The mood is unremittingly bitter, bleak, harsh and unpleasant, showing in viscerally thick verbal detail the absolute horror endured by the men who fought in World War 1.
Within that, though, I think there a few separate moods in the various stanzas of the poem. The first stanza is heavy, tired, almost asleep with weariness, and the rhythms drop in heavily, slowly, painfully...
All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of tired, outstripped Five-Nines that dropped behind.
Then, as the gas explodes, the poem picks up pace and the mood becomes one of terrified, gasping panic:
Gas! Gas! Quick, boys! – An ecstasy of fumbling, Fitting the clumsy helmets just in time...
There's then another painfully bloody and gruesome description of a man dying from gas, and the mood is almost wincingly painful, emphasising little details. And the final stanza then changes into a new, more angry, more ironic mode: the mood is aggressive, hostile to the reader, and it hammers home its final ironic point:
You would not tell...
The old Lie; Dulce et Decorum est
Pro patria mori.
An exceptional, wonderful poem, I think. Hope it helps!
In the first stanza of the poem, the mood is gloomy and depressing. This is because Owen creates an image of soldiers who are battle-worn, fatigued and weary. They cough like "hags" and are missing vital pieces of equipment, like boots. For Owen, this is the reality of war: it is not glamorous or noble; it is desperate and difficult and transforms men into shadows of their former selves.
When Owen describes the gas attack, the mood of the poem changes. It becomes violent, as Owen describes in detail the "guttering" and "choking" of such an attack. Moreover, he uses imagery to reinforce this violence: he talks about the "gargling" caused by the gas attacking the lungs and "incurable" sores on the tongue.
In the final lines of the poem, the mood is bitter and ironic as Owen reveals his message: that war is not glorious at all and that fighting for one's country is not sweet, as the title suggests. This idea is, in fact, the greatest lie told to men and boys.
Wilfred owen/ telling about his war experience.
Death, WWI, gory, and sick.
Dulce et decorum est.
Fear, violent, and ghastly.
Death, violence, war, and misery.