Military prosecutors have reached into a section of military law seldom used since World War II in the politically fraught case against Army Sgt. Bowe Bergdahl, the soldier held prisoner for years by the Taliban after leaving his post in Afghanistan. Observers wondered for months if Bergdahl would be charged with desertion after the deal brokered by the U.S. to bring him home. He was — but he was also charged with misbehavior before the enemy, a much rarer offense that carries a stiffer potential penalty in this case.