For the Union Dead
Lowell is letting it flow! He doesn't use any formal constraints in this poem, which is fitting for a poem like this. With so much jumping around in time and subject matter, this poem benefits from the breathing room that free verse allows.
While he doesn't use any official formal elements, he does keep his stanzas consistent with four lines each. The steady length of the stanzas could be reminiscent of soldiers marching along in an orderly fashion, or Lowell could have just chosen it because he packs so much information into the poem that he decided to offer it up in organized snippets. Because the poem doesn't have any other formal elements to keep it together, though, Lowell mainly relies on the subject matter. As you can check out in the "Line by Line Summary," he juggles and returns to several different subjects in the poem to make it seem more unified—even though, let's be real, it's still pretty complicated.
Still, that complication is lessened—to a degree—by the choice of free verse, and through the use of a first-person speaker. This poem, as much as it jumps around through time and space, reads more like a one-sided conversation. Since we have free verse—which best mimics the natural patterns of our speech (when's the last time you chatted at length in iambic pentameter?), it's as though we as readers are overhearing someone discussing his gripes with modern society. The imagery and the time setting may be a bit tricky, but the free verse reminds us that the speaker is really just trying to get something off his chest here.