The 54th regiment was definitely the little engine that could. They kept on trucking even after losing so many men. They fought with strength and courage despite all of the criticism, non-believers, and haters. "For the Union Dead" salutes them for their valiant effort. The perseverance of African Americans continued from the emancipation to one of its strongest points, the Civil Rights movement. If we were to admit that this poem is pretty pessimistic in tone (and we should; it's a total Debbie Downer), then the perseverance of African Americans is its only high point. Lowell seems to be writing from a dual standpoint: one of disappointment in his countrymen for their lack of progress, and one of respectful awe of those that persevere despite slim chances and adversity.
Silver lining alert: highlighting the perseverance of the Union soldiers is the only positive aspect of this poem.
Time for some time travel. The soldiers' perseverance foreshadows and parallels the perseverance of African Americans during the Civil Rights Movement.