In Wilfred Owen's poem Dulce Et Decorum Est, what poetic techniques are used?

Wilfred Owen was a major war poet, writing about his experiences during World War I. He died days before Armistice Day, the ending of that brutal, bloody war. Unlike today's "surgical strikes," World War I was fought in trenches. Seventeen million soldiers and civilians died between 1914 and 1918. 

Owen's poetry reflects the horror of that war and Dulce et Decorum Est is part of that representation. The poem begins and ends with a major irony. "Dulce et decorum est" is Latin, meaning "sweet and beautiful it is." What follows this title is anything but sweet and beautiful. In describing what happened to a victim of gassing, the poem states:

If in some smothering dreams, you too could pace
Behind the wagon that we flung him in,
And watch the white eyes writhing in his face,
His hanging face, like a devil’s sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues,—

There is nothing sweet and beautiful about this. 

The irony continues in the last four lines of the poem, stating that if we were to truly understand the horror of war, we would not be telling children, particularly British school boys of the early 20th century and their descendants, the old Latin saying, "Dulce et decorum est pro patria mori," which means, "sweet and beautiful it is to die for one's country." People choose to go to war and die for their countries, but that process is ugly and profane. 

A second major poetic device Owen uses is simile. Owen's challenge as a writer is to portray the level of horror he felt during this situation to readers who had never been in a trench, never been shot at, never been subject to gas, never watched friends die in agony. Simile offers him a chance to compare what he saw in war with horrible things people experience in everyday life, such as a devil, cancer, and bitter cud (cud is something to be chewed on for awhile; cows chew on cud). 

Finally, like many poets, Owen uses the sound of words as a means of unifying the poem. Rhyme and assonance make the poem hold together, since words that share sounds feel like they belong with each other. The rhyme scheme is a, b, a, b, which means that every other line rhymes. The verses vary in length but this rhyme scheme is carried through the poem. 

Assonance is the use of the same vowel sound across words. Here is an example of the use of the short /u/; words with this sound are italicized:

Gas! GAS! Quick, boys!—An ecstasy of fumbling
Fitting the clumsy helmets just in time,
But someone still was yelling out and stumbling

The sound /m/ in this passage is an example of consonance, the use of the same consonant sound, even if this does not occur at the beginning of the word (which would be alliteration). 

It is almost impossible to relate how terrible war is to experience; yet Owen makes a mighty effort to persuade his readers that war is not glorious or something to be romanticized.