[1988:] The front-line soldiers of 1914-18 saw things that people should not see. Among them were hideously wounded men. [...] But what made at least as deep an impression was [...] the persistent presence of the dead. In previous wars battles had lasted a few days at most. [...] But this war was different: combat went on for months; artillery fire dismembered men in a flash; and the front line hardly moved at all. Consequently, the line of trenches stretching from Switzerland to the English Channel was littered with the remains of perhaps one million men. Soldiers ate with the dead, made jokes about them, and rifled their possessions. [...] Those buried would reappear during bombardments, and be reinterred, at times to help support, quite literally, the trenches in which they had fought. Many soldiers recalled the stench of decomposition, and the swarms of flies on corpses [...]. Everyone execrated the rats. It is difficult to imagine the nature of this ghastly environment.
[The] war in the trenches was terrifyingly new. Not only were there the innovations in weaponry but also the unprecedented degree of stress faced by hundreds of thousands of men. [...]
But where could a soldier go when he had reached the limits of his endurance at Verdun or Passchendaele? It is true that most soldiers saw limited and intermittent duty in the trenches, but eight days could last a lifetime. And the fact of prior experience may not have made it any easier [...]. What is most remarkable is not that some broke under the strain, but that so many did not. Their resilience is one of the mysteries of the war. [...]
The horrors of trench warfare also combined to produce a debilitating illness that had not previously been described. Called shell shock, it was only gradually accepted as a psychological condition. Sufferers became hysterical, disorientated, were paralyzed, or ceased to obey orders and had to be hospitalized away from the front. (J. M. Winter, The Experience of World War I, 145ff)