From The Art and Popular Culture Encyclopedia
Shell shock was the reaction of soldiers in World War I to the trauma of battle. It has been described as a reaction to the intensity of the bombardment and fighting that produced a helplessness appearing variously as panic, or flight, an inability to reason, sleep, walk or talk.
During the War, the concept of shell shock was ill-defined. Cases of "shell shock" could be interpreted as either a physical or psychological injury, or simply as a lack of moral fibre. While the term shell shock is no longer used in either medical or military discourse, it has entered into popular imagination and memory, and is often identified as the signature injury of the War.
In World War II and thereafter, diagnosis of "shell shock" was replaced by that of Combat stress reaction, a similar but not identical response to the trauma of warfare.