World War I was a horrific war. There was much disillusionment as expressed by Erich Maria Remarque in his novel All Quiet on the Western Front. Regarding Ernest Hemingway, his themes often express this disillusionment and a certain futility.
Some of Hemingway's plain, simple, no-nonsense method of writing may have been influenced by his experiences in the war -- a war that did not lend itself to romanticization. Hemingway's style was partly shaped by his experience as a journalist, but little in World War I would have encouraged him to adopt a more flowery, elaborate style.
An interesting book to consider here is Ragtime by E.L. Doctorow (It was made into a musical). It opens in 1902 and ends with the death of Arch Duke Ferdinand. Could be an interesting piece to study by way of comparing life before WWI and life in the years after--BIG changes! Also, there may be nothing better for pre-war, war, post-war literature than the USA trilogy by John Dos Passos. Almost no one reads him and the material and style is brilliant.
To be sure, Remarque's novel is a must read, as is Hemingway. As it gets traction with critics and schools consider that The Great Gatsby (Fitzgerald) is also post WWI. Keep in mind the later folks mentioned here expatriated themselves to Europe for a time following the war.