The atmosphere or mood of a given work of literature is defined as the feeling that this work of literature evokes. Often atmosphere is linked to the setting of a story, as emotions are often created from details such as the time of day and the sounds that can be heard, for example. Given the nature of this excellent story and how the setting shifts, we could argue that a number of different atmospheres are actually created, but let us focus on the dominant mood of fear and terror. Note the following description as the platoon goes on one of their missions:
The morning was hot and very still. Not good, Kiowa said. He looked at the tunnel opening, then out across a dry paddy toward the village of Than Khe. Nothing moved. No clouds or birds or people.
Note the way that the setting creates an atmosphere of impending danger and fear. It is of course during this part of the short story that Ted Lavender is shot suddenly and killed. Thus it is that we can say O'Brien creates an atmosphere of insecurity, where the soldiers are constantly in danger and where death can spring up out of any situation.