"Dulce et Decorum Est" is one of Wilfred Owen's most famous poems. It details the suffering of soldiers caught in a chlorine gas attack and cites "the old lie: Dulce Et Decorum Est, Pro Patria Mori," which means "it is sweet and good to die for one's country."
Like his mentor, Siegfried Sassoon, Owen disdained the old imperialistic idea that dying for one's country was a glorious and soldierly way to die. In the First World War, soldiers died in ways no soldiers ever had before: namely, in chemical attacks which resulted in massive damage to their lungs that, in many cases, lasted for decades after the war (although of course Owen was not to know this). Owen addresses those at home who do not know of the horrors of war, suggesting that they would not declaim the "old lie" with "such zest" if they truly knew how things were in France.
As an anti-war poem, "Dulce et Decorum Est" is unequivocal. The men described in the midst of chlorine attacks have lost their boots and are "lame" or "blind," while those who do not reach their masks in time are seen "flound'ring" as if in "lime." If, Owen says, others could see men die as he has, they would no longer suggest that a warrior's death is a good one.