I don't believe he was a practitioner of "black flag" warfare. He did, indeed, believe in "total" war, insuring as many belligerents as possible were soon unable to wage war lacking food, weapons, horses, and so forth. But, "black flag", take no prisoners, kill all enemy? That was more a Southern thing when fighting black Union Army units. Fort Pillow? Ring a bell? I've never even seen a suggestion, North or South, that Sherman was a proponent of "Black Flag" tactics. "Total War"? Certainly. A shorter war meant fewer casualties.It is the practice of black flag warfare that makes him so divisive,
His fighting career did indeed continue after the "War of the Rebellion." A lot of the Indian Wars were fought under his direction.