Eamon Bradley said he went to Syria so he could tell Allah he was there.
The Muslim convert from Londonderry insisted he went to help the people suffering during a catastrophic civil war.
Londonderry Crown Court heard Bradley told police: "The killing of babies, this is what brought me there."
His lengthy interviews with officers constituted the "bedrock" of the prosecution case.
The accused was arrested by the Police Service of Northern Ireland (PSNI) after photos on his Facebook account appeared to show him posing with weapons, although an expert said they could have been decommissioned.
When he was questioned he spoke at length with detectives about his alleged role.
His defence attempted to undermine some key details of his account.
In 2013 he was watching the devastation of the Syrian civil war, as rebels fought a bloody war to depose Syrian president Bashar Assad from power.
He used a Facebook page to research the fighting, convert to Islam and make contact with people who told him how to get there, his evidence at interview showed.
They pinpointed a village on the Turkey/Syria border and he was given a WhatsApp mobile messaging number to meet with the rebels once there, his account said.
He flew from Dublin to Turkey in February 2014, and was then smuggled across the border with Syria, crossing a river using a tractor wheel as a makeshift raft.
At the time he did not know which group he was with but was taken to a training camp in the mountains and learned that it was Jaysh al Islam or Army of Islam.
Jaysh were described in court as a more moderate rebel group opposed to the regime which wanted to introduce sharia law in Syria.
It was well armed, supplied by individuals in Qatar and Saudi Arabia, an expert in the conflict told the jury.
Bradley told police he signed a letter in Arabic which he understood to mean that he was a "mujahid" fighter, the court was told.
During months of training as the only western European at a camp, his interviews revealed he was taught how to use an AK 47 assault rifle, other firearms and a grenade.
He told police: "I went to help the people. I wanted to be among those who were being bombed.
"I just wanted to be there and then I could say to Allah at least I was there to do something."
He said he was present at three battles in Aleppo, Idlib and Hama but never fired a bullet.
Bradley said sitting around doing nothing was the worst part of it, adding: "You could hear the bullets overhead."
During evidence in court it was suggested the battle in Aleppo never happened and that a police weapons expert should have more closely interrogated Bradley over his alleged training in using the AK 47.
The police interviewer suggested Bradley was withholding something from police, which his defence said showed detectives too had their doubts about his evidence.
The prosecution argued that the inclusion of details about how much he was paid and the leadership of the rebels meant the interviews were reliable, that he "agreed" that he committed the terrorism offences he was charged with.
Financial records showed he used bank machines in Turkey near the Syrian border. Then there was a gap corresponding to the period when he told police he was in Syria.
His defence said the uncertainty and ambiguity around key areas like getting into and out of Syria, his weapons training and the battles he allegedly took part in created doubt about his account.
Bradley became disillusioned and asked his commanders for permission to leave.
His mother paid for his flight home from Istanbul.
His defence said he had suffered a nightmare in the intervening period.