The Battle at Big Black was the shortest of the entire war. Vaughn's brigade fired one volley then pulled sheets of cotton from their "fort" and quit. These East Tennesseans were mostly Unionists to begin with. I have found evidence that the brevity of the battle, the quick surrender, and the change of sides in less than a month were all pre-arranged days before the "battle". Grant and Secertary Stanton opposed this side swapping but Andrew Johnson had Lincoln's ear so Abe "with a wink and a nod" approved of Johnson's plan "so long as they are Tennesseans". Parson Brownlow's rhetoric incited a riot at Camp Morton in 1863 (It is not clear to me who on which side were killed.
Most of E. Tn was Unionist but Cocke and Rhea Counties were able to support a plantation economy and hence held CS lienings. East Tennessee was "hell done broke open" and in my view was not yet resolved in my 1950's childhood.