The title of the poem ‘Dulce Et Decorum Est’ refers to the popular latin phrase at that time “Dulce et Decorum est Pro patria mori” (It is sweet and fitting to die for one’s country.). It was originally a part of the Roman Poet Horace’s Ode 3.2. Owen ends the poem with these lines to accentuate the fact that participation in war may not at all be decorous. The poet can in no way justify the sufferings of war.
 
The use of the phrase in the title to mean the opposite of what it actually means is an irony. Though the phrase glorifies war, the poet has used it to criticize its purpose. Moreover, it is a reference to an earlier literary work of Horace. In that sense, it is an allusion.