The poem centrally tries to convince its audience that war is not the glittering arena of strong uniformed heroes who valiantly meet in armed combat and die glorious deaths for the sake of their nation. This is something that is achieved through stanzas of varying line lengths that generally are pentameters (have ten syllables). It is important though to see how Owen subverts a traditional poetic form by using the pentameters to express his message.
Firstly, note the way that each stanza has a varying number of lines. This is most clearly shown in the third stanza which only contains two lines for emphasis. Owen clearly uses this to highlight the importance of this short stanza, as it describes the full haunting horror of his memories of watching his fellow soldier die from gas, away from the front lines, dying a terrible death full of suffering and indignity.
This is something that structurally we can observe as he expresses his bitterness in the final stanza. The "old lie" that he criticises so intensely is focused in the Latin phrase that ends the poem. What is important to note is the run-on line or enjambement that is used to leave "Pro patria mori" danging by itself at the end of the poem, perhaps signalling it out for particular scorn from the perspective of the author. To die for one's country is precisely the lie that the poet is trying to expose as being anything but sweet and fitting.