First World War - enthusiasm, mobilisation, technology, trenches.

Wilfred Owen's poetry comes in the latter stages of the war when bitterness at the incompetence of leaders, the horrific waste of young lives.
Another important point here was  the realization that the optimism of MODERNISM, the Industrial Revolution, the spread of literacy and education, the advances in Science and Technology were not serving to improve humanity but to kill our fellow man more brutally, ruthlessly and efficiently. This is one of the sources of a general pessimism in twentieth century literature.

The fact that we had a second world war twenty years later with even more efficient brutality, aerial bombing of cities, the atomic bombs, the mass murders in the concentration camps helped create the literature of THE ABSURD, Existentialism, Postmodernism etc You can see this in "Waiting for Godot" and Bukowsky.