young men have been utterly dehumanized by their experiences, reduced to cattle as they are slaughtered in their droves by the "monstrous anger of the guns."

For these men, there are no angelic voices to serenade them to their graves, no prayers or bells; just the "shrill, demented choirs" of wailing shells. Here, as elsewhere in his work, Owen is attacking the romanticization of World War I, the widespread belief that there is something inherently noble about the mechanized slaughter of the trenches. In contrasting bells, choirs, and prayers with the "rapid rattle" of rifle fire and the "monstrous anger of the guns," he is showing us the stark difference between the romanticized view of war and its sordid reality.

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.

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.

Wildred Owen's "Anthem for Doomed Youth" describes the juxtaposition of what youth is meant to be and what young soldiers get instead. Owen begins his poem by posing the question, "What passing-bells for these who die as cattle?" A passing bell is a bell that is rung after someone dies to signal that it's time for prayer. In war, the soldiers die so frequently and quickly that rather than a passing bell, each death is marked with more death.

Only the monstrous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.

Owen paints a picture of desolate sadness where instead of being loved and cared for, boys are killed and left behind. They may be mourned upon their death, but they aren't around to know it's happened.

War is described as a heartbreaking alternative to life. The soldiers sent to war are described as doomed, facing death in lieu of life. Moreover, their deaths are not at home, in comfort with their loved ones. They are killed, one after another, in a cold and desolate place where those around them fall to their deaths moments later.

Yes...  I mean it doesn't come out and say "war is bad" but here are the relevant lines.  Seems to me to be sort of saying that it's dumb to go off to war and all you get is dead young men.

Where have all the young men gone?
Long time passing
Where have all the young men gone?
Long time ago
Where have all the young men gone?
Gone for soldiers every one
When will they ever learn?
When will they ever learn?

Where have all the soldiers gone?
Long time passing
Where have all the soldiers gone?
Long time ago
Where have all the soldiers gone?
Gone to graveyards every one
When will they ever learn?
When will they ever learn?

Both of these are anti-war works, but one is a poem and the other is a song.

"Anthem for Doomed Youth" was a poem written by the British poet Wilfred Owen during WWI.  Owen was a soldier in that war and was killed in 1918.

"Where Have All the Flowers Gone" is from a completely different genre and era.  It was written by the American folk singer Pete Seeger in 1961.  Seeger was a prominent singer in the 1950s and 1960s and was very much involved in opposition to the Vietnam War.