Wilfred Owen wrote "Dulce et Decorum Est" during World War One. He fought in the war himself, until he was killed in 1918, just days before the end of the war. At the start of the war, in 1914, the British government distributed propaganda to try to persuade young men to join the army and fight in the war. The propaganda boasted that the war would be over by Christmas, and that it would be an exciting adventure and an opportunity for young men to achieve honor and glory in the name of their country.
But as young soldiers like Wilfred Owen quickly discovered, the claims made by recruiting propaganda were misleading. He knew that the war was brutal, painful, and sickening. Indeed, Owen himself was hospitalized in 1917 to be treated for shell-shock. In the poem "Dulce et Decorum Est," Owen tries to challenge the claims made by the war's advocates by presenting the harsh reality of war. He describes young soldiers aged by the relentlessness of war, "bent double, like old beggars" and "coughing like hags." These images are deliberately very unromantic, and offer an impression of the reality of war which is in stark contrast to the impressions created by those who promised the soldiers honor and glory.
Owen also describes in the poem how painful the death of a soldier can be. In the second stanza, he describes the victim of a gas attack, who is left "flound'ring like a man in fire … guttering, choking, drowning" until he finally dies. The reference to fire here evokes a sense of the intolerable, burning pain in the soldier's lungs. After the soldier dies he is unceremoniously "flung" into a wagon with other dead soldiers, the blood still "gargling" from his lungs. There is nothing heroic, glorious or romantic about this soldier's death. He is simply "flung" into the back of a wagon.
At the end of the poem Owen addresses directly the aforementioned propaganda, and specifically the promise of glory and honor. He calls this promise "the old lie," a lie which he sought to expose through writing this poem.