After a trial lasting more than three years, involving more than 60 witnesses and more than a thousand items of evidence, the three judges of Chamber I at the International Criminal Court came to a unanimous decision.
Presiding judge Adrian Fulford delivered the verdict. "Chamber concludes that the prosecution has proved beyond reasonable doubt that Mr. Thomas Lubanga Dyilo is guilty of the crimes of conscripting and enlisting children under the age of 15 years into the FPLC and using them to participate actively in hostilities," he said.
An ethnic Hema, Thomas Lubanga led the rebel group, the Union of Congolese Patriots, and its military wing, the FPLC. He was found guilty of recruiting child soldiers under the age of 15 to kill rival ethnic Lendus in the gold-rich Ituri region during 2002-2003.
At the beginning of the trial in 2009, ICC Chief Prosecutor Luis Moreno-Ocampo claimed Lubanga used children as young as nine.
"Lubanga's armed group recruited, trained and used hundreds of young children to kill, pillage and rape," he said.
Videos shown in court by the prosecution showed Lubanga addressing fighters, including child soldiers.
The trial focused solely on the use of children. Human rights groups have called for a wider court investigation into abuses during the Ituri conflict. It is estimated more than 50,000 people were killed and hundreds of thousands were forced to flee their homes.
Lubanga was arrested in March 2005 and transferred to The Hague a year later.
"Lubanga really, ultimately, is a very middle-ranking official in the rebel movement and a bit-player in the Congolese conflict as a whole," said Phil Clark, a specialist in African politics and the role of the International Criminal Court, at the School of Oriental and African Studies in London. "So many Congolese who've lived through this conflict are surprised that the International Criminal Court isn't going after more senior suspects, including actors in the Ugandan and Congolese governments."
This is the International Criminal Court's first verdict since it was set up 10 years ago.
The judges criticized the prosecution for submitting some unreliable testimony - which cost the ICC time and money.
Analyst Phil Clark says the ruling is a big step for the court.
"Certainly this is a landmark," he said. "I think the ICC has waited a long time to complete its first case. But relatively speaking this has still been quite a long and drawn out affair, with various stops and starts. It's a trial that has almost collapsed on three occasions because the prosecution's investigations were very weak in Congo, they relied on third hand evidence, they didn't do investigations on their own independently on the ground and that's really weakened this case."
A sentencing hearing will take place at the ICC in the coming months and judges will also decide on possible reparations for Lubanga's victims.
In the meantime, Lubanga has 30 days to appeal the verdict.