Ebrahim B. and Ayoub B. went to fight for Isis in June 2014. After three months, disillusioned and scared for their own lives, they fled the war zone and made their way back to Germany, they say.
They were arrested in January and now face a charge of joining a foreign terrorist organisation, a crime that comes with a jail term of up to ten years.
Ayoub B. faces an additional charge of training in the use of weaponry and planning an act of violence against the state, an offence which also carries a ten-year sentence.
This is not the first case against former Isis recruits to come to trial. But the defendants' decisions to speak openly about their experiences is set to provide an intimate glimpse inside the workings of the terrorist cell.
"When you want to stop [Isis] from deep in your heart, you go public and you talk about it," Ebrahim B. told NDR before the trial in the first such interview an ex-Isis fighter has given to a journalist.
The experience was so horrific that jail in Germany was preferable to freedom in Syria, he said.
Ebrahim B., who like Ayoub B. is the son of Tunisian immigrants, says he was deceived by a "false preacher" who worked as an Isis recruiter in Wolfsburg, the two young men's hometown.
He promised him that in Syria he would drive a sports car and have multiple wives.
But the reality was one of imprisonment and fear, he said.
Immediately upon arriving in Syria his phone and passport were confiscated and he was locked into a guarded facility. Later, after completing initial training he was given a choice between being a fighter or a suicide bomber - "a choice between death and death,“ he told NDR.
Ebrahim B. was sent into Iraq and placed in a villa where future suicide bombers were housed to the west of the capital, Baghdad.
There he claims to have heard a Saudi man being tortured to death on suspicion of being a spy. His corpse was taken and put in front of the others to scare them, he said.
"[Foreign fighters] are used as cannon fodder," he told the television channel. "There is massive pressure put on you to go and die. When they can't convince you to do it they force you."
Ebrahim B. claims that he managed to escape this fate by hiding on a roof. From there he escaped from the country, but how is still not clear. Prosecutors charge that Iraqi police stopped him on his way to carrying out a suicide attack.
Ayoub B. spoke at the trial on Tuesday of the man - referred to by Ebrahim B. as the 'false preacher' - who convinced as many as twenty young men from Wolfsburg to join the jihad, the Hamburger Abendbalatt reports.
They called him 'the learned brother' and he used to speak more about Allah than about battle, Ayoub said.
"You will see the lights of Turkey and you can come back whenever you want," he claimed the man told him.
Ayoub said that he traveled to Syria to learn more about his religion, but when he was there he was forced to learn how to fight.
Texts sent to family and friends from inside Syria suggest Ayoub B quickly came to regret the decision.
"I'm afraid of being caught. They think I'm a spy," he texted his brother.
"I needed to get out. It was enough what I experienced," he messaged another friend after making it back to Turkey.
Radicalization in jail
How best to deal with returning jihadis is still an open question with some calling for an amnesty for fighters who willingly leave Syria - a possible means of encouraging them to reverse a bad decision.
Meanwhile ex-fighters who can talk honestly about their regret at what they did and the reality of the situation are also seen as a key weapon in tackling Isis propaganda.
But Germany offers no amnesty to returning fighters, Cornelia Lotthammer of the Violence Prevention Network - an NGO which works to de-radicalize youth - told The Local, arguing that such a policy is "not necessary."
De-radicalization is a long process, she explains, estimating that it takes around two years.
"One must check to see that the regret is genuine," she said, adding that "if they have killed some one they must go to jail."
But it is important that NGOs are there to keep a dialogue open while they are incarcerated.
"There is a risk of radicalization or recruitment while they are in jail, or of them becoming martyrs."