What characteristics of Wilfred Owen's poetry changed after his experience in the war?

The main characteristics of Owen's poetry show the results of the poet's own trauma, both physical and emotional, while serving in World War I. Owen was wounded multiple times and experienced what was then called "shell shock" and is now known as Post-Traumatic Stress Disorder (PTSD). The poetry for which he is principally known today was written in the last year of his life and published only after his death. Poems such as "Dulce et Decorum est" and "Strange Meeting" express the horror of the battlefield and the senselessness of war.
British soldiers, like their American allies who joined them in the last two years of the war, had been encouraged to enlist based on a need to safeguard freedom and democratic values. The actual experience of the war was disillusioning to most of these men. The casualties were enormous, and new methods of warfare such as the use of poison gas made conditions even more horrific than in previous wars. There were incompetent battlefield commanders who made tactical blunders, sending waves of troops against entrenched enemy positions and causing a massive number of deaths and injuries. As in the American Civil War fifty years earlier, the firepower and accuracy of the weaponry had gone beyond what battlefield tacticians were able to deal with. The European commanders had learned little in the period from the middle of the nineteenth century to the Great War.
The horrifying conditions in the field made Owen and millions of others feel they had been tricked by their country into serving in a useless, futile cause. "Dulce et Decorum est" derives its title from a line of the Latin poet Horace, meaning "it is sweet and fitting to die for one's country" (dulce et decorum est pro patria mori). Owen quotes it ironically, saying that given the condition of men gassed on the battlefield, if the leaders could see this, they never would have told men to fight and die for the homeland. In "Strange Meeting" the speaker encounters a dead soldier who talks to him and finally reveals that "I am the enemy you killed." The poem "Disabled" tells of a hospital ward in which men who have been maimed in battle are suffering, without hope of any kind.
Owen himself was killed in battle only a week before the Armistice in 1918. His fate was thus similar to that of Paul, the disillusioned German soldier in the novel All Quiet on the Western Front, who is killed shortly before the war's end.