Combat stress reaction (CSR) is acute behavioral disorganization as a direct result of the trauma of war. Also known as "combat fatigue", "battle fatigue", or "battle neurosis", it has some overlap with the diagnosis of acute stress reaction used in civilian psychiatry. It is historically linked to shell shock and can sometimes precurse post-traumatic stress disorder. Combat stress reaction is an acute reaction that includes a range of behaviors resulting from the stress of battle that decrease the combatants fighting efficiency. The most common symptoms are fatigue, slower reaction times, indecision, disconnection from ones surroundings, and the inability to prioritize. Combat stress reaction is generally short-term and should not be confused with acute stress disorder, post-traumatic stress disorder, or other long-term disorders attributable to combat stress, although any of these may commence as a combat stress reaction. The US Army uses the term/acronym COSR (Combat Stress Reaction) in official medical reports. This term can be applied to any stress reaction in the military unit environment. Many reactions look like symptoms of mental illness (such as panic, extreme anxiety, depression, and hallucinations), but they are only transient reactions to the traumatic stress of combat and the cumulative stresses of military operations.In World War I, shell shock was considered a psychiatric illness resulting from injury to the nerves during combat. The horrors of trench warfare meant that about 10% of the fighting soldiers were killed (compared to 4.5% during World War II) and the total proportion of troops who became casualties (killed or wounded) was about 57%.
Flight surgeons reported that the men who had been at jungle airfields longest were in bad shape:

Many have chronic dysentery or other disease, and almost all show chronic fatigue states. … They appear listless, unkempt, careless, and apathetic with almost mask-like facial expression. Speech is slow, thought content is poor, they complain of chronic headaches, insomnia, memory defect, feel forgotten, worry about themselves, are afraid of new assignments, have no sense of responsibility, and are hopeless about the future. British
Unlike the Americans, the British leaders firmly held the lessons of World War I. It was estimated that aerial bombardment would kill up to 35,000 a day, but the Blitz killed only 40,000 in total. The expected torrent of civilian mental breakdown did not occur. The Government turned to World War I doctors for advice on those who did have problems. The PIE principles were generally used. However, in the British Army, since most of the World War I doctors were too old for the job, young, analytically trained psychiatrists were employed. Army doctors "appeared to have no conception of breakdown in war and its treatment, though many of them had served in the 1914–1918 war." The first Middle East Force psychiatric hospital was set up in 1942. With D-Day for the first month there was a policy of holding casualties for only 48 hours before they were sent back over the Channel.