It is clear that this is one of many World War I poems that represent a crushing indictment on the whole military enterprise, presenting another image than one that views war as something noble and heroic. Things to notice in this excellent poem are the strident, angry (at times almost sarcastic) tone that presents a picture of war that is anything but glorious. Also note the choice of words and figurative language that is used to underline this message and tone.
Note how the poem begins:
What passing-bells for these who die as cattle?
Owen here presents the harsh reality of soldier-life. They do not die a glorious, heroic death, but here he uses a simile to reveal the real way most soldiers meet their death--as if they were cattle being herded towards a slaughterhouse.
What is also interesting is how Owen uses the sounds of war to make their funeral service. There are lots of examples of onomatopoeia here, for example:
Nor any voice of mourning save the choirs--
The shrill, demented choirs of wailing shells
Note here how Owen makes the "wailing shells" into the only choir that will remember the lives of these dying soldiers.
Consider too how the poem ends:
And each slow dusk a drawing down of blinds.
Clearly the onset of dusk is symbolic, as is the drawing down of blinds: the dusk represents the dying of so many soldiers, and during this time, houses would draw down their blinds to show that someone had died in that household. This then is the only remembrance for their wasted lives--and it is clear that the theme of this poem is to speak out against the horror of war.
The overarching theme or message from the poem is the futility of war. The language of the poem is one that helps to construct the grim landscape of death as an intrinsic part of war. Constructed as part of the modernist approach to World War I, the poem is a repudiation of the rationale and rationalizations behind the First World War. At a time when many young people were galvanized into fighting for nation, for religion, for protection of political identity, or for many other reasons, the poem is a bleak reminder that there is a level of futility underlying all of these proposed rationalizations. With describing elements such as “shrill, demented choirs of wailing shells” or “passing bells for those who die as cattle,” the thematic implications of lines and images such as these are that war is a fruitless endeavor. It's only accomplishment is the destruction of a nation’s youth.