Syrian terrorism accused Eamon Bradley has been found not guilty of possessing grenades with the intent to endanger life.
But a jury has been unable to return a verdict on whether he attended a training camp in the Middle East war zone where he received instructions on the use of AK47 assault rifles, two other firearms and a grenade.
Northern Ireland's director of public prosecutions is expected to inform the court within a fortnight whether the Londonderry man could face a retrial.
This was the first case of its type taken in Northern Ireland and it took the jury around six hours of deliberations at Londonderry Crown Court.
Mr Bradley, 28, originally from Melmore Gardens in Creggan, was alleged to have been involved with a Syrian rebel group opposed to the government of President Bashar Assad and Islamic State.
The "bedrock" of the prosecution case surrounded interviews he gave to police after he was arrested in Northern Ireland over images of him apparently posing with guns posted on social media.
He allegedly told a detective he was spirited into Syria from Turkey in a makeshift raft and joined the forces of the Army of Islam in 2014.
He described, according to police notes of his interview, attending a training camp and carrying an assault rifle and grenade as a junior infantryman or mujahid fighter. He was found not guilty of possessing a grenade.
The accused told detectives he denied firing a single bullet and allegedly returned home disillusioned after initially going to help the Syrian people, interview notes revealed.
Judge Brian Sherrard said Mr Bradley's defence had attempted to introduce doubts about the police questioning centring on his apparent lack of knowledge about military matters and about the Army of Islam.
An expert witness testified that the AK 47 assault rifles Mr Bradley was photographed in front of could have been deactivated.
A lawyer for the defence said one of the battles the defendant told police he was present at did not happen; it was a flimsy and threadbare case and there was no evidence to corroborate that he was even in Syria.
He said his client was not Derry's answer to Britain's First World War desert adventurer Lawrence of Arabia and was simply in Turkey on holiday.
According to the prosecution, he had truthfully agreed that he committed terrorism offences when questioned by detectives and was not a fantasist.
Barrister Ciaran Murphy QC said he was referring the case back to the director of public prosecutions Barra McGrory QC.
It is due to be mentioned in Belfast on March 3.