Hasan, who is confined to a wheelchair after being shot by military police during the attack, is representing himself in the trial at the Texas army base.
In a brief opening statement on the first day of the trial, he said witnesses would testify that "war is an ugly thing," with death and devastation on both sides.
"Evidence from this trial will only show one side. I was on the wrong side but I switched sides," Hasan, an American-born Muslim, said in a roughly two minute-long opening statement.
Hasan, 42, who carried out the shooting on Nov. 5, 2009, just days before he was to be deployed to Afghanistan, has said he shot the soldiers to try to stop what he has called a US war on Islam. Thirty-two people were wounded in the attack and some of them are expected to testify.
"Evidence will show that Hasan didn't want to deploy and he possessed a jihad duty to kill as many soldiers as possible," military prosecutor Colonel Steve Henricks said in laying out the case. Some Muslims define jihad as a holy war.
Hasan could be sentenced to death if convicted. The military judge for the court-martial, Colonel Tara Osborn, has rejected Hasan's offer to plead guilty in return for being spared the death penalty.
An Army psychiatrist at Fort Hood at the time of the shooting, Hasan has since apologized for being in the US military and helping the US response to the Sept. 11, 2001 attacks. He has tried to renounce his US citizenship.
Hasan spoke very little during the opening statements and as testimony was heard from eight witnesses during the first morning session of the trial on the sprawling military base between Dallas and Austin, Texas.
He asked questions of two witnesses - his Army psychiatrist supervisor, and a man who accompanied him to Muslim services at a mosque two or three times a week.
Hasan had been rated on his performance appraisal, "Outstanding performance, must promote," said supervisor Ben Phillips, adding that everyone got that rating.
"I checked off that as I do with every officer because if you check any other, you basically end that officer's career," Phillips said.
Pat Sonti testified that on the morning of the shooting he accompanied Hasan to prayers at a mosque. Hasan took over the microphone to lead the call to prayer at the mosque even though it was Sonti's turn.
Hasan asked Sonti if there was a protocol for determining who leads prayer and Sonti said the Imam signals the chosen person. Hasan was not chosen but did it anyway, Sonti said.
Three of the witnesses were from a gun store near the base, where Hasan bought the pistol used in the shooting. Store manager David Cheadle said he showed Hasan how to assemble the pistol while Hasan recorded him on video.
Frederick Brannen, a former sales clerk at the store, testified that he sold Hasan the gun.
When the weapon was presented as evidence, Hasan said: "Your honor, that is my weapon."
A review by a former FBI director found Hasan had exchanged emails with Anwar al-Awlaki, a US-born cleric linked to al Qaeda's Yemen-based wing. Awlaki was killed in a US drone strike.
Judge Osborn ruled on Friday that prosecutors may present evidence that Hasan was on the Internet in the days, and even hours, before the attack, searching terms such as "Taliban" and "jihad."
Hasan has said he plans to call only two witnesses at trial, according to Fort Hood officials. The witnesses were not identified. Hasan may cross-examine any witness, including survivors of the attack.
He faces 13 charges of premeditated murder and 32 charges of attempted premeditated murder. The dead included 12 active duty soldiers and a retired chief warrant officer who worked as a civilian employee at the base.
The jury of 13 Army officers includes nine colonels, three lieutenant colonels and a major.
The trial had been delayed repeatedly over procedural issues, such as whether he would be allowed to keep a beard that violates military grooming regulations, which he has said he wears for religious reasons.
Hasan had sought to use a "defense of others" strategy at trial, arguing that his actions were taken to protect Muslims and the Taliban in Afghanistan from US assaults. Osborn denied that request.
A unanimous verdict of guilty is required for execution to be an option. The last execution carried out by the US military was in 1961.
Witnesses scheduled for the afternoon include a soldier who was shot by Hasan and survived.
