Over the past few years, Mr. Kony’s group is reported to have dwindled to 100 or so men. But the Obama administration once considered him such a threat to peace in the region that it offered a reward of up to $5 million for his capture, even though the United States is not a member of the International Criminal Court. The rebel group’s reign of terror, which included its practice of mutilating and burning victims, spread beyond Uganda to southern Sudan, the Democratic Republic of Congo and the Central African Republic.

Mr. Ongwen was brought to the court early last year, after being detained in the Central African Republic by American Special Forces who had been helping to track Mr. Kony and his men. Mr. Ongwen, long a top commander in the Lord’s Resistance Army, is believed to have fallen from favor with Mr. Kony and to have been held captive by him before breaking free and eventually handing himself in. Some analysts said Mr. Ongwen’s escape and surrender had probably been motivated by the fear that Mr. Kony would kill him, as he had done with others.

Mr. Ongwen told the court last year that he was born in Uganda in 1975 and that he was abducted by the Lord’s Resistance Army when he was 14. People who knew him as a child soldier said he had been taught that his destiny was to fight for the rights of the Acholi people, a group to which both he and Mr. Kony belonged. He rose to become one of Mr. Kony’s trusted commanders, joining the control altar, as the warlord’s headquarters were called.

Mr. Ongwen has been charged with 70 counts of war crimes and crimes against humanity in connection with atrocities committed in northern Uganda, many of them victimizing the Acholi people he was said to have felt called to protect. Even though his fighting career lasted more than 25 years, his trial focuses on a series of four attacks on refugee camps in northern Uganda between 2002 and 2005, for which prosecutors apparently have the best evidence.

As his trial opened, with Mr. Ongwen in a court-issued gray suit, an official read out the list of charges against him, including murder, torture, rape, sexual slavery and conscription of children under 15.