The main theme of this harrowing poem is the dehumanizing influence or destructiveness of war. Other themes include death and man's utter impotence in the face of relentless physical assault.
The poet highlights his themes through visual, tactile, and auditory imagery which culminates in the proclamation of what he calls the 'old Lie Dulce et Decorum est Pro patria mori.' Translated, this literally means ' It is sweet and seemly to die for one's fatherland.'
In the poem, the soldiers are 'bent double, like old beggars under sacks,/Knock-kneed, coughing like hags...' Here, we have the visual imagery of soldiers who have been reduced to pitiful caricatures of manhood; under relentless physical assault, they can only hope to 'trudge' back to their 'distant rest' or camp in a stupor of sorts. Fatigued beyond belief, they march as if asleep. The poet tells us that 'all went lame, all blind.' The word 'all' is significant. Aside from the obvious physical implications of blindness and physical impotence, a metaphorical blindness can allude to how all the soldiers are blinded or rendered defenseless by the overwhelming forces against them. In the face of relentless pressure, the morale of the whole troop has been crippled; the mental and physical paralysis are evident in the phrase 'Drunk with fatigue.'
Even though the soldiers present a demoralized formation marching back to camp, they soon find themselves scrambling to fit on their 'helmets' or gas masks when shells explode in their vicinity. Some versions of the poem describe 'Five-Nines' exploding near the soldiers. In World War One, these 'Five-Nine' shells came from German field howitzers, 15 cm schwere Feldhaubitze 13s, which decimated whole Allied platoons mercilessly and systematically during the beginning of the war. The 'Five-Nine' reference comes from the internal measurement of the barrel which is 5.9 inches.
Take a look at the German howitzer here and here.
The poet continues the poem by describing what happens to anyone who suffers a chemical gas attack from these shells.
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,
During World War One, the Germans filled artillery shells with various deadly chemicals. These deadly chemicals caused acute and terrible physical suffering and/or death on Allied troops; they included mustard gas, chlorine gas, and diphosgene gas. All these were toxic, and many caused burning in the eyes, throats, and lungs. Compare the passage above about the physical suffering of the soldier with what happened to any soldier unfortunate enough to be caught without his gas mask when one of these shells exploded near him during the war.
"I wish those people who write so glibly about this being a holy war and the orators who talk so much about going on no matter how long the war lasts and what it may mean, could see a case - to say nothing of ten cases - of mustard gas in its early stages - could see the poor things burnt and blistered all over with great mustard-colored suppurating blisters, with blind eyes . . .all sticky and stuck together, always fighting for breath, with voices a mere whisper, saying that their throats are closing and they know they will choke."
Source: Chemical Warfare During the First World War
So, you can see that the horrific visual imagery of 'vile incurable sores' and 'froth-corrupted lungs' reinforces the themes of death and the destructiveness of war. The poet thus contends that 'Dulce et Decorum est Pro patria mori' is cold comfort in the face of agony and torture; he wishes the ugliness of war to be made known to all those who would sacrifice for freedom's sake. The battlefield is not a place to secure some 'desperate glory;' rather, it is a place of suffering and death.