In poetry, war was usually depicted as an
honourable,
glorious affair.
Many soldiers turned to writing and it became modern. Naturalistic and painfully realistic, with shocking images and language, intending to show what the war really like, the war poetry showed the mud, the trenches, death, and sometimes even compassion for soldiers.
The overall massage through such poetry is that the war is brutal, vicious, meaningless, stupid and barbarous; there was nothing honourable, glorious, or decorous about the war. The daily experience of soldiers on the front was of mental disorders, nervous breakdown, caused by constant fear and pressure.
The Trench Poets question the very notion of war, heroism, patriotism, etc., they attacked the values of the pre-war society in general.