Spc. Ryan Anderson, 27, could get life in prison without parole if convicted.
Anderson, a Muslim convert, pleaded innocent Aug. 9 to five counts of trying to provide the al Qaeda terrorist network with information about U.S. troop strength and tactics, and methods for killing American soldiers.
"This is a case about betrayal — betrayal of our country, betrayal of our Army, betrayal of his fellow soldiers," Maj. Melvin Jenks said Monday in his opening statement.
Anderson's attorney, Maj. Joseph Morse, argued the government has no proof the 27-year-old tank crewman had criminal intent when he contacted people he thought were members of al Qaeda.
He said Anderson "likes to pretend he's something he's not," often embellishing the truth or lying to impress people. "Evidence will show someone who isn't a very good soldier ... isn't a very good Muslim," Morse said.
Pfc. Scott Specht testified Anderson once told him he had joined the Army so he could "go to the motherland and help liberate Muslim brothers." The pair were in boot camp together at Fort Knox in January 2003.
"I was taken back by his statement. I was somewhat startled by it," he said. "I decided it was probably best that I kept my distance."
Shannen Rossmiller, a city judge in Montana, testified she contacted Anderson after coming across a posting in October on a Muslim-oriented Web site she was monitoring for signs of extremist or terrorist activity.
She said when she posted a phony call to jihad against the United States, Rashid wrote back, saying he was "curious if a brother fighting on the wrong side could join or defect."
Rossmiller contacted the Homeland Security Department, which put her in touch with the FBI.
Undercover investigators met with Anderson at a parking lot near the Space Needle in Seattle, where an hour-long discussion was secretly recorded on Feb. 9, just days before Anderson was to leave for Iraq with his unit, the Washington National Guard's 81st Armored Brigade.
On the video, Anderson offers sketches and information about weaknesses in the M1A1 Abrams, the Army's primary battle tank, authorities said. He was arrested at Fort Lewis three days after the meeting.
Capt. Jay Stephenson, a spokesman for the military prosecutors, said the charges against Anderson amount to attempted treason.
"We're at war now," Stephenson said Monday. "This is a big thing. This is a very serious and grievous offense."
The trial at Fort Lewis, an Army base south of Seattle, is expected to last five days. A conviction requires agreement by at least six of nine commissioned officers on the panel, unlike a criminal trial in a civilian court, which requires a unanimous verdict.