Several of the actors who played soldiers, including Wes Chatham (Corporal Steve Penning), Jake McLaughlin (Specialist Gordon Bonner), and Sean Huze (Captain Jim Osher), had actually served in the Iraq War themselves.
The online news article about the Mexican drug cartel that Charlize Theron reads is authored by Greg Hooper and William Kent, who are really the movie's Art Director and Visual Effects Supervisor, respectively.
The trivia items below may give away important plot points.
Based on the true story of a soldier, Richard Davis, who, after returning from a tour in Iraq, disappeared. His father, who spent twenty years in the Army, began an investigation. It turned out that Davis was murdered by four other soldiers, who served with him on the tour. The four men have been sentenced to prison, two for life, another for twenty years, and another for five years.
References to I Samuel 17 where the story of David facing Goliath include 'Come to me, and I will give thy flesh unto the fowls of the air, and to the beasts of the field' as well as beheading, both plot points.