Sassoon was hospitalised at Somerville College when he became ill
Along with the likes of Wilfred Owen, Isaac Rosenberg and Vera Brittain, he was one of World War I's most important poets.
Now the works of Siegfried Sassoon will reach a new audience.
The University of Oxford is marking the Armistice by launching the first ever online collection of his war poetry.
The previously unseen material forms a large part of the First World War Poetry Digital Archive, gathered from collections all over the world.
Sassoon was in the 3rd Battalion, Royal Welch Fusiliers in France during the War, serving as a transport officer. He was given the nickname 'Mad Jack', and was well known for his bravery; he was awarded with a Military Cross for his actions during a raiding party in 1916.
When Sassoon was struck with illness and sent back from the Front in August 1916, he was treated at Somerville College in Oxford which had turned into a military hospital.
During his recovery period he was introduced to the influential aristocrat Lady Ottoline Morrell and visited her at Garsington Manor. He returned to the front line again in 1917 but was injured by a sniper.
Encouraged by Morrell and mathematician and fellow pacifist Bertrand Russell he then wrote his declaration against the war.
In the damning letter he proclaimed: "I have seen and endured the sufferings of the troops, and I can no longer be a party to prolong these sufferings for ends which I believe to be evil and unjust."
Original manuscripts for poems like "The General" are now available
Sassoon narrowly escaped a court martial and was instead sent to Craiglockhart Military Hospital to be treated for shell shock. It was at the hospital that he met another poet, Wilfred Owen.
By May 1918 he was back in France helping the war effort, but he was accidentally shot in the head by one of his own troops, putting an end to his military career.
During this time he published a great deal of work inspired by the War including The Old Huntsman, Counter-Attack and Other Poems, and Picture-Show.
After the War, Sassoon moved in literary circles in and around Oxford and later in life he received an honorary degree from Oxford University.
He died at the age of 80 but his name remains on a slab in Poets' Corner in Westminster Abbey.
"Siegfried Sassoon ranked alongside Wilfred Owen as the most widely read of all of the poets of the First World War," said Dr Stuart Lee, Director of the Archive.
"Technology has enabled us to virtually reassemble Sassoon's manuscripts, which for years have been dispersed across the UK and USA, so they are now freely available for anyone to read.
"It is fascinating being able to see the corrections and crossings-out he made to the manuscripts, invaluable to researchers studying the literature of the War, and provides a rich resource to enhance both teaching and learning of the period."