We can absolutely read “Dulce et Decorum Est” as an anti-war poem. The speaker describes the idea that it is sweet and becoming to die for one’s country—via a quotation in Latin from the Roman poet Horace—as “the old lie.” It is, he implies, the lie that countries, or leaders of countries, tell their citizens to compel them to go to war and fight the country’s enemies. The speaker suggests that people are quite willing to prey on young citizens’ youthful desire “for some desperate glory” and glorify war so that it sounds like a beautiful opportunity to achieve that glory.
Ultimately, however, war is a horrifying experience in which death is anything but sweet and becoming. To prove this, the speaker describes the death of one of his fellow soldiers, a man who did not get his gas mask on in time and inhaled the poison gas used as a weapon by their enemies. This man, upon inhaling the gas, appeared to be “flound’ring like a man in fire or lime,” “drowning” under a “green sea” of haze and mist; it is not a quick death. The speaker and his fellows “flung” the man into the back of a wagon and watched “the white eyes writhing in his face.” They listened and watched in horror, helplessly, as his blood came “gargling from the froth-corrupted lungs.” It is painful, torturous even, and not at all glorious.