At the height of its influence, the LRA numbered thousands of combatants, women and children.
After the LRA was driven out of Uganda in the mid-2000s, it continued to prey on civilian populations in Congo, the Central African Republic and parts of what became South Sudan.
Ongwen is facing trial at the ICC on charges of crimes against humanity and war crimes for his role in the brutal 30-year campaign by the LRA.
He appeared briefly before the court yesterday, dressed in dark suit, white shirt and checked tie, to confirm his identity and that he understood the charges.
Ongwen was abducted by the Lord's Resistance Army as he walked home from primary school as a child according to his family. He told the court yesterday he was 14 at the time.
Kony's atrocities
The LRA is believed to have abducted at least 30,000 children to recruit as soldiers.
Anneke Van Woudenberg, a senior researcher at Human Rights Watch, said emotions surrounding Ongwen's trial were highly mixed.
Lord's Resistance Army leader Joseph Kony, who is now thought to be in hiding, in 2006. Photo / AP
"This is an important moment for accountability for the LRA's crimes, when the victims will finally get their day in court," she said.
"But it's also a difficult moment. Here is a child who suffered, who was himself abducted. And he will now have to answer for his crimes."
But while Ongwen may have been a victim, his own record as an adult is one of clear brutality.
After his abduction, Ongwen rose quickly through the ranks of the LRA, being moved to South Sudan in 1994 and becoming a platoon commander soon after.
His reputation as a hardline leader brought him into favour with Kony and he was promoted to the rank of brigadier at the age of 28.
"They found me a very sharp recruit," Mr Ongwen said of the time after his kidnap, in a video seen by the Associated Press.
He became the leader of the Sinia Brigade, one of the four main LRA units responsible for atrocities across northern Uganda, South Sudan, Congo and CAR. The group has been responsible for the murder of thousands across the region, as well as torture, kidnappings and the use of sex slaves.
Forces acting under Ongwen are believed to have been responsible for a brutal four-day massacre in the Makombo area of north-eastern Democratic Republic of Congo in 2009, in which more than 300 civilians, including 80 children, were brutally killed.
Men were hacked to death with machetes or had their skulls crushed with axes and heavy wooden sticks, according to a Human Rights Watch report.
Ongwen's arrest
Relations between Ongwen and Kony appear to have soured during negotiations with the Ugandan government, when Ongwen stood by his former commander, Vincent Otti, after Kony ordered his death.
It was at a meeting with Kony last December when, after he rejected a plan to abduct people in the Central African Republic, Kony reportedly forced him out of the group. He was later handed over to US soldiers and then on to the ICC.
Ongwen is one of five commanders indicted by the ICC in 2005, including Kony. The other three - Raska Lukwiya, Okot Odhiambo and Otti - are either known or presumed to be dead.
It is Kony, said Ms Van Woudenberg yesterday, that the victims of the LRA would truly like to see before the court.
"He is the man who should really have to face justice," she said. "This is not an armed group that anyone goes to join because they agree with its ideology or with Joseph Kony."
Recruits are brutalised from an early age, forced to witness extreme violence and taught to kill, she said.
It is thought most likely that Kony is hiding out, along with a much depleted LRA, in the Kafia Kinji region, a disputed area on southern Sudan's border with South Sudan and the Central African Republic.
Another hearing is set for August 24 when judges will hear prosecutors' evidence and decide if it is strong enough to merit a full trial.
- Independent
