The title of this poem about World War I comes from a longer Latin phrase: Dulce et decorum est pro patria mori. In English, it means "it is sweet and proper to die for one's country," much the same line touted by those who tried to get young men like the speaker to go off to war by portraying it as a glorious, heroic struggle.
But the scenes of war the speaker describes seem to contradict this. In the first stanza of the poem, the speaker describes the soldiers fighting in the Great War as "bent double" and compares them to "old beggars under sacks." The implication here is that the soldiers are weighed down and appear to be aged by the hardships of war. The speaker also says that the soldiers are "coughing like hags," which implies that the war has made them sick.
In the second stanza of the poem, the speaker describes a gas attack. He describes one soldier "flound'ring like a man in fire" after inhaling the poisonous gas. This simile suggests that war is excruciatingly painful, and anything but "sweet and proper" as the title suggests. This impression, and thus the tragic irony of the title, is compounded when the speaker describes the same soldier "guttering, choking, drowning."
In the third and final stanza the speaker describes the dead body of the soldier who inhaled the poisonous gas. The descriptions are deliberately visceral and graphic. The dead soldier's "hanging face" is "like a devil's sick of sin," the implication here being that even the devil would be "sick," or tired of the sins of the war. The speaker also describes the "blood / Come gargling" from the soldier's lungs, and "the white eyes writhing" in the soldier's face.
At the end of the poem the speaker addresses the reader directly, referring back to the phrase from the title:
My friend, you would not tell with such high zest
To children ardent for some desperate glory,
The old lie: Dulce et decorum est
Pro patria mori.
The propaganda leading up to World War I suggested that the war represented a chance for young men to achieve glory and honor, and it also suggested, as indicated by the title of the poem, that it was "sweet and proper" to give one's life for one's country. The speaker at the end of the poem says that these claims made by the propaganda are lies. He points out that the reality of war is neither "sweet" nor "proper," but rather hideous and brutal. This is the tragic irony of the poem: that the reality or war is so far removed from the ideas put forward by propaganda.