Death sneaks up on us in "Beat! Beat! Drums!" The first nineteen lines focus on the living, and then suddenly, in the final verses, we're confronted with the image of the dead under their shrouds, waiting for the hearse. After we read these words, the rest of the poem takes on a different, darker meaning. Goes to show you: death is such a forceful theme that one brief mention of it can tinge an entire poem.
The image of the dead at the end of the poem is a metaphor: it suggests that the American way of life could also be brought to a sudden end.
The image of the dead at the end of the poem foreshadows the deaths of soldiers that will be caused by the Civil War.