The International Criminal Court in the Hague has convicted Congolese warlord Germain Katanga of being an accessory to war crimes including murder.
He now risks up to 30 years behind bars when sentenced, although he was acquitted of other charges of rape and using child soldiers.
His was only the second conviction by the ICC, which was set up 12 years ago.
Katanga can appeal against his conviction.
The case stemmed from violence in the diamond-rich northeast Democratic Republic of Congo in the early 2000s. Katanga was convicted for his role in an attack on the village of Bogoro in 2003 that left some 200 people dead.
“The timing of the attack and the methods used – encircling the village while its inhabitants were still asleep, the use of machetes to attack them, and shooting indiscriminately – led the chamber to find that combatants intended to target the civilian population,” said presiding judge Bruno Cotte.
The violence was a localised ethnic clash over land and resources among myriad conflicts that spun out of the wider war in the Democratic Republic of Congo from 1998 to 2003.
The judgment, albeit by a split verdict of two to one after a six-year trial, brings some relief to prosecutors who have faced growing criticism, notably for at least five cases that collapsed either pre-trial or pre-verdict because of a lack of evidence.
Apart from accusations of working too slowly, the ICC has also faced criticism for so far only bringing charges against Africans, while atrocities in conflicts in the politically-charged Middle East and elsewhere go unpunished.
The court is now preparing to try Kenyan President Uhuru Kenyatta, accused of orchestrating clashes in which 1,200 died after elections in 2007.
Kenyatta denies the charges but will cooperate, even as his government lobbies hard to have his trial dropped or postponed.