Lee's army routed and captured at Antietam?

[The Mikado]

In this scenario, McClellan aggressively pursues Lee's army after Antietam and prevents them from crossing the Potomac back into Maryland, and eventually encircles them and forces their surrender.

Does Lee's surrender end the war (especially since it predates a lot of the decisive battles on the Western front)? Does a Confederate defeat in 1862 still result in the abolition of slavery? With slavery not abolished at the end of the Civil War, how does the newly-reunited US (only split up for a year and a half) adjust to its reunion?

Presumably, if he'd successfully orchestrated the defeat of the CSA, McClellan would be a pretty strong Presidential candidate in 1864.

[True Federalist (진정한 연방 주의자)]

It's difficult to imagine the circumstances that would have led McClellan to be aggressive.  If he had been capable of being aggressive, then Seven Days would have turned out much differently.  So a different result at Antietam will require a change of command, presumably because George got wounded or killed. (Maybe the USS Galena strikes a Confederate torpedo and is sunk with McClellan drowning or being blown up.)

But if McClellan is lost to the Union during Seven Days, then even if that leads to no major butterflies, it means whoever replaces him is likely to acquiesce to Halleck's command to evacuate the Army of the Potomac from the Peninsula, meaning the Union won't be in as bad a shape during Second Manassas. Without a near disaster for the Union then, the Maryland campaign likely turns out much differently in both planning and execution.

So let's try a different tack. Porter's caution was a factor in keeping McClellan from going all out at Antietam.  Perhaps Porter is killed or wounded during Second Manassas so that Little Mac can be more aggressive.  Even committing one of his two reserve Corps likely smashes Lee, tho at a heavy cost in soldier's lives.

However, even a smashing success won't gain as much as one might hope.  With the prisoner exchange cartel still in operation, The South will get many of its gallant men back, while the heavier Union casualties mean the North ends up slightly weaker in manpower than in OTL.  The draft likely gets enacted sooner. Conversely, lack of elan and materiel probably keeps whoever leads the Confederate forces in the Eastern theatre grounded in reality in 1863.