https://en.wikipedia.org/wiki/William_Tecumseh_Sherman#Breakdown
Breakdown
Having succeeded Anderson at Louisville, Sherman now had principal military responsibility for Kentucky, a border state in which Confederate troops held Columbus and Bowling Green and were present near the Cumberland Gap.[42]
He became exceedingly pessimistic about the outlook for his command and he complained frequently to Washington, D.C. about shortages while providing exaggerated estimates of the strength of the rebel forces. Very critical press reports appeared about him after an October visit to Louisville by the Secretary of War, Simon Cameron, and in early November, Sherman insisted that he be relieved.[43]
He was promptly replaced by Don Carlos Buell and transferred to St. Louis, Missouri. In December, he was put on leave by Maj. Gen. Henry W. Halleck, commander of the Department of the Missouri, who considered him unfit for duty. Sherman went to Lancaster, Ohio, to recuperate.
Some scholars believe that, in Kentucky and Missouri, Sherman was in the midst of what today would be described as a nervous breakdown. While he was at home, his wife Ellen wrote to his brother, Senator John Sherman, seeking advice. She complained of “that melancholy insanity to which your family is subject.”[44]
Sherman later wrote that the concerns of command “broke me down,” and he admitted contemplating suicide.[45] His problems were compounded when the Cincinnati Commercial described him as “insane.”[46]
I think it’s more likely that Sherman was bipolar and just suffered a particularly serious episode during that period. It didn’t prevent him from winning in the end, any more than it prevented Winston Churchill from leading Britain to victory during World War II.
I have nothing nice to say about that...