Alliteration is an example of a literary device where, in a series of words that may or may not be next to each other, the initial consonant sound is repeated to create a special sound effect. If you think about it, alliteration is commonly found on a day-by-day basis in newspapers and their headlines, which are more impactful because of the alliteration they employ. Quickly scanning the first four lines of this excellent poem therefore, it is clear that line 3 contains the best example of alliteration. Let us examine the first four lines:
What passing-bells for these who die as cattle?
Only the monstrous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
Note how the repetition of the "r" sound operates in line 3 to create alliteration in "rifle's rapid rattle," which itself seems to be onomatopoeic as it enacts the "stuttering" and "rapid" sound of the rifles rattling as men shoot each other and men die as if they were nothing more than "cattle."