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.
The sonnet opens with a  rhetorical question  which draws the reader in. 
 Passing-bells : bells are said to toll to announce that a soul is passing, or has passed, from its body. Also, traditionally, bells toll during the passing of a funeral procession to the grave, or during the funeral ceremony. 
 
 The use of the word “cattle” is an example of  zoomorphism  (think anthropomorphism in reverse). The men are dying so rapidly and in such numbers that death is almost industrialized, as in a modern slaughterhouse. The reference indicates that the humans killed in this struggle have become no more significant than animals killed for food