In "Dulce et Decorum Est," how does Owen use these horrific images of a poison gas attack to comment upon the saying alluded to in the title?

Dulce et decorum est pro patria mori
is from a poem by Horace that was often quoted by World War I era patriots, and means
roughly "it is sweet and fitting to die for one's country." Owen describes combat as far
from heroic or glorious, with soldiers bent over "like beggars" by their packs,
"knock-kneed," "cursing through sludge." Then the gas attack comes and one young man who
is unlucky enough not to get his mask on in time suffers a horrible death, literally
drowning in the fluid from his lungs. Those back home in Britain, claiming that war was
heroic and patriotic, Owen asserts, would feel differently if they saw the realities of
war--for him it was "blood frothing" from the boy's lungs with "white eyes writhing in
his face." In short, like others who followed, most notably Erich Maria Remarque, Owen
tries to use gritty realism to portray the horrors of war, a very effective means of
conveying an anti-war message.