Sassoon wrote his scathing letter criticizing the war in June of 1917 – see link below:
https://en.wikisource.org/wiki/Finished_with_the_War:_A_Soldier’s_Declaration
Sassoon’s letter was forwarded to the press and read allowed in the House of Commons. This letter was seen by some as treasonous and Sassoon was lucky he was not executed. Instead Sassoon was sent to Craiglockhart War Hospital where officers with shell shock would convalesce. Sassoon met fellow patient Wilfred Owen and encouraged him to write poetry. The hospital’s monthly publication, “The Hydra”, contains some of Sassoon’s work. It offers an interesting record of life at the hospital in 1917–18, having been produced by the officers who were being treated there. Sassoon (1886-1967) would live to be 80 but Wilfred Owen was killed at the age of 25 just one week before the Armistice.
See Link below for free online access to this rare and interesting look at the writing talent of the “Lost Generation.”