What passing-bells for these who die as cattle? (1)
Being killed like an animal isn't anyone's idea of glory, and this line makes sure that we don't approach the idea of dying in war with any undue excitement. Any patriotic idea these soldiers might have had about how heroic it is to die for their country is totally undercut by this simile. There's the suggestion, through the comparison to cattle, that the soldiers are trapped and at the mercy of others. It doesn't leave much hope of escaping death. How many cattle have you heard of escaping from the slaughterhouse?
Can patter out their hasty orisons. (4)
That word "hasty," plus the haphazardness of "patter," relates to us the suddenness of death on the battlefield. Not only is death fairly certain, but it doesn't necessarily announce itself far in advance. It's not stately, heroic, or glory-filled. It's quick and dirty.
And bugles calling for them from sad shires. (8)
This is the first time the poem reaches out beyond the death of the soldiers to mention the suffering it causes others: relatives, neighbors, and so on. Those "sad shires" really are pitiful, calling for their dead young men. The only way they seem to get them back is for a funeral.