What Sherman finally decided on was the annihilation of the city itself—an instructive example, as it were, for other Southern cities; or if you will, an act of terrorism. Earlier he had warned Atlantans to "prepare for my coming." In his written orders he couched the warning in terms of obliterating everything of military value, but, as in so many other places his army visited, the reality was destruction of the town by fire—the 19th century's version of carpet-bombing.
This kind of devastation was relatively unprecedented for Sherman's time; the burning and sacking of cities had more or less gone out of fashion as the customs of "civilized" warfare had generally foreclosed the molesting of civilians.
Sherman defied this sense of military restraint almost from the beginning; in fact, his earliest pyromaniacal urges in connection with Southerners and their property seem to have developed in 1862, while he was in charge of the recently captured city of Memphis. There, in retaliation for Confederates shooting at Union steamboats from the Arkansas side of the Mississippi, Sherman ordered the torching of all towns, villages, farms and homes for 15 miles up and down the river. ...
It is hard to reconcile the peculiar psychology of Sherman's military tactics with the fact that these were his fellow Americans whose homes were being burned—mostly women, children and old men, at that. For despite all his hard-bitten declarations against the Confederacy and its supporters, Sherman, in his private correspondence, often made a point of expressing an abiding fondness for the South and the Southern people.
With his victory at Atlanta, Sherman solidified himself as an American hero—in the North, at least—and ensured what Lincoln's ally Sen. Zachary Chandler called "the most extraordinary change in publick opinion here that ever was known." The South's hopes to exploit Northern discontent and wring a "political victory" from the war vanished.
Eventually, Sherman's scorched-earth tactics validated a new standard for military operations—the notion of "hard war" or "total war," in which civilians were no longer treated as innocent bystanders and their property became fair game. This policy was incorporated, improved and refined over the ensuing decades, reaching its most pitiless apogee at Hiroshima in 1945.