The theme of this poem is the noun "Union".  Can it exist?  Lowell describes the monument in his city of Boston that has been braced against the building of a parking garage.  The monument is for Colonel Shaw, the leader of a black infantry unit during the Civil War.  Lowell considers how black and white are still not united in this country, 100 years after a war that fought to end the inequality. 

The second theme of the poem, still revolved around "union", is the death of the Union of the United States.  Lowell complains that the city of Boston, which has so much history and which was so central to the creation of the United States, is easily ignoring its history in favor of technology and the fast-pace life of automobiles.  In other words, the idea of "union" that started this country and that caused the Civil War, is being abandoned because people are too concerned with their own convenience.

The tone is both nostalgic and critical.  The be nostalgic is think kindly on a time/person/event from the past.  Lowell does this in describing Boston and in describing the efforts of all those that fought against racism and discrimination.  The tone turns critical when Lowell describes the state of the world today:  "The ditch is nearer".  We are burying what we out to be honoring and remembering.