Maj. Nidal Hasan is representing himself but told the judge Wednesday that he wouldn't be calling any witnesses in his defense.
Hasan has sat mostly silent during the trial's first two weeks, raising few objections and briefly questioning only three of prosecutors' nearly 90 witnesses.
During his brief opening statement, he said evidence would show he was the shooter, and he said he'd "switched sides."
The American-born Muslim suggested before trial that he wanted to argue the killings were in defense of Taliban fighters in Afghanistan. A judge rejected that strategy.
Hasan is accused of killing 13 people at the Texas military base. He faces the death penalty if convicted.