'Shell shock' or combat stress reaction is psychological condition suffered primarily by soldiers who have seen combat. The trauma and stress of such situations causes great mental fatigue that often translates into physical illness. Today known as post-traumatic stress disorder, many soldiers who experienced this disorder after the first World War were left untreated due to general lack of knowledge about mental illnesses. Some, who deserted or committed other wartime crimes in their distress, were executed. Most were accused of cowardice.
Symptoms of shell shock vary widely and can include the 'thousand yard gaze',, fatigue, disconnection from one's surroundings, shaking, trembling, skittishness, headaches, palpitations, anxiety, incontinence, insomnia and panic attacks. Among other things.
World War I saw possibly the most notable and dramatic cases of shell shock due to the newer, more intense, explosive and chaotic artillery style of warfare which caused unanticipated and extreme psychological stress though such responses to warfare have been documented throughout history.
Here is a pretty interesting (and short) video about the subject featuring archival footage of shell shocked soldiers. Kinda creepy. Are videos allowed here, btw?
-source,
Wikipedia and a few other military history sites