and enabled much of the fighting, died in March 2006 in his cell in The Hague before the end of his trial before the United Nations tribunal. Ratko Mladic, who was Mr. Karadzic’s military chief during the campaign, is being tried separately.
Mr. Karadzic was convicted of genocide for the Srebrenica massacr e, which aimed to kill “every able-bodied male” in the town and systematically exterminate the Bosnian Muslim population there.
He was also found guilty of persecution, extermination, deportation, forcible transfer and murder in connection with a campaign to drive Bosnian Muslims and Croats out of villages claimed by Serb forces during the country’s civil war from 1992 to 1995.
He avoided conviction on a second count of genocide in seven Bosnian towns, but was found guilty in that case on a reduced charge of extermination.
In addition, Mr. Karadzic was found to have been “instrumental” in a campaign of sniping and shelling that terrorized the civilian population of Sarajevo, the Bosnian capital. And he was convicted of leading the seizure of United Nations employees as hostages, to obstruct NATO from carrying out airstrikes on behalf of besieged Bosnian Muslim civilians.
Mr. Hussein said the tribunal’s judgment “strips away the pretense that what he did was anything more than political manipulation, and exposes him for what he really was: the architect of destruction and murder on a massive scale.”
Peter Robinson, an American lawyer who was Mr. Karadzic’s chief legal adviser, said his client “was disappointed and astonished by his conviction and the judges’ reasoning, and he asked us to appeal his sentence.”
Relatives of the victims were disappointed as well, but only because they felt the sentence was too lenient. Kada Hotic of Srebrenica, who said she had lost her only son, husband, cousins and all the other men from her family, cried with anger.
“He got the verdict of an ordinary soldier,” she said. “He should have had a life sentence. He is guilty of all the killings in Bosnia because he pushed all sides to go to war.”
Some critics complained that Mr. Karadzic could conceivably walk out of prison one day. Typically, those convicted at the tribunal have served two-thirds of their sentences. With a credit of eight years for time served, that means he could be freed after a little more than 20 years.
The atrocities in Bosnia were a source of lasting regret for President Bill Clinton, and spurred his administration to broker the Dayton peace accords and, subsequently, support the NATO bombing of Belgrade in 1999 to prevent similar atrocities in Kosovo.
The decisions were read out by the presiding judge, O-Gon Kwon of South Korea, who took more than an hour as he calmly but precisely recounted the series of atrocities Mr. Karadzic was accused of.
His brother Luka and other relatives watched from the public gallery during the long reading. The defendant looked tense, tapping his hands and feet beneath his desk.
The judges took more than a year to deliberate — after being in session for nearly 500 days, spread out over four years — reflecting the ambition of the prosecution and the complexity of conducting a criminal trial covering a lengthy civil war.
Mr. Karadzic acted as his own lawyer in the proceedings, portraying himself as a man of peace who was driven solely by his desire to protect Serbs.
In his closing statement, he said that he took “moral responsibility” for crimes committed by Bosnian Serb “citizens and forces.” But he denied having ordered killings and said he was not aware that a massacre would take place at Srebrenica.
During the trial, to the amazement of experts following the case, he described himself as a “true friend to Muslims” who had tried to make them feel safe, despite his fiery speeches leading up to the war.
He mounted a zealous defense, bringing 238 witnesses to attest to his innocence. He based his defense on the premise that the Bosnian war broke out because Serbs had no choice but to defend themselves against a Bosnian Muslim separatist regime that intended to create an Islamic state.
The prosecution contended that Mr. Karadzic was the commander of a separatist Serb government bent on removing all non-Serbs from all areas of Bosnia that had been traditionally Serb.
Prosecutors presented electronic intercepts, written orders, video recordings and a long line of witnesses — fighters, politicians, peacekeepers, survivors of prison camps and rape victims — to demonstrate his central role in the conflict.
In a policy that came to be known as ethnic cleansing, hundreds of thousands of Muslims and ethnic Croats, largely Roman Catholic, were driven from their villages, their homes looted and mosques and churches demolished.
In 1992, the height of the ethnic cleansing campaign, close to 45,000 people were killed or missing, almost half of the 100,000 who died in the Bosnian war.
Men and boys were held in concentration camps, where prosecutors said thousands were tortured, were killed or died of starvation, and women were said to have been raped and used as sex slaves.
Mr. Karadzic was arrested on a public bus in 2008, more than 10 years after effectively vanishing. He had taken a new identity, posing as a faith healer and using the alias Dr. Dragan Dabic. He had grown a bushy beard and had long hair fastened in a topknot. His arrest led to the creation of yet another persona — zealous defense lawyer, who worked long nights in his cell to prepare his case.
The son of a modest family in Montenegro, he began his career as a psychiatrist and a poet who showed no political leanings.
But as Communism collapsed in Yugoslavia and the Soviet Union and the Warsaw Pact came undone, he joined the Serb nationalists and became a populist demagogue, delivering bombastic, jingoistic speeches to incite Serbs.
On Thursday in Belgrade, the Serbian capital, a nationalist group protested the sentence, saying Mr. Karadzic was convicted only because he was a Serb.
Natasha Kandic, a Serbian human rights lawyer, called the verdict just and said she hoped it would be an “obstacle for future revisions of history, for what really happened” in Bosnia.
Nick Cumming-Bruce contributed reporting from Geneva.
A version of this article appears in print on March 25, 2016, on page A8 of the New York edition with the headline: Bosnian Serb Leader Karadzic Convicted of Genocide and War Crimes.