Tolkien was finishing his time at Oxford when the war errupted. For a time he served in a kind of ROTC, training on weekends while finishing up his studies. In July 1915 he entered full service. at the rank of Second Lieutenant, and was assigned to the 13th battalion of the Lancashire Fusiliers. His fascination with words led him to chose to be a Signalling Officer.
He married Edith Bratt in March of 1916, feeling he would soon be sent to the front. He was right, on June 4, 1916 his regiment was sent to France, reaching the front near Bouzincourt by June 30. The next day, July 1, was the beginning of the murderous confrontation known as the Battle of the Somme: On that first day the British lost 20,000 men.
Tolkien's B Company first went "over the top" (climbing out the trenches and advancing through the mud and uncut barbed wire of "no man's land" into German machine gun fire, on July 14. He saw action for 48 hours, brief rest , 48 more hours, then relieved for some four days in the trenches, then back into action for another four. This was the pattern until he came down with "trench fever" or Pyrexia and was sent to the rear on October 27.

How did this experience shape Tolkien's life and work. I only saw three references in _Letters_ as well as a chapter in Humphrey Carpenter's biography.
1. Tolkien had been very close to 3 other young men, who had formed a kind of literary club/clique at their school, the TCBS (I get the impression that the TCBS was much more important to Tolkien than the later, more celebrated Inklings). Two of them, G.B.mith and Rob Gilson also served in the Somme. Gilson was killed that first day. Tolkien would see Smith from time to time. Smith was later killed when a shell exploded as he was walking along a road well behind the lines. Tolkien eludes to these losses in the Foreward to the _Fellowship_.
Smith had written Tolkien (by then back in England in a Birmingham hospital), "may you say the things I have tried to say long after I am not there to say them, if such be my lot." Is it a coincidence that Tolkien began to put on paper his earliest "lost tale," that of Earendel, while in that very Birmingham hospital?

Tolkien directly credited his war experience for his description of the Dead Marshes and the wastes before the Morannon. The unburied, half-rotted slain with their corpse candles are a very powerful image. Tolkien would have seen such things on the Somme.

He also wrote "My Sam Gamgee is indeed a reflection of the English soldier, of the privates and batmen I knew in the 1914 war, and recognised as so far superior to myself."

Tolkien knew the "animal horrors" of war, his treatment of battles, while containing a heroic element he couldn't have known on the Somme is nuanced. Eomer knows (or thinks he knows) that he is riding to his death. There is a chill to that scene an author who hadn't known war could not convey.