THE HAGUE — Charles G. Taylor, the former president of Liberia and once a powerful warlord, was convicted by an international tribunal on Thursday of arming, supporting and guiding a brutal rebel movement that committed mass atrocities in Sierra Leone during its civil war in the 1990s. He is the first head of state to be convicted by an international court since the Nuremberg trials after World War II.


After 13 months of deliberation, a panel of three judges from Ireland, Samoa and Uganda found Mr. Taylor guilty of crimes against humanity and war crimes, including murder, rape, slavery and the use of child soldiers. But the judges said the prosecution failed to prove that Mr. Taylor directly commanded the rebels responsible for the atrocities.


The conflict in Sierra Leone became notorious for its gruesome tactics, including the calculated mutilation of thousands of civilians, the widespread use of drugged child soldiers and the mining of diamonds to pay for guns and ammunition. A new, sinister rebel vocabulary pointed to the horrors: applying “a smile” meant cutting off the upper and lower lips of a victim; giving “long sleeves” meant hacking off the hands; and giving “short sleeves” meant cutting the arm above the elbow.


Ten years after the war ended, Sierra Leone is still struggling to rebuild. An estimated 50,000 people died, while countless others fled the country or took refuge in camps. A broad swath of the nation’s young missed their educations. Unemployment, particularly among the young men who emerged from the war with few skills, is crushing. Electricity is scant, even in the capital. The country has returned to democracy, but many educated Sierra Leoneans remain abroad, literacy is low and some industries, like mining iron ore, are just starting to return.


“Charles Taylor is guilty, what he has done,” said Osman Turay, one of several amputees playing soccer on crutches in the concrete shell of an unfinished building in Freetown, Sierra Leone’s capital, on Thursday. “He is the one who started this.”


Prosecutors said Mr. Taylor’s part in the devastation was motivated not by ideology, but by a quest for power and money — “pure avarice,” in the words of David M. Crane, the American prosecutor who indicted him in 2003. Rebels supplied Mr. Taylor with “a continuous supply” of diamonds, often in exchange for arms and ammunition, the court found, allowing him to send what prosecutors said amounted to hundreds of millions of dollars to off-shore companies.


Yet investigators never unraveled the web hiding this presumed fortune and Mr. Taylor pleaded penury, leaving the court to foot the bill for a defense that cost $100,000 per month in lawyers, staff and rent.


Still, the trial has brought “a sense of relief,” said Ibrahim Tommy, who leads the Center for Accountability and Rule of Law, a rights group in Freetown. “I’m not sure it will bring closure to the victims,” he said, but the trial was “a genuine effort to ensure accountability for the crimes in Sierra Leone.”


The tribunal, called the Special Court for Sierra Leone, has already sentenced eight other leading members from different forces and rebel groups. Mr. Taylor, who has maintained his innocence, will be sentenced in the coming weeks. There is no death penalty in international criminal law and any jail term would be served in a British prison.


The fighting for control over one of the world’s poorest regions also involved Liberia, where many more died, and threatened to spill over into neighboring Guinea and Ivory Coast. But only crimes in Sierra Leone between 1996 and 2002 are within the court’s mandate, and Mr. Taylor is the special court’s last defendant. His trial was moved here, to The Hague and a second court nearby, for fear of causing unrest in the region, where he still has followers.


Not since Karl Doenitz, the German admiral who briefly succeeded Hitler upon his death, was tried and sentenced by the International Military Tribunal has a head of state been convicted by an international court. 


Slobodan Milosevic, the former president of Serbia, died in his cell before his war crimes trial ended. Jean Kambanda, the first person sentenced for the crime of genocide, received a life sentence for his role in the 1994 Rwandan genocide, but he was a former prime minister, not the head of state. The former president of Ivory Coast, Laurent Gbagbo, has been charged with crimes against humanity by the International Criminal Court, but his trial has not yet begun. Similarly, President Omar Hassan al-Bashir is wanted by the court on genocide charges for atrocities in Darfur, but he has long evaded trial.


During Mr. Taylor’s lengthy trial, which began in 2006, the judges heard testimony from 115 witnesses. Before the formally robed court officers, they spoke of slave labor in captured diamond mines, rape, severed heads displayed on stakes to terrorize people and captured villagers lining up, waiting to have their limbs hacked off.


There were many chilling moments, as witnesses described the barbarism of the rebels, gesticulating with the stumps of amputated limbs swaddled in bandages.


Mustapha Mansary, a villager, was twice asked by the defense lawyer if he could read and write English, until he held up his two bandaged stumps.


“I have no hands to write anything,” Mr. Mansary replied.