Beat! beat! drums!—Blow! bugles! blow! Through the windows—through the doors—burst like a force of armed men, (1-2)
These drums and bugles are military instruments and our speaker makes their connection to the war pretty clear by comparing their sound to "armed men." Mr. Whitman wanted to be sure that we had the image soldiers and battle in the back of our minds as we read the poem. So why didn't he just come out and say it? Why so subtle?
Nor the peaceful farmer any peace plowing his field or gathering his grain; (6)
That word "peace" – and the way our speaker repeats it – again reminds us of the war. After all, if there is no peace, there must be… you guessed it: war.
Mind not the old man beseeching the young man; Let not the child's voice be heard, nor the mother's entreaties. Recruit! recruit! (18-19)
Toward the end of the poem, our speaker finally makes it clear what these men with the drums and bugles are doing. They're recruiting men (and probably boys) for the army. And from what we've seen in the poem, there's a good chance they're not just asking politely for volunteers.
Make the very trestles shake under the dead, where they lie in their shrouds awaiting the hearses. (20)
Why is this whole war and recruiting thing such a big deal to these people? Oh, yeah: people die in war. This line at the end of the poem makes sure that we don't forget that tiny detail. It also suggests that the war (or war in general) is so momentous that even the dead are shaken by it.