In a historic ruling, an international court convicted Charles Taylor for supporting notoriously brutal rebels in return for blood diamonds. (Peter Dejong, Pool/Associated Press)

Charles Taylor, the U.S.-educated guerrilla leader who fought his way to the presidency of Liberia, was convicted Thursday of war crimes and crimes against humanity — including murder, rape and slavery — for his role in assisting a bloody rebel movement in neighboring Sierra Leone.

The conviction, in the U.N. Special Court for Sierra Leone, was hailed by chief prosecutor Brenda J. Hollis as a triumph for the idea that political leaders should be held accountable for their deeds in “the new reality” of an international justice system composed of a half-dozen U.N. courts headquartered in this verdant Dutch city.

“This judgment confirms that with leadership comes not only power, but also responsibility,” the U.S. jurist declared at a news conference after the verdict was read out over two hours by the presiding judge, Richard Lussick of Samoa.

Elise Keppler, senior counsel in Human Rights Watch’s international justice program, said the verdict marked the first such judgment against a former head of state.

“This is a victory for Sierra Leonean victims and all those seeking justice when the worst abuses are committed,” she said.

But chief defense lawyer Courtenay Griffiths, a silver-tongued London barrister, challenged the court’s decision as a political gesture that he qualified as “inevitable,” implying that it was not based on the evidence. At a news conference, Griffiths said the verdict grew out of “tainted and corrupt” testimony because witnesses were paid to come from Sierra Leone, in West Africa, to appear before the three-judge panel with evidence of what happened in their country’s atrocity-ridden civil war from 1991 to 2002.

Griffiths depicted Taylor, 64, as the legitimate president of a sovereign nation who assisted a rebel movement in a neighboring country but who should not be held accountable for crimes that the rebels might have committed. If that were the standard, he suggested, U.S. leaders should be tried for abuses committed when they assisted rebels in Nicaragua and Afghanistan and financed a brutal military in El Salvador in the 1980s.

“If such behavior is being deemed illegal, then I’d like to see it deemed illegal across the board,” Griffiths said.

Griffiths expressed outrage at the court’s treatment of alternate judge Malick Sow of Senegal when he tried to voice a dissenting opinion after the verdict was read. Sow, who as an alternate did not have a vote, rose to complain that the evidence against Taylor was too flimsy for a conviction. But the other three judges turned their backs on him and walked away, while court technicians immediately cut off an in-house video feed to reporters.

“My worry is that the whole [international justice] system is not consistent with the values we know and love,” Griffiths quoted Sow as saying, reading from a prepared statement. “I am afraid the whole thing is headed for failure.”

Taylor, who has been held by the court since his arrest in 2006, appeared in a blue pinstriped suit with a maroon tie and a white handkerchief in his breast pocket. During the reading, he sat expressionless, occasionally taking notes with a yellow ballpoint pen.

Griffiths said Taylor remained “stoic” as he was led back to his cell after the proceedings. The defense will decide whether to appeal only after sentencing, which is scheduled for May 30, he added. Under court rules, the three judges may not condemn him to death but can send him to prison for a term they consider commensurate with his crimes, according to a court spokesman, Solomon Moriba.

The court dismissed prosecution allegations that Taylor became involved in the Sierra Leone war after hatching a plan with rebel leaders when they were all being trained in Libya in the 1980s. It also dismissed charges that Taylor was in effect the senior commander of the rebel forces, using his prestige as president and his connections with arms merchants to exercise command-and-control functions and arrange arms-for-diamonds deals.

But it upheld the prosecution’s contention that, by supplying advice, arms, ammunition, communications and transport to the rebels, he bore criminal responsibility for atrocities they committed in Sierra Leone. The judges ruled that the assistance was “sustained and significant” during the war and that Taylor knew full well of the bloodletting through reports from newspapers, U.N. agencies and his own staff.

In particular, the judges singled out attacks in the late 1990s on Freetown, Sierra Leone’s capital, and on diamond-producing regions as the occasions for particularly abhorrent rebel abuses. These, they said, included using pubescent girls as sex slaves, forcing young boys to go into battle under the influence of drugs and killing or maiming civilians to terrorize people living in areas under their control.