Wilfred Owen's "Dulce et decorum est" is explicitly framed as a rebuttal to a poem by Horace. The tradition of epic poetry was to glorify war and the heroism of those who died for their country. Death in war is portrayed romantically in much war poetry in the context of noble individual combat (Homer, Arthurian romance, etc.). The reality of World War I trench warfare (and the slightly earlier US civil war) was quite different, with death being more impersonal, and offen as the result of disease or long suffering after injury. Owens attempts a controversial anti-war stance by portraying thde realities of war.
The phrase from Horace comes from an ode which begins:
It is sweet and proper to die for one's country
and death pursues even the man who flees
nor spares the hamstrings or cowardly
backs of battle-shy youths.
Owens argues directly against Horace, with the ending of his poem:
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud(12)
Of vile, incurable sores on innocent tongues,
My friend, you would not tell with such high zest(13)
To children ardent(14) for some desperate glory,
The old Lie; Dulce et Decorum est
Pro patria mori.(15)