In 1917 Sassoon decided to make a stand against the conduct of the war. One of the reasons for his violent anti-war feeling was the death of his friend David Cuthbert Thomas, who appears as "Dick Tiltwood" in the Sherston trilogy. Sassoon would spend years trying to overcome his grief.
In August 1916, Sassoon found himself at Somerville College, Oxford, which was used as a hospital for convalescing officers. Sassoon was suffering with gastric fever. At the end of a spell of convalescent leave, Sassoon declined to return to duty; instead, encouraged by pacifist friends such as Bertrand Russell and Lady Ottoline Morrell, he sent a letter to his commanding officer entitled “Finished with the War: A Soldier’s Declaration”. Forwarded to the press and read out in the House of Commons on 30th July by a sympathetic Member of Parliament, Hastings Lees-Smith, the letter received very mixed reviews! Even his good friend Robert Graves was shocked and disagreed with the whole idea. Sassoon himself was very unsure of the action he had taken and worried about it a great deal, particularly regarding what the men of his battalion still fighting at the front would say. Nevertheless, there was no going back once the letter had been sent.
Rather than court-martial Sassoon for cowardice, which would have proved difficult with Sassoon already having been awarded the Military Cross for bravery, the Under-Secretary of State for War, Ian Macpherson, not to be outwitted, decided to send Sassoon to Craiglockhart War Hospital near Edinburgh, where he was officially treated for neurasthenia ("shell shock"). The authorities could then show that they understood Sassoon’s protest was the result of a severe nervous condition brought about by his terrible experiences in the trenches, and rather than court martial him, they would take care of him and give him the best possible treatment for this obvious ‘mental illness’.
At Craiglockhart Sassoon was very fortunate to find himself being treated by the eminent neurologist and psychiatrist W. H. R. Rivers, with whom he remained a close friend until Rivers’ untimely death in June 1922. Rivers soon deduced that Sassoon was not suffering from shell shock and knew about his protest. However, as he had been sent to him to be treated, that is what he would do.
Sassoon was allowed to do basically what he liked at Craiglockhart but still had to attend sessions with Rivers. For Rivers, there was a considerable dilemma involved in "curing" his patients simply in order that they could be sent back to the Western Front to die. He did not wish to "break" his patients, but at the same time he knew that it was their duty to return to the front and his duty to send them.
Following his appointment at Craiglockhart War Hospital, Rivers published the results of his experimental treatment of patients in The Lancet, "On the Repression of War Experience", and began to record interesting cases in his book Conflict and Dream, which was published in 1923.
Before that, in 1920, he published ”Instinct And The Unconscious” giving the substance of lectures he delivered in the Psychological Laboratory in Cambridge in the summer of 1919. The book also included appendices in which were published occasional papers written as a result of clinical experience gained during the war. The aim of the book was to put into a biological setting the system of psycho-therapy which came to be generally adopted in Great Britain in the treatment of psycho-neurosis of war. Among the subjects included were ‘Instinct and Suppression’, ‘The Repression of War Experience’, ‘War-Neurosis and Military Training’, ‘Wind-up’ and ‘Psychology and the War’. All of this was the result of his work at Craiglockhart.
In July 1917, before being sent to Craiglockhart, Sassoon wrote a poem about a man sitting at home unable to put the horrors of the war out of his mind. He retrospectively entitled this poem ‘Repression of War Experience’. He had learned a lot from his talks with W. H. R. Rivers.