He's alleged to have committed the most atrocious war crime of the Afghan conflict, arrested and charged with 17 murders. Now, the lawyers representing Staff Sgt. Robert Bales need to come up with a strategy to defend their client -- currently being held at Fort Leavenworth -- from a sentence that threatens to be as severe as the death penalty.

But according to lawyers with a background in military law, that defense team -- led by civilian lawyer John Henry Browne -- has very, very few options. Staff Sgt. Bales is quite likely facing several years behind bars. In the best-case scenario.

"You can't argue self-defense, or combatant immunity, the way you might in other instances of soldier violence," Morris Davis, a professor at Howard University Law School and formerly the chief prosecutor for the terrorism trials at Guantanamo Bay, tells Danger Room. "I'd say that insanity is really the only card in their deck."

Browne seems ready to play the cards of mental instability, combat-induced stress, and, perhaps, insanity. "[PTSD] is commonly used in military defense," he told Reuters earlier this month, before being retained by Bales. Last week, referring to his client, Browne noted that "[Bales] has no memory," of the night he allegedly committed the murders. And earlier this week, he told the AP that Bales "suffered tremendous depression" after enduring a traumatic brain injury (TBI) during his third deployment.

But PTSD -- whose symptoms can include outbursts of rage, insomnia and emotional numbness -- rarely, if ever, qualifies a soldier as insane. In fact, not a single service member accused of murder has ever been found not guilty by reason of insanity, according to a report from Stars and Stripes.

There's still a lot we don't know about Bales, his mental health, and the circumstances surrounding the night in question. It remains unclear whether or not he was, in fact, treated for PTSD or a Traumatic Brain Injury. There's no reliable report yet on Bales' mental state -- before he went to war or after. Plus, questions persist about whether military medical staffers, or Bales' own colleagues, missed key warning signs or were more involved in the massacre than it initially appears. Any new information could entirely transform how Bales' defense strategy will play out.

If it does play out as Browne suggests, with an emphasis on Bales' mental wellness, then legal experts are emphasizing that it's an extremely difficult strategy -- especially for defense teams working in military courts -- to successfully cobble together.

In short, a defense team would need to prove that Bales was "so detached from reality that he didn't know right from wrong," Davis says, and therefore couldn't appreciate -- either before or after committing the alleged murders -- the wrongfulness of those actions.

In a civilian court, that's tough enough. For those tried by military court martial, as Bales will be, two key factors make that defense strategy even harder. First of all, military personnel typically live in close quarters and operate under a rigid hierarchical command -- making it tough to sell a jury on the idea that someone with severe mental health problems went unnoticed. "Bales wasn't out in the woods, letting things fester," Davis says.

Second, military courts abide by a particularly rigid set of criteria to determine insanity. "A PTSD diagnosis by itself typically is not sufficient to create a defense based on insanity," says Michael Navarre, a special council for the D.C.-based law firm Steptoe & Johnson and a former Lieutenant Commander in the U.S. Navy Judge Advocate General's Corps.

The same appears true of traumatic brain injury (TBI), which Bales is reported to have suffered in 2010 and which has been linked to violent behavior in several studies.

The defense team's likely goal in spotlighting Bales' TBI and, possibly, his PTSD? It wouldn't even be to get their client off the hook -- it'd be to save him from death, by proving that while he may have slaughtered 17 people, he did it without premeditation because of a mental state weakened by brain injury and the rigors of four deployments.

"[Browne] is most likely making statements about PTSD and TBI that are part of an effort to negate the capacity for premeditation," Navarre explains. "He's trying to build a case that'd leave the death penalty off the table."

Unfortunately for Bales, it's starting to look like his own behavior -- and comments made by his own loved ones -- will compromise that strategy. Most important is that, as reported earlier this week, Bales may have walked off base to commit some of the murders, returned, and then gone back out to kill yet again. "That's a big, big problem," Navarre says, and it might hinder lawyers from crafting a narrative that portrays Bales as out-of-touch with his own intentions and actions.

And while uncertainty persists over whether Bales had ever been diagnosed with PTSD, the AP reported his own wife stating that "her husband showed no signs of PTSD before he deployed," and "she feels he was mentally fit when he was deployed." Of course, studies suggest that repeat deployments do increase one's risk of PTSD, but a seeming dearth of any diagnosed, preexisting mental health condition will force Bales' legal team to fight an even tougher battle.

Then there's the question of who's going to pay for Bales' legal defense. Veterans groups are typically vital in helping fellow soldiers who find themselves in court. But with those groups already calling foul on Browne's narrative -- that Bales, scarred by the rigors of war, committed these atrocious acts -- there's a chance the veterans won't shell out. "Obviously, the team thought about what strategy they were going to use," Navarre says. "But by going hard on this angle, they're definitely taking a risk of alienating veteran's groups."

Before any of the defense strategy plays out, Bales will likely undergo examination by a sanity board. Comprised of military psychiatric experts, that panel will help determine whether Bales is fit to stand trial, and might identify any "mitigating" issues (like PTSD or alcohol use, for example) that may have affected his behavior the night of the alleged crimes.

Even if Bales is fit to stand trial, it might be years before he does. The trial of alleged Fort Hood shooter Nidal Hasan, for example, still hasn't even started, nearly 15 months after the deadly incident. "Bales' case is so complicated, I would be very, very surprised," Davis says, "if it's wrapped up even within a few years."

*Update: This story has been tweaked to reflect an important distinction made by Michael Navarre in the 10th paragraph, which is that PTSD diagnoses have been used in civilian cases to convince a jury of an insanity defense. We apologize for the inaccuracy. *