Did you notice while reading the poem that the odd-numbered stanzas are kind of different than the even-numbered stanzas? Did you also notice that in the even-numbered stanzas the speaker really tries to make it seem like we're physically on the battlefield, watching soldiers fight? If you did, you were on to something. The odd-numbered stanzas give us one setting, and the even-numbered stanzas another, for the most part.
Let's start with the former (the odd-numbered ones). In each one of those the speaker addresses somebody who has lost a loved one in battle: a maiden (stanza 1), a babe (stanza 3), and a mother (stanza 5). You know how in the movies, some guy usually delivers the news in person that a loved one has died in battle? Well, we think that's pretty much what's going on in stanzas 1, 3, and 5. A well-dressed military man is coming to visit the maiden, the babe, and the mother at their homes to tell them not to weep because war is kind (and that their lover, father, and son are all dead as well).
While we get glimpses of the battlefield in the odd-numbered stanzas (the trenches of line 13, for example), in the even-numbered stanzas we get some pretty specific descriptions. The second stanza, for example, gives us a vivid picture of booming drums, a regiment marching, a flag flying above the soldiers, and a field of a thousand corpses. The fourth stanza gives us pretty much the same sequence: the flag, slaughter, killing, and again that field of a thousand corpses. We're not exactly in a grieving widow's living room anymore.
Now we know you're wondering why there is no clear divide between these two settings. Why, for example, are there pretty gruesome depictions in the "home" stanzas? And grandiose, bright language in the battlefield ones? Well, the point there is that war infects every aspect of life; war still manages to penetrate very deeply into the places that are supposedly away from the war—the home that is nowhere near the front. This is also why, incidentally, the speaker shifts back and forth between home and the battlefield. The two are more related than we might think.