What was the main ghost in Vietnam?

“The Ghost Soldiers” is all about the revenge O’Brien wreaks on a young medic who he thinks didn’t treat him in time when he was wounded. (One of the most gripping qualities of the book is the narrator’s honesty about his own shortcomings.) His greed for revenge is fueled by an unattractive sense of his own worth. It blinds him, distorts his morals, and makes him temporarily seem a monster in the reader’s eyes. But he does not flinch at recalling his actions, nor does he decline to tell them to the reader. Ghost was a term that they used for the enemy.

O'Brien and Azar rattled the cans to scare Jorgenson who ended up outside with a shotgun crying O'Brien's name.