Craig Baxam, 24, from a Maryland suburb outside Washington, served in Iraq and Korea for the US military over four years. He received cryptology and intelligence training.
He enlisted in the US Army in 2007 and was deployed to Iraq. He re-enlisted in August 2010 and was deployed to South Korea for one year, but he left a month before completing his term, according to an FBI affidavit.
Prosecutors say he converted to Islam days before he left the army last July.
He was arrested in Kenya last month and has been accused of attempting to travel to Somalia with hundreds of dollars to join the terrorist organisation Al Shabaab.
Prosecutors say he wanted to be a fighter for the rebel group, which has links with Al Qaeda.
He was interviewed by FBI agents in Nairobi in late December. Baxam said he wanted to die "with a gun in my hand" and that he would be happy to die defending Islam, according to the affidavit.
He was arrested when he arrived back in the US on January 6, charged with trying to provide material support to Al Shabaab.
If convicted he could face a 15-year prison sentence.
The US government has designated the Al Qaeda-affiliated organisation a terrorist group and Al Shabaab has sought to recruit Americans to fight for them in Somalia.
The group claimed responsibility for a deadly 2010 bombing in Uganda and has waged a long, violent battle to control Somalia.
