Gettysburg sort of time I think, so 1863.
- BNC
A myth that. The only way the Rebels win at Gettysburg is on Day 1, in which we are talking the mangling of the AotP's I & XI Corps. Winning on Day 2 requires Unspeakable Seamammal levels of incompetence by the North and equal levels of luck and derring-do by the South.
Besides, in the end defeat the AotP and they simply fall back on their interior lines to the Pipe Creek Line, which in terms of defensibility makes Fredricksburg look like an open door. Unflankable, and forcing Lee to frontally assault the AotP with his entire army. IOW, Pickett's Charge x3. War ends in 1863 with a Northern Victory. George Meade becomes the next President!
When they opened fire on Fort Sumpter.
Same then, you ever read Shelby Foote? The great Southern War historian? Guy who is buried next to N.B. Forrest? He said the South lost and had no chance of winning because the North fought the war with one hand tied behind it's back. That the North could have raised and equipped army after army. The North lost 5% if it's white population the South 18%!
I suppose in fairness to your question, the moment they lost any chance of losing reelection. Even then he would have been in office until March of 1865. Even McClellan finishes it. Lincoln was never going to quit.
You don't need to be Bruce Catton to understand all this. Being good at 1st grade arithmetic is good enough. McClellan was a War Democrat. (1) The thought of being POTUS when the South is overrun would be a dream scenario for him. The 13th, 14th, and 15th amendments would be stillborn, yes. The Emancipation Proclamation, no. Slavery was dead in too many areas of the Occupied South by this time, plus the flood of runaways to Union lines had become a torrent.
1) Too much is made by people who seem to not understand the differences between the American republican system and European parliamentarian systems. If the US were a parliamentarian system, it is POSSIBLE that the Copperheads might force a "Vote of No Confidence" on any given day against the "McClellan Government". But that's not how it works here. No matter how well the Copperheads do, the "McClellan
Administration" (2) can still get a working congressional majority of War Democrats, Whig Republicans, and Radical Republicans to at least get Congress to pay the bills to keep the war going.
2) McClellan will still remain President until March 1869, and unless he goes full bore Andrew Johnson doesn't face the threat of Impeachment. Even less so with a Democratic Congress.
Sherman yes, I agree. If Atlanta had not been Lincoln's Christmas (3) gift then you have a problem.
3) Election Day gift. Sherman's Christmas Day gift to Lincoln was completing his March to the Sea and taking Savannah, cutting the Confederacy into thirds.
But Lincoln was going no where before March of 65. How was the South doing at that point? You think Sherman was going to sit outside Atlanta forever?
Sherman was not the greatest of tacticians. It is not ASB that continuing and inexplicable failures outside Atlanta might have forced his removal or larger forces to be sent to him (canceling the Mobile Campaign, perhaps?). In the end, the chances for the South to stop the North stone cold outside Atlanta are small. Johnston MIGHT have done it, but its not just that he was replaced by Hood. You needed Davis to make that mistake. Davis' sole measuring rod for talent was their level of loyalty/ability to suck up. And the Army of the Tennessee was low on talent. The only first rate commander they had left other than the cavalryman Forrest was the abolitionist P.R. Cleburne!
Or Lee could hold Petersburg? What changes by March of 1865? Lincoln was NEVER going to quit, ever.
Lee could and did hold out by March. The Spring Muds insured that.
Early had as about the chance of taking DC as Hitler does London. Not going to happen, ever. At best Early's raid causes a Corps or two to reboard and sail North. But, Lee's mean old man was taking nothing.
The Union XXII Corps may have been a collection of clerks, but behind those works they were like the Rebel Militia at Bunker Hill with vastly more training, artillery and fortifications up the yin-yang, and most importantly ammunition reserves that the farmers at Bunker Hill could only have dreamed of. Give the Rebels at Bunker Hill unlimited ammo, and instead of merely suffering the worst percentile loss of forces engaged in a victorious battle in the history of British arms the British would have lost outright!
Not really. A CSA victory is one of the GIGANTIC clichés of alternate history. It goes beyond cliché, it was a regular go to.
It's also massively improbable, simply considering the differential in resources. This is unpleasant to hear, but typically, any fight where one side has a three to one weight advantage over the other side.... the big guy wins, 99.9% of the time.
Actually, its not 3:1. Minus 4 million slaves from 9 million Southerners. Now its 27 million Yankees against 5 million Southern Whites. Now deduct 1 million Southern White Unionists. Now its 27 million Yankees against 4 million Southern White Rebels. Now add 4 million slaves (whether as a hostile internal force or runaways/liberated slaves) to the Yankee pool of manpower. Now you have 31 million Unionists to 4 million Southern White Rebels. Finally, you add 1 million White Southern Unionist to the party, and you have 32 million Unionists facing 4 million Southern White Rebels.
8:1 odds.

Now THOSE are some impressive numbers. Both for what the Rebels faced and for how long they held out regardless of the odds.
And yes, its true that the Confederacy was pretty morally repugnant. That makes it harder to sing Dixie.
Impossible to sing if you Google the second verse of Dixie.
But the reality is simply the reality. The Confederacy can't win, unless it is impossibly lucky, and the north is impossibly incompetent.
The North was plenty incompetent enough as it was before Fort Donelson. And continued to be incompetent in the Eastern Theater of Operations until Meade's appointment. McClellan's original appointment left a malignant influence of vicious rivalry almost as bad as the Confederate Army of Tennessee! There WERE other AotP corps commanders besides Meade who could have done a respectable job. But Washington politics kept incompetents in their place and better men down in the lower ranks.
It won't surprise people that, IMHO, it was the moment that Jefferson Davis decided to replace Joseph Johnston with John Bell Hood.
IMHO it was when the Confederacy picked Jefferson Davis as its President. I'm hard pressed to think of any US President as bad a national leader as he was.