The Latin phrase from which the poem takes its title—Dulce et decorum est pro patria mor—says that "it is sweet and proper to die for one's country." In 1914, at the beginning of World War One, much of the propaganda put out by the British government followed this same line, suggesting that men should join the army and help win the war for their country because this was the "sweet and proper" thing to do. To die for one's country, the propaganda suggested, was "sweet" because it was honorable and glorious and "proper" because everyone should be a good patriot and put the needs of their country before their own wellbeing.
At the end of the poem, Owen calls this idea that it is "sweet and proper" to die for one's country "the old lie." He shows throughout the poem that the reality of war is violent and brutal rather than "sweet." To this end, he describes soldiers "coughing like hags" and limping through the mud, "blood-shod" and "blind" and "drunk with fatigue." He also describes a soldier who inhales poisonous gas as "guttering, choking, drowning," his lungs burning as if "in fire." These visceral images are all the more powerful when we know that Owen himself fought in World War One. Knowing this, the horrors he describes become all the more real, and thus all the more horrifying.
Possibly the most apt image in the poem to counter the "old lie" that to die for one's country is "proper" is the image of a dead soldier being "flung" unceremoniously onto the back of a wagon. The word "flung" suggests carelessness. This soldier's death is not "proper" and is barely marked at all. Instead, his dead body is treated like an animal carcass. Confronted with this image, it is difficult to believe that dying for one's country in these conditions is even remotely "sweet" or "proper."