The speaker of "Dulce et Decorum Est" is a soldier who has fought on the front lines in World War I. He is bitter and disillusioned by all he has seen.
Though the real Wilfred Owen never attended university, he/the speaker shows some familiarity with the classics by quoting a famous Latin phrase, which also serves as the poem's title: Dulce et decorum est pro patria mori ("it is a sweet and fitting thing to die for one's country"). Yet as Owen's poem shows, the realities of war are anything but "sweet and proper."
Through his position on the front lines, the speaker is able to give an eyewitness account of what the modern battlefield is truly like. Despite what ideas people might have gathered from reading the heroic Greek or Roman accounts of war, the modern reality is that the soldiers are exhausted, limping, and "bent double, like old beggars." Rather than engaging in heroic battles, they are subjected to sneaky and vicious gas attacks which take a terrible toll on people who cannot get their gas masks on in time. The speaker describes one man who breathes in gas as having
white eyes writhing in his face,
… at every jolt, the blood
Come[s] gargling from the froth-corrupted lungs.
The speaker has been deeply shocked by all he has seen. War is far different from what he has been led to believe. As a result, he has taken on the mission of letting the people back home know what war is really like so that they will stop telling the "old lie" that it is patriotic and glorious.