The historical context of this poem rests on Colonel Robert Shaw, who was recruited in 1863 by Governor John Andrew to lead one of the first troops of all–African American men in the Civil War. This group became the 54th Massachusetts Infantry. Shaw grew passionate about the equality of his soldiers, and he even led a boycott against unequal pay when he learned that his African American soldiers were paid less than their white counterparts. The 54th paraded through Boston before going to South Carolina, where Shaw was killed near Charleston.

In the first stanza, the reader is jolted a bit from the title to the topic. Expecting to read about the "Union Dead," it is a perplexing to read initially about the South Boston Aquarium instead. This is part of the point Lowell is making. The contrast between the ideals Shaw fought for and the world Lowell sees is a bit jolting. Lowell uses alliteration twice in the second line ("Sahara . . . snow" and "broken . . . boarded") to further emphasize the harsh and cold environment of his modern Boston. He ends that stanza with fish tanks that are dry. Just as the tanks have lost the purpose for which they were created, Boston may have lost its way in the calling issued by Colonel Shaw to create a more integrated world.

In the second stanza, Lowell reflects on memories from his childhood, when he watched fish and reptiles in the aquarium, and he connects this through enjambment to the next stanza, in which construction is going on all around him. The machinery seems to be eating the earth, and there are negative connotations in the verb choices—"grunting," "cropped," and "gouged." Lowell indicates that everything touted as progress may not be so.

The scene seems ostentatious compared to Shaw's statue. Lowell uses the metaphor of an "orange girdle" to describe the supporting beams. Not only are girdles (a type of women's underwear) not supposed to be seen, but these are also orange, a symbol of warning. This scene directly contrasts the quiet and solemn accomplishments recognized by Shaw's statue.

Lowell then uses this simile: "Their monument sticks like a fishbone / in the city's throat." Swallowing a fishbone is an uncomfortable sensation at best. This simile provides further indication that modern Boston has forgotten the sacrifices of the past. They have lost Shaw's vision of racial equality in their supposed forward progression, and visually, this reminder makes Boston uncomfortable.

This sentiment is continued in these lines:

frayed flags
quilt the graveyards of the Grand Army of the Republic.
The stone statues of the abstract Union Soldier
grow slimmer and younger each year—

The memories of the past no longer guide the present. The flags are fraying, showing signs of being forgotten. The soldiers now seem "abstract," almost existing in another world, their personal battles and sacrifices forgotten.

Lowell places the word "n*****s" in quotes in the poem not to reflect his own voice, but rather to reflect the voice of the Confederate soldiers who tossed Colonel Shaw in a ditch with his soldiers upon his death. Ironically, Shaw's own father noted that this is exactly what his son would have wanted; the Confederates' efforts to dishonor Shaw worked exactly the opposite.

Looking to the future in the last stanza, Lowell uses alliteration in the line "giant finned cars nose forward like fish," drawing attention to the almost grotesque visions of the future. The s alliteration in the final two lines is reflective of a snake:

a savage servility
slides by on grease.

This leaves the reader with an evil connotation of the future being created each day, one that neglects to honor the personal sacrifices of the past as we slide into a future like savages who consume the history left to us.