The most important theme in Dulce et Decorum Est is the difference between the bitter realities of war and the idealized version of war perpetuated by people who send young men off to fight them. One device that Owen uses to show this theme is the line from the poem by Horace from which the title is taken. One line from this poem reads:
Dulce et Decorum est, pro patria mori.
This line serves as a sort of epigram for Owen's poem. It is a pithy summation of precisely the idea that he is writing to dispel. In order to do so, he uses graphic imagery to describe a gas attack on his platoon:
But someone still was yelling out and stumbling
And floundering like a man in fire or lime.--
Dim, through the misty panes and thick green light
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
In describing the man's horrible death, Owen takes on a bitter tone to ridicule the idea that war is an ennobling, honorable aspect of the human condition. He uses words like compares the man's face to a "devil's, sick of sin." He describes the blood the man coughs up as:
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues...
This grotesque imagery and diction is juxtaposed with the soaring rhetoric or Horace (and, by implication, those in England who sought to glorify war) to highlight the absurdity and brutality of war.