On February 18, 1865, Charleston, South Carolina — the spiritual capital of the Confederacy and the cradle of secession — surrendered to Union troops. The first federal unit to enter the conquered city was the Twenty-First United States Colored Infantry Regiment. Men, some of whom who had not long before been South Carolina slaves, returned as emancipators.

Leading a column down the main thoroughfare, a mounted black soldier carried a banner proclaiming, simply, “Liberty.” As Union troops strode through the streets, black residents flocked to their side. Two weeks later they celebrated freedom with a massive procession of their own. Among the revelers were almost two thousand children who sang the lyrics to the abolitionist marching song “John Brown’s Body.” Local whites wondered aloud “whether they are actually in another world, or whether this one is turned wrong side out.”

Within six weeks, similar scenes greeted the entry of the Army of the Potomac into Richmond, Virginia, the Confederacy’s capital. General Godfrey Weitzel, commander of the Twenty-Fifth Corps of the United States Colored Troops, accepted the city’s formal surrender. Black cavalry troopers brandished their sabers and cheered triumphantly. As they surged into Richmond, former slaves stood atop shacks and waved their hats and cheered while well-to-do whites retreated into their homes, bolted their doors, and peered anxiously, indignantly, and incredulously through shuttered windows.

These scenes, hardly imaginable five years earlier, captured the revolutionary denouement of the American Civil War.

Prior to the war, one out of every three Southern residents was enslaved — nearly four million people. Their labor had made those who owned them immensely rich and powerful. Indeed, most of the richest families in the United States were then slaveholders.

Since the country’s founding, they and their representatives had controlled not only state governments but the federal government in Washington as well. That political power was used to secure their control over their slaves, to expand the territory in which slavery was legal and protected, and to advance their own special interests elsewhere. It was a state of affairs that seemed likely to continue indefinitely.

The former slave and abolitionist leader Frederick Douglass noted that the South’s “peculiar institution” seemed politically “impregnable” as late as 1850. Leaders of both major parties (Whigs and Democrats) were congratulating themselves for having suppressed the slavery controversy once and for all through a recent legislative deal.

But history can take dramatic turns, turns that catch both conservatives and radicals by surprise, turns that rattle anyone who assumes that the future will look like the recent past. Just such a development occurred in the mid 1850s, when slave owners and their allies moved to legalize slavery in a part of the Midwest — the Kansas Territory — that had been closed to it by law since 1820.

That aggressive attempt outraged much of the Northern population and precipitated a political crisis that gave birth to the Republicans, a mass political party that opposed slavery and that was committed to preventing its further expansion. In 1860, Northern voters elected that party’s candidate to the presidency.

Convinced that Abraham Lincoln’s victory spelled the beginning of the end of slavery in the United States, slave owners rose in armed rebellion. They set out to break up the federal union and to form a new, homogeneously slave-holding country, the Confederate States of America. It was that reaction that spawned the April 1861 attack on Fort Sumter in Charleston Harbor, beginning four years of civil war.

Although the white South embarked on that path to preserve slave society, the war inaugurated a titanic revolution that within a few years destroyed the system and broke the once-imposing political supremacy of the planter class.

This was not Lincoln’s intention in 1861. But what Frederick Douglass called “the inexorable logic of events,” the Republican Party’s eventual recognition and acceptance of that logic, and the active intervention of slaves, all combined to make such a revolution a reality.

Lincoln’s war program in 1861 was not at first revolutionary in intent. Although many Republicans had warned the South that secession risked emancipation, a host of considerations initially limited the Republican government’s willingness to directly attack slavery.

Lincoln was keenly aware that nearly half the electorate in the country’s free states had not supported him but one of his three far more conservative opponents (Stephen A. Douglas, John C. Breckinridge, or John Bell). Lincoln would need the active support of many of these non-Republican Northerners in order to win the war. He believed he could keep the politically heterogeneous Union solidly behind him and his armies only if he limited his war aims to suppressing secession.

Lincoln also knew that his party’s political support was almost nonexistent in the four slave states — Kentucky, Delaware, Maryland, and Missouri — that remained within the Union after Fort Sumter. He considered the retention of those states absolutely crucial. They — especially Kentucky and Maryland — contained men, material, and geography that, if lost, could lead to defeat. The president feared that a boldly antislavery war policy would “alarm our Southern [pro-]Union friends, and turn them against us.”

But most surprisingly, Lincoln and most other Republicans presumed that most whites even in the Confederacy, including many large slave owners, were actually loyal to the Union at heart. They had, he believed, simply been outmaneuvered, stampeded, or bullied by a minority of well-organized political extremists. From this premise, too, Lincoln deduced the need to defend the Union without giving unnecessary offense to the Southern white majority.

Lincoln and most of his party thus underestimated how firmly attached slave-owners were to their “property” and to the society they built around human bondage. To put it another way, Lincoln and his allies underestimated the class-consciousness, self-confidence, and political cohesion of the South’s slave-owning elite. Republicans also underestimated the racially minded support that most non-slaveholding Southern whites gave to slavery and the slaveholders’ Confederacy.

Karl Marx pointed to the mistake in a Vienna newspaper. Lincoln “errs,” he wrote, “if he imagines that the ‘loyal’ slaveholders are to be moved by benevolent speeches and rational arguments. They will yield only to force.” Abolitionists such as Douglass had made the same point. “The ties that bind slaveholders together are stronger than all other ties,” he stressed. Counting on any significant fraction of them to help save the Union was hopeless.

Eliza Frances Andrews, the daughter of Georgia plantation owners, later underlined the practical significance of this fact. The Southern “aristocracy” to which her family had belonged, she recalled, was “intensely ‘class conscious,’ ” united by “a solidarity of feeling and sentiment.”

So it was that after a full year of war, and despite Lincoln’s efforts to spare their property and sensibilities, US troops were encountering precious few whites in the Confederacy who were displaying any active sympathy with them or with the Union cause. This was all the more worrisome in light of the bad news coming from Virginia battlefields. There and elsewhere in the South, moreover, slave labor was critically undergirding both the Confederate society and war effort, with slaves performing myriad tasks essential to sustain the home front and to field, feed, transport and otherwise support the Confederacy’s armies.

By the spring and summer of 1862, as Lincoln would later recall, “things had gone on from bad to worse, until I felt that we had reached the end of our rope. We had about played our last card, and must change our tactics, or lose the game!”

It was time to change course. “We must think anew, and act anew,” he said. “We must disenthrall ourselves, and then we shall save our country.” The Union must once and for all give up trying to wage war without excessively angering its enemies; it must begin instead to assault those enemies more aggressively and more determinedly to strip them of the slave labor that helped make them so formidable. “We know how to save the Union . . . In giving freedom to the slave, we assure freedom to the free.”

Marx grasped and anticipated this change of direction in late summer 1862. “So far,” he wrote, “we have only witnessed the first act of the Civil War — the constitutional waging of war. The second act, the revolutionary waging of war, is at hand.” This second act culminated in September 1862’s preliminary emancipation proclamation and the final proclamation of January 1863.

The same kind of dynamic transformed the Republican Party’s policy regarding blacks serving as soldiers. During the first phase of the war, Lincoln’s government categorically rebuffed black men’s attempts to join Union armies. “This Department,” Secretary of War Simon Cameron announced as the conflict began, “has no intention to call into the service of the Government any colored soldiers.”

Local political authorities in the Union drove home his point by flatly prohibiting black recruitment meetings as “disorderly gatherings.” The Cincinnati police warned would-be black soldiers, “We want you damned niggers to keep out of this; this is a white man’s war!” Racist mobs attacked some Northern free blacks who organized recruitment meetings at their own initiative.

This policy, too, reflected Lincoln’s fear of antagonizing racist white Northerners, in uniform and out, and of losing the four slave states still in the Union to the enemy. As late as August 1862, the president was still worrying aloud that “to arm the negroes would turn 50,000 bayonets from the loyal Border states against us that were for us.”

But military necessity — the need for more soldiers to fight the war — proved decisive. Under that pressure, articulated most clearly by free blacks, white abolitionists, and more radical Republicans, Union policy evolved from adamantly excluding blacks in 1861 and 1862 to recruiting them as non-combat duty soldiers in 1863. The courageous conduct of those black soldiers whom circumstances nonetheless pulled into the fighting — as at Jacksonville, Florida; Milliken’s Bend and Port Hudson, Louisiana; Battery Wagner, in South Carolina’s Charleston harbor; and Honey Springs, in what is now Oklahoma — eventually led the Union to modify its policy again. They now welcomed black troops into combat duty.

By the end of the war, some 200,000 black men had served in the Union army or navy, most of them recruited in slave states. Black soldiers took part in about 450 military engagements, some 40 of which were major battles, and they provided the Union with 120 infantry regiments, 12 heavy artillery regiments, 10 batteries of light artillery, and seven cavalry regiments. They were crucial to the eventual victory.

Freeing and recruiting them, Lincoln repeatedly explained, was “the only” policy that “can or could save the Union. Any substantial departure from it insures the success of the rebellion.” In spring 1865, General Ulysses Grant’s forces besieging Petersburg and Richmond in the confrontation that finally brought the war to an end included thirty-three black regiments, one in every eight Union soldiers in the campaign.

In the American Civil War, as in other revolutions, the radicalization of methods and the escalation of stakes paralleled shifts in the popular base. In the middle of 1861, when the Lincoln government was still defining its aims narrowly, its war effort enjoyed strong bipartisan support across the North. As the army and administration moved toward a more aggressive and especially toward a more deliberately emancipationist policy, however, the Northern population fragmented politically. As conservative advisors had insistently warned Lincoln, Democrats, especially those in the “loyal” slave states, furiously denounced the Union war program’s radical turn.

But by the middle of 1862, the Republican president was concluding that the continued support of his own party, including its more radical wing, and securing the active aid of slaves was more vital to success in the war than continuing to appease conservatives. He now acknowledged that courting those conservatives in the past had hobbled the Union war effort. Such “professed friends,” he growled in a sharply worded letter to a Southern unionist, had in fact “paralyzed me more in this struggle than any other one thing.”

As he abandoned his original, limited war policy, Lincoln consciously turned away from conservatives and toward slaves and their champions. His administration and armies would henceforth count for victory far more on the efforts of African Americans (especially but not only black soldiers) than on those who opposed a revolutionary war policy. Lincoln alluded to that change in the summer of 1863 when he praised the newly recruited black soldiers — the “black men” who “with silent tongue, and clenched teeth, and steady eye, and well-poised bayonet . . . have helped mankind on to this great consummation.” At the same time he scorned those “white ones” in the North who “with malignant heart, and deceitful speech . . . strove to hinder it.”

This shift in the Union war effort’s effective social base, in turn, influenced the subsequent course and content of the Second American Revolution, accelerating and deepening its radicalization. Lincoln’s growing reliance on emancipated black soldiers and laborers led him to cease urging freed blacks to emigrate abroad, as he had previously done. It also made it impossible, as Lincoln acknowledged, to retreat from the new emancipationist policy, even when battlefield setbacks and waning electoral support in spring 1863 and summer 1864 seemed to demand that he do so.

“Any different policy in regard to the colored man deprives us of his help, and this is more than we can bear,” Lincoln explained. “We can not spare the hundred and forty or fifty thousand now serving us as soldiers, seamen, and laborers.” And “if they stake their lives for us, they must be prompted by the strongest motive,” including “the promise of freedom. And the promise being made, must be kept.”

In 1863, abolitionists and radical Republicans began pressing for the adoption of a constitutional amendment to finally outlaw slavery throughout the United States. In November 1864, the platform on which Lincoln was reelected pledged his party to that goal, and by the end of 1865 the Thirteenth Amendment was ratified.

The shattering of the Confederacy, the reconstitution of the Union, and the destruction of slavery radically transformed social and economic relations and the distribution and exercise of political power in the United States. The defeated Southern elite, which had dominated all branches of the national government throughout most of the prewar era, was now driven into a corner politically and remained there for decades.

Half a century would now pass before a man born in the South would sit in the White House or preside over the United States Senate. The Supreme Court remained in the hands of non-Southerners, too. Control of the federal government now rested instead in the hands of those who represented the interests of Northern industrial and commercial capital.

But it was in the South that the Civil War and its aftermath left its deepest enduring imprint. It is true that during the 1870s and afterward, champions of white supremacy succeeded through a vicious terror campaign in denying former slaves and their descendants many of the rights that they had won in the war’s immediate aftermath. The brutally oppressive Jim Crow system that was then created remained in full force down past the middle of the twentieth century.

But the destruction of slavery remained a central, immovable fact of postwar life. Some of the families torn apart under slavery reconstituted themselves, never to be separated again by slave markets. Meanwhile, the average living standards of black people rose by half during the fifteen years following the Civil War. Even in the pit of the late nineteenth century, landowners could never compel their laborers to work with the brutal intensity that slavery had once exacted as a norm.

Just as important, the fruits of slavery’s destruction then helped advance the cause of liberty and equality. The greater freedom of action that emancipation brought enabled black people to forge stronger family ties, build strong organizations, and thereby organize and fight more effectively for equal rights when changed conditions made that possible in later years.

For those who experienced, heard, or read about what slavery’s foes had accomplished during the 1860s, the memory of the Second American Revolution provided hope and inspiration. It should do the same for us today.