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.