The speaker in Wilfred Owen's "Dulce et Decorum Est" is a soldier who has experienced shell shock, or as we would now call it, Post-Traumatic Stress Disorder (PTSD), after taking part in trench warfare during World War I. Although it is tempting to view the poem as autobiographical because Wilfred Owen himself served as a military officer for the British armed forces, there is no historical indication that Owen experienced the exact scene described in the poem. Nevertheless, Owen experienced nightmares for months when he was hospitalized for shell shock.
The speaker in the poem relates an incident of seeing a fellow soldier succumb to chlorine gas poisoning when he was unable to get his gas mask on in time. In the poem, the speaker was one of the soldiers who was "bent double," coughing and limping, "drunk with fatigue," and generally miserable with the conditions of war. In such a state, almost semi-conscious, he is helpless to assist the man who can't get his helmet on. The experience of watching the man "drown" on "the wagon that we flung him in" is one that severely traumatized the speaker, causing "smothering dreams."
The speaker turns his bitterness for the incident not at the enemy, nor at the concept of war itself, but at those who recruit young men by holding military service out as some glamorous prospect. He addresses "my friend," a woman who wrote jingoistic war poetry, saying that if she suffered from nightmares like he does, she would not be so quick to spout the "old lie" to young men that it is sweet and fitting to die for one's country.
The fact that the speaker is plagued by having observed the death of a fellow soldier while he stood by helplessly and the fact that he now speaks out against dishonest recruiting methods shows that he is a compassionate person who values truth. He isn't against war, but he wants young men to go into the experience with their eyes open, and he wants society to understand and appreciate the level of sacrifice the soldiers are making for Great Britain.
The speaker in this person is a World War I soldier who witnessed another soldier's horrible death because he did not get his gas mask on in time when the "hooting" gas shells fell. Inhaling the caustic chlorine gas destroys his lungs and causes him to drown in fluid from his own lungs. Other soldiers can only watch helplessly.
Observing this horrid scene after experiencing the exhausting, miserable march that he describes at the beginning of the poem has embittered the speaker. He declares that Horace's adage "it is sweet and fitting to die for one's country," a translation of the title, is wrong. Clearly this speaker strongly disapproves of war, which he depicts as a hellish experience.