How does the influence of Unbroken's setting relate to the story?

The novel, Unbroken, is set during WWII in the Pacific Theatre and tells the story of Olympic hopeful, Louis Zamperini. As the survivor of a plane crash, 40 days of living on a raft, and later, a prisoner of war, Zamperini is the epitome of the term "unbroken". Not that the horrors left him unscathed.... his return to the States was difficult, moving on with his life was even harder. His marriage suffered, flashbacks of his time as a POW permeated his life, and he self-medicated with alcohol. In the end, however, Louis' life was changed by a religious revival, a Southern preacher (Billy Graham), and the Grace of Jesus, who he accepted as his savior. In the end, his marriage was saved, and he escaped his memories..... unbroken.