Perched at the top of Boston Common there's a piece of public artwork every American should see. It's a massive bronze high-relief sculpture by August Saint-Gaudens, and it depicts what at first glance seems like a fairly standard military tableau: a group of Union soldiers marching resolutely forward.

Their leader is on horseback, in their midst, and floating near his left shoulder, hovering just over the field of the soldiers' upturned rifles, is the spirit of victory, guarding the whole enterprise. Thanks to the amazingly supple detailed work Saint-Gaudens employed in the muscles of the horse and the trousers of the troops, the whole tableau almost moves while the viewer watches.

It's only on closer examination that the sculpture's key detail leaps out: These Union soldiers are black men. This is the Massachusetts 54th Volunteer Infantry, the first all-volunteer black regiment to fight in the Union cause. It was created in 1863, and Colonel Robert Gould Shaw, a young son of a wealthy and prominent Boston Brahmin family, was its commander.

Eventually, nearly 200,000 black troops would join the Union ranks before the end of the war, and the story of those regiments is at the heart of Thunder at the Gates, the new book by prolific historian Douglas R. Egerton. It's a fantastic performance throughout, and although it pulls other black units such as the Massachusetts 55th Infantry and the 5th Massachusetts Cavalry into the larger narrative, its pride of place goes to Col. Shaw and the 54th.

A significant percentage of the 54th's personnel, both its officers and its troops, were recruited by Boston abolitionists eager to see former slaves finally taking up arms in President Lincoln's army. But perhaps like many of those officers, Col. Shaw initially had reservations about the whole project. Before he began his command, for instance, he admitted to the uncle of one of his officers that although in his opinion black troops had proven “effective in bush-fighting,” he doubted they'd do as well in battle, and he suggested the possibility of keeping armed white soldiers in their rear to cut off retreat.

Time with his men changed that opinion, and Egerton charts that change and others like it with sensitivity and remarkable readability. This is a landmark study of the true crucible of the American Civil War, when former slaves took up uniforms and weapons in order to fight for their own freedom. Egerton's story ranges pleasingly far from the battlefield – he's equally concerned with the vast and easily underestimated ripple effect news of black regiments had on all precincts of civilian society. (On this aspect another must-read of the season is Chandra Manning's outstanding new book "Trouble Refuge").

He also gives very satisfying accounts of the lives his principal characters led after the war. He follows, for instance, the career of James Monroe Trotter, formerly a second lieutenant of the 54th (and also formerly a slave), who moved to Boston and enmeshed himself in state and national politics, eventually being appointed recorder of deeds in Washington by President Grover Cleveland, a move that, as Egerton quotes the old Boston Evening Transcript, managed to displease people on both sides of the aisle: “The Democrats and Republicans in Congress are almost equally perturbed by it,” the paper wrote, “the former objecting to Mr. Trotter's color, and the Republicans objecting to his politics.”

Trotter, Egerton reminds his readers, was the first black New Englander to receive a high-ranking federal position.

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But from a dramatic standpoint, this part of the long history of black emancipation in America will always revolve around the moment that Massachusetts senator and abolitionist Charles Sumner looked back on as “their Bunker Hill,” the moment in which those immortal soldiers of Saint-Gaudens are forever marching up on Beacon Hill: the assault on Fort Wagner in South Carolina on July 18, 1863, when the 54th tried in vain to take a heavily-fortified Confederate position and suffered grievously for their efforts. “Shaw was waving his sword and shouting, 'Forward, my brave boys!' when a bullet caught him in the breast,” Egerton writes. “He was hit five or six more times before he dropped.” Twenty-nine of his troops were also killed, and 149 were wounded, many mortally. The South held Fort Wagner, but in every important way, the confrontation was a Union victory; it gave a graphic, bitter demonstration – to those on either side who required such demonstrations – that valor is indifferent to skin color, and that the war was actually about shared humanity.

"Thunder at the Gates" is acutely aware that its subject is in many ways ongoing. Egerton's concluding chapter traces the role black soldiers have played in the long shadow of Fort Wagner, through the world wars of the 20th century, to Executive Order 9981, in which President Truman in 1948 ordered the desegregation the US armed forces. But the centerpiece of his tale is the modern-era birth of that story, under fire.