In The Valley of Elah (2007)

Tommy Lee Jones plays Hank Deerfield, an ex military police, who searches his soldier son who has been back from Iraq and is now going AWOL. He finds his son has died, multilated in a military jurisdiction, but the army seems to stay silent. Later it turns out that his son is murdered outside the area and he gets helps from Detective Sanders (Charlize Theron).

The Valley of Elah is the valley where David kills Goliath. Deerfield tells David, Sander's son, the story about David. "Do you know where your name comes from?" However Deerfield's version is abridged, for when David asks why the king let such a boy went to the war, he couldn't answer. Also when David asks where David got his courage from. If David knows the whole story (his mother doesn't believe it is a true story.), he would know that before going to the war, David had killed a lion and a bear. David was not afraid because he knew God was with him. As for King Saul, why he let such a boy went to war, was because he knew God was with David.

Why this movie is called 'In the Valley of Elah' - I think because Deerfield is David, only one man against the army, to find the truth behind his son's death.