The trial of the first Lords Resistance Army (LRA) commander to appear before the International Criminal Court (ICC) is under way in The Hague.
Dominic Ongwen is also the first child soldier to be tried by the ICC.
He faces 70 counts of war crimes and crimes against humanity in Uganda, more charges than any other suspect.
The BBC’s Anna Holligan says his past may present ethical and legal dilemmas, and his lawyers may use his traumatised youth to plea for leniency.
Dominic Ongwen was a boy when he was abducted and conscripted by the notoriously ruthless rebel cult.
Our correspondent in The Hague says his story encapsulates many of the complexities of the conflict which has claimed more than 100,000 lives – he was a child groomed in the image of his oppressors.
Who is Dominic Ongwen?
- Said to have been abducted by LRA at the age of 10 as he walked to school in northern Uganda
- Rose to become a top commander
- Accused of crimes against humanity, including enslavement
- ICC issued arrest warrant in 2005
- Rumoured to have been killed in the same year
- US offered $5m (£3.3m) reward for information leading to his arrest in 2013
Mr Ongwen is accused of leading attacks on four camps for internally displaced people in northern Uganda, murdering and torturing civilians, and forcing women into marriage and children to take part in the fighting.
He was captured in the Central African Republic in January 2015, after being sought by US and African forces since 2011.
He is said to be the deputy to LRA commander Joseph Kony, who is still on the run.
Uganda agreed that Mr Ongwen should be tried by the ICC despite being a fierce critic of the court.
Big screens have been set up in the Ugandan capital Kampala, where people are following the trial via video link.
The LRA rebellion began more than two decades ago in northern Uganda and its estimated 200-500 fighters – many of them child soldiers – have since terrorised large swathes of central Africa.