The two men face charges of war crimes and crimes against humanity committed in the Central African Republic. These include murder, torture, mutilation, persecution and the conscription of child soldiers.
The former head of the Central African Republic football association goes on trial at the International Criminal Court (ICC) Tuesday.
Patrice-Edouard Ngaissona and co-defendant Alfred Yekatom face charges of war crimes and crimes against humanity.
At the time of his arrest in 2018,Ngaissona was also a board member of the Confederation of African Football (CAF).
What are the charges in detail?
Prosecutors allege the two men committed the crimes while leading Christian-dominated anti-Balaka militias in the Central African Republic.
They formed after a coalition of predominantly Muslim rebels known as Seleka seized power in March 2013, ousting then-president Francois Bozize, a Christian.
The militias carried out widespread attacks on Muslims during 2013 and 2014.
Charges against Ngaissona include murder, rape, attempted rape, persecution and torture.
Prosecutors say he committed the crimes while he was a senior leader and national coordinator of the anti-Balaka militias.
Co-accused Alfred Yekatom, 46, a former militia commander who styled himself as Rambo after a movie character, faces additional counts for his alleged use of child soldiers. He does not face rape charges.
Both men have said they are innocent.
Why is the trial important?
Ngaissona and Yekatom, 46, are the “highest ranking anti-Balaka leaders to face trial, and the first at the ICC”, NGO Human Rights Watch said.
“The opening of the Yekatom and Ngaissona trial is a milestone for justice for victims of brutal crimes,” said Elise Keppler, associate international justice director at HRW.
What is the current situation in the CAR?
The CAR is one of the world’s poorest countries despite its rich natural resources. Its history of unrest stretches back to independence from France in 1960.
The trial in The Hague is going ahead against a backdrop of continuing unrest in the CAR. Rebels are waging an offensive against the government of current president Faustin Archange Touadera.
A counter-offensive has seen pro-government forces retake a series of towns from the rebels in recent days.