They all speak of fighting a terrible war at a terrible cost. Many of the poems also speak of dying. In "Anthem for a Doomed Youth," Wilfred Owen writes, "What passing-bells for those who die as cattle? / Only the monstrous anger of the guns. / Only the stuttering rifles' rapid rattle / Can patter out their hasty orisons" ("Anthem"). Sassoon writes of death in "The General," "Now the soldiers he smiled at are most of 'em dead," ("The General"), and Robert Brooke writes in "The Soldier," "If I should die, think only this of me: / That there's some corner of a foreign field / That is forever England" ("The Soldier.)...

It was a dark, painful, and horrible subject, and that is why their poems are so dark and disturbing.
References

Brooke, Rupert. "The Soldier" YOU NEED TO ADD TEXT AND PUBLISHER TO THESE REFERENCES p. 2050

Owen, Wilfred. "Anthem for Doomed Youth," p. 2066;

Dulce Et Decorum Est," p. 2069;

Disabled," p. 2071, from "Owen's Letters to His Mother," p. 2072

Sassoon, Siegfried. "They," p. 2055;

The General," p. 2056;

Glory of Women," p.…