This week a young man in Texas became the first American to plead guilty to terrorism charges related to the recent fighting in Iraq.
Michael Wolfe, 23, was arrested just before he boarded a plane. He was on his way to join ISIS, the Islamic State of Iraq and Syria, the Sunni extremist group that has been storming its way across Iraq for the past two weeks.
ISIS and hundreds of other rebel groups in Syria have inspired thousands of young men around the world to leave their homes and join the fight.
As Westerners find their way to the battlefield, officials are worried that the militants will use their new skills when they return home.
An Elaborate Recruitment System
The road to violent jihad has never been so smooth: All aspiring fighters have to do is get to Turkey.
Parallels
Behind ISIS Battle In Iraq, A Clash Between Two Arch-Terrorists