Glenn David Brasher is an instructor of history at the University of Alabama and the author of “The Peninsula Campaign and the Necessity of Emancipation: African Americans and the Fight for Freedom.”

“Today we recognize the right of every man … to be a MAN and a citizen,” Gov. John Andrew of Massachusetts proclaimed on May 18, 1863, to a crowd gathered around the 54th Massachusetts, the first African-American regiment raised in the North. They fight “not for themselves alone,” he insisted, but also for their race. Their military service would refute “the foul aspersion that they [are] not men,” proving that African-Americans deserved their nation’s citizenship rights.

The regiment then proudly accepted its official flag, and Andrew told its commander, Col. Robert Gould Shaw, to report to the Department of the South, which included the coastal areas of South Carolina and Georgia. The soldiers would thus begin their war against slavery deep in the institution’s bowels. We are “on our way to Dixie,” Cpl. James Henry Gooding proclaimed, and “the greatest difficulty will be [for anyone] to stop [us.]”

On May 28, the regiment marched through Boston to Battery Wharf as a crowd of 20,000 lined the streets to watch. Detractors hissed, but supporters outnumbered the critics. Souvenirs were sold that included a quote from Byron: “Those who would be free, themselves must strike the blow.” The sight of the handsomely dressed soldiers marching in perfect order, with the sun glinting off their meticulously polished bayonets and buttons, was thrilling. Leading the march from horseback was Shaw, and he, too, made a lasting impression. The poet and abolitionist John Greenleaf Whittier recalled that Shaw looked as “beautiful and awful” as an avenging angel sent to lead the men. But no one was more impressed than Shaw’s mother: “If I never see him again,” she wrote, “I shall feel that he has not lived in vain.”...