Ntaganda, 45, was found guilty for acts committed when he was military operations chief at the Union of Congolese Patriots (UPC) militia in east Democratic Republic of Congo in 2002-2003.
His conviction is a rare success for prosecutors at the ICC, an international court set up in 2002 to prosecute war crimes and crimes against humanity when member states are unable or unwilling to do so.
Ntaganda’s sentence will be determined at a later hearing.
“The chamber ... having heard all the evidence mentioned by the parties, finds you as concerns count one, murder as a crime against humanity, guilty,” said judge Robert Fremr, reading a summary of the ruling.
The court then found him guilty on all 18 charges.
His lawyers argued Ntaganda had sought to keep order among troops, punishing those who violated rules of war.
Ntaganda, in a dark blue suit, showed no emotion as the sentence was read out. He has 30 days to appeal.
In the conflict in Congo, Ntaganda’s UPC, dominated by the Hema clan, targeted rival Lendu people for expulsion from the mineral-rich Ituri region. Hundreds of civilians were killed and many thousands were forced to flee.
“This ICC decision comforts the victims and the whole population of Ituri province, which was bereaved by the atrocities of Bosco Ntaganda’s rebellion,” said Xavier Macky, director of Justice Plus, a rights group based in the provincial capital Bunia.
He called the conviction a “contribution to the war against impunity.”
Ntaganda’s boss, UPC leader Thomas Lubanga, is serving a 14-year prison sentence after his conviction at the ICC.
The court has also convicted one of their wartime opponents, Germain Katanga.
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