Major Nidal Malik Hasan, 42, could face the death penalty if convicted of carrying out the mass shooting at a processing center where hundreds of soldiers were preparing to deploy to Iraq and Afghanistan.
The November 2009 incident is the worst shooting on a military base in US history. It shocked the military and led to an outcry for greater security from "homegrown" terror acts.
Hasan is also accused of wounding 32 fellow soldiers during the Fort Hood massacre -- people who may end up being cross-examined by Hasan after he fired his lawyers and opted to represent himself.
In a copy of Hasan's intended opening statement obtained by AFP, he acknowledged that "it's not going to be easy" to convince a jury made up of soldiers that his actions were justified.
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Hasan wrote that he planned to prove that the United States had engaged in an "illegal war" in Afghanistan and that "I did what I thought was necessary to help."
"I want to convey to you that my actions were meant to defend a people who were attacked by the United States," Hasan wrote.
It is not clear what Hasan will say when the court martial gets underway in a few weeks now that he has been barred from offering any evidence or testimony indicating he believed he was saving the lives of Muslims in Afghanistan when he admittedly opened fire.
Many military law experts agreed with the judge's ban, but Hasan's advocates argue that it violates his constitutional rights to a fair trial.
"I think that defense is obscene," said Richard Rosen, a military law professor at Texas Tech University. "That would justify shooting the president because he is targeting the Taliban and leaders of Al-Qaeda. It's offensive."
Hasan's former defense attorney John Galligan, a retired Army colonel, called the trial a "kangaroo court."
"She's essentially said you'll never hear from Nidal Hasan," Galligan told AFP. "It's a major, major error.