The title and last line translated from the Latin mean It is sweet and honorable, to die for one's country. The use of Latin is appropriate for this classical, epic view of war.
It is a view Owen completely rejects and he uses his own experience of modern warfare to do so. Straightaway, there is an ironic distance between the opening of the poem and its title.They don't march as soldiers should, they 'trudge .....through sludge'. The anti - heroic view of war is very apparent, the rhyme on this occasion suggesting its dull,repetitive nature.
When Owen describes the death throes of the victim of the gas attack, the contrast between reality and the high minded sentiment of the title becomes not just ironic, but 'Obscene' , 'bitter', words Owen uses to describe the victim's now infected lungs. It is also description which coveys the horror, not the glory of war.
The poem's ending reveals that the whole thing is actually what Owen is saying to someone who chose to repeat the lie that is the dulce et decorum est sentiment. The fact that the poem is part of a conversation, allows Owen to fully vent his anger at the sentiment, the poem's title. It is the most effective anti-war poem I have ever read.