Let's say Halleck succeeds in court-martialling Grant. Sherman becomes an Army commander. After the debacle at Fredericksburg, Lincoln sends George Thomas east to command the Army of the Potomac. Sherman launches a series of offensive operations in the west over a 4-year period and succeeds in burning down pretty much every Confederate city outside Virginia except for Savanna and Montgomery. ITTL, Sherman has no scruples against what today would be considered violating the Geneva conventions, and as long as it shortens the war he will do it. By the time Lee Surrenders around 1867, the south is a complete wasteland- no functioning rail lines outside Virginia, no farms manufacturing
any crops, no functioning businesses, no plantations that haven't been totaled, all the trees have been burned, all the livestock have been killed, all the women (even girls who were
as young as 11 when the war began!!!

) have borne children fathered by Union troops, and there are dozens of towns that were burned to the ground, plowed over, salted, and abandoned (some of whom had their
entire populations put to the sword!

).
This was what Sherman called "Hard War"- when the entire enemy country is a combatant, and the despoiling and desolation of the enemy is required in order to win the war. Johnny Reb will lose hope if his town was burned down and there is no living thing within a 3-mile radius of it, his family is in a refugee camp, his daughters are all pregnant with Union troops' children, and all his life savings and investments are gone, and he will have to travel to what is left of his home on foot. Even the most die-hard Confederates will give up if they are hungry, humiliated, and utterly ruined this way.
George Thomas manages to capture Robert E Lee and his Army of Northern Virginia, and then realizes that, thanks to Sherman,
the most optimistic future for the non-Virginian elements of the AoNV is for them
to be POWs for life.
If you think TTL Sherman's apparent acceptance of his men's actions toward civilians is ASB Vlad Tepes Award bait, I took a page from
a different chapter of military history... (you
did ask for the
worst possible ACW after all!)
Please don't ban me if the content of this post is too extreme. I
DID put a warning on it!