The night of September 1, Lieutenant General William J. Hardee (South) did the one thing left to do: he slipped away. Undetected in the dark, he pulled his broken corps south down the railroad to Lovejoy’s Station and was gone before the Federals knew it.

That same night, the news everyone had been marching toward for four months finally arrived inside Atlanta. With the last railroad cut at Jonesborough, the city could not be supplied and could not be held, and General John Bell Hood (South) ordered it abandoned. His garrison marched out to the north of the battlefield, by the McDonough Road, in the dark. Before they left, the Confederates destroyed what they could not carry, torching their own stores and ordnance so the Union could not have them. The most spectacular casualty of that demolition was a trapped ammunition train: locomotives and boxcars loaded with ammunition, set ablaze in the yard, that went up in a chain of explosions so enormous the ground shook and the sound carried for miles into the night. It is the war’s iconic image of a city being abandoned to the enemy, the one later generations would recognize from its dramatization in Gone with the Wind, a sky full of fire over Atlanta as the army that held it walked away.

On September 2, the Union walked in. The XX Corps under Major General Henry W. Slocum (North), the force Sherman had left behind north of the city when he swung the rest of his armies south, occupied Atlanta, and Slocum sent off the first word that the city had fallen. The thing four months of campaigning had been for was done: Atlanta was in Union hands.

And then Sherman sent the telegram. To Washington, in the first days of September, went the four words that did as much for the Union cause as any victory of the year:

“Atlanta is ours, and fairly won.”

Sherman, to Washington, early September 1864

The North was living through a hard summer when those four words arrived. The summer of 1864 had been a catastrophe of morale. The Union’s overall commander, Ulysses S. Grant, had driven his Overland Campaign in Virginia to a bloody stalemate in the trenches around Petersburg, with casualty lists longer than anything the country had ever seen and no victory to show for them. The North was sick of the war. And Abraham Lincoln was facing reelection in November and fully expected to lose. As late as August 23, barely a week before Jonesborough, Lincoln wrote privately that it seemed exceedingly probable he would not be reelected. He had his cabinet sign, sight unseen, a memorandum pledging to cooperate with whoever beat him.

Atlanta’s fall reversed all of it, overnight. The most-covered Northern victory of the year landed in the middle of a presidential campaign and detonated the case against the war. The opposition Democratic Party had built its platform on a single declaration, that the war was a failure, and demanded a negotiated peace. Their nominee, George B. McClellan (North), the army’s own former commander, repudiated the peace plank, but the platform was the message. And the platform meant something specific: a negotiated peace in 1864 would have meant leaving the Confederacy in existence and slavery intact. The Union the Democrats offered was a Union that stopped fighting before it finished the thing it had started, the abolition of slavery.

The fall of Atlanta blew that platform to pieces. Northern morale surged on the news, Lincoln made emancipation a central issue of the campaign rather than hiding from it, and in November he won not narrowly but decisively, 212 electoral votes to 21, with around 55 percent of the popular vote. The man who had expected to lose carried the country. And because he carried it, the war would be fought through to the end of slavery rather than negotiated to preserve it; the Thirteenth Amendment, abolishing slavery, followed. The battle of Jonesborough, by taking Atlanta, secured that outcome. A rail line cut south of a Georgia town turned a presidential election, and the election decided whether four million people would be freed or left in bondage.