5 August 2013 Last updated at 22:27 ET 
Major Nidal Hasan, who is defending himself, has admitted the killings, but argues he was protecting Muslims and the Taliban in Afghanistan.
However, a military judge has already rejected his “defence of others” claim.
If convicted on 13 counts of murder and 32 counts of attempted murder, the 42-year-old faces the death penalty.
The killings are viewed as the deadliest ever non-combat attack on an American military base.
Witnesses say that on 5 November 2009, Maj Hasan entered a crowded Fort Hood medical building, where deploying soldiers were awaiting check-ups and vaccines.
He then climbed on to a desk, shouted an Islamic benediction and opened fire with two handguns, pausing only to reload, witnesses said.
His trial will take place at the same military base, near Killeen, Texas, where he carried out the attacks.
‘Workplace violence’A jury of 13 – nine colonels, three lieutenant colonels and one major – will consider evidence in the case.
Maj Hasan was about to be deployed to Afghanistan when he carried out the attacks.
The prosecution is expected to present evidence of the US-born soldier’s radicalisation, including internet searches for “Jihad” and “Taliban” hours before the shootings.
Retired US Army Staff Sgt Shawn Manning is among the wounded who will testify at the trial The Pentagon has treated the case as workplace violence, rather than an act of terrorism, a legal distinction that has angered victims and family members, says the BBC’s Nick Bryant at Fort Hood.
Many of those wounded in the attack are expected to testify.
Maj Hasan will represent himself at the court martial, which means he will have the opportunity to cross-examine his own victims.
He uses a wheel chair after being paralysed when he was shot in the back by a base police sergeant who responded to the shootings.
Judge Colonel Tara Osborn has denied him the “defence of others” strategy, in which he would have argued that he was protecting insurgents fighting American soldiers in Afghanistan.
She ruled there was no evidence of “any immediate threat to others” from US soldiers on base.
Judge Osborn has also said he will not be able to make speeches about his beliefs or try to testify himself when he is questioning witnesses.
The trial has been delayed by a series of requests for preparation or other issues, including an objection to Maj Hasan’s beard, which was ruled to violate military regulations.
Analysts say the military prosecutors are being extra cautious in order to avoid anything that could lead to a reversal of a guilty verdict.
If found guilty and sentenced to death, Maj Hasan would have multiple automatic appeals available to him.
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Fort Hood shooting trial set to open
