Staff Sgt. Camilo Mejia, a reservist in the Florida National Guard, is charged with desertion after failing to return to his unit in Iraq after a two-week furlough in October.
He turned himself in to the Army in March after being gone five months, saying he did not want to fight in an "oil-driven war."
Mejia, 28, faces a year in prison and a bad-conduct discharge if convicted of desertion, which military law defines as leaving the military with no intention to return or to "avoid hazardous duty or to shirk important service."
His attorney, Louis Font, has argued that the Army illegally extended Mejia's service. His enlistment was originally set to end in March 2003, but he was ordered to stay after his unit was called up for the war in Iraq.
Mejia's attorney said he should have been exempt because a 19th-century treaty between the United States and Costa Rica exempts Costa Rican citizens from "compulsory service" in the U.S. military.
Capt. A.J. Balbo, the lead prosecutor, said Mejia never requested an exemption before his court-martial and voluntarily went to fight in Iraq, where he accepted a promotion.
Mejia joined the Army as an active-duty soldier in 1995 and entered the National Guard in 1998.
The judge did not immediately rule on the request.
After returning in March to Fort Stewart, Mejia applied to become a conscientious objector, saying his experiences in Iraq had made him opposed to war.
Mejia said he became upset after seeing civilians hit by gunfire during an ambush on his unit and at seeing an Iraqi boy die after confusion over which military doctor should treat him.
He also claims he saw Iraqi prisoners treated "with great cruelty" when he was put in charge of processing detainees last May at al-Assad, an Iraqi air base occupied by U.S. forces.
Mejia's objector application is being considered separately from his court-martial on the desertion charge.
By Russ Bynum