Eugene Fidell, Bergdahl's attorney, told the Washington Post that Bergdahl will will face. charges under articles 85 and 99 of the military's Uniform Code of Military Justice. From the Washington Post:
Article 85, desertion, can be used to punish a variety of offenses, including anyone who "quits his unit, organization, or place of duty with intent to avoid hazardous duty or to shirk important service." The sentence for those convicted can include death, although that is highly uncommon and has not occurred since 1944, when Pvt. Eddie Slovik was executed by a firing squad after running away from combat duty in France.
Article 99, misbehavior before the enemy, is used to charge a service member who has run away in the face of the enemy, abandoned his unit, cast aside his weapon or ammunition or willfully failed "to do his utmost to encounter, engage, capture, or destroy any enemy troops, combatants, vessels, aircraft, or any other thing, which it is his duty so to encounter, engage, capture, or destroy."
UPDATE 3:37 pm: Bergdahl has been formally charged with desertion with intention to shirk hazardous or important duty and misbehavior before the enemy. If convicted of misbehavor, he faces life in prison; the desertion charge carries a maximum sentence of five years.