What is told about the background to Bromden’s psychological problems?

Chief Bromden acts primarily as a narrator who describes external conditions rather than his own psychology. But in Chapter Fifteen, he provides some indication of the origin of his psychological problems. Bromden compares the imaginary “fog machine” of the mental institution to the real fog that apparently surrounded him during wartime as a matter of military tactics. This tale indicates that Chief Bromden likely suffers from some sort of shell-shock caused by his war experience.