“While there is no specific U.S. law against fighting with a force like the peshmerga, the State Department has explicitly discouraged it. After he left, Smith said he called the FBI, notifying the agency of his whereabouts. ‘They knew I was here,’ he insisted, ‘but they still felt the need to wake up my mother in the middle of the night and give her a heart attack.’”
Source: The Americans Who Volunteer to Fight ISIS – Defense One