John Ronald Reuel Tolkien lost just about all his friends in WWI. One of his closest, Geoffrey B. Smith, passed away on 3 December 1916, succumbing to injuries received from shell bursting.
Not long before, he had written to Tolkien:
My chief consolation is that if I am scuppered tonight – I am off on duty in a few minutes – there will still be left a member of the great T.C.B.S., to voice what I dreamed and what we all agreed upon. For the death of one of its members cannot, I am determined, dissolve the T.C.B.S. Death can make us loathsome and helpless as individuals, but it cannot put an end to the immortal four! A discovery I am going to communicate to Rob before I go off to-night. And do you also write it to Christopher. May God bless you, my dear John Ronald, and may you say the things I have tried to say long after I am not there to say them, if such be my lot.
~ quoted after Humphrey Carpenter: J.R.R. Tolkien. A Biography.
Tolkien arrived at the Somme in early July 1916. In between terms behind the lines at Bouzincourt, he participated in the assaults on the Schwaben Redoubt and the Leipzig salient. (…) On 27 October 1916, as his battalion attacked Regina Trench, Tolkien came down with trench fever, a disease carried by the lice. He was invalided to England on 8 November 1916. Many of his dearest school friends were killed in the war. Among their number were Rob Gilson of the Tea Club and Barrovian Society, who was killed on the first day of the Somme while leading his men in the assault on Beaumont Hamel. Fellow T.C.B.S. member Geoffrey Smith was killed during the same battle when a German artillery shell landed on a first aid post. Tolkien’s battalion was almost completely wiped out following his return to England.
~ according to the Wiki