Why was Sherman's "total war" a major change from earlier forms of fighting?

Sherman's concept of total war was a continuation of the tradition of the "nation at arms" established by the French Revolution. In the wake of the bitter conflicts that engulfed Europe for decades after the Revolution, war was no longer regarded as a gentlemanly pursuit, conducted on a particular field of battle by professionally-trained soldiers. Instead, it was held to involve the whole of society, civilians as well as soldiers, everyone pulling together in pursuit of a common goal.
For Sherman, that goal was the holding together of the Union by defeating the South. And he was utterly ruthless in its pursuit, illustrating his famous remark that war was an uncivilized game that couldn't be civilized. It wasn't enough for Sherman that the Confederacy be defeated on the field of battle; its people had to be broken psychologically as well. To that end, on his famous March to the Sea, he set about terrifying the civilian population of Georgia into abandoning the Confederate cause, stealing food and livestock, and burning the homes and barns of those who resisted.
Sherman's destruction of Atlanta can be seen as an act of psychological warfare as much as anything else. He knew that the city was a symbol of Confederate pride and strength. The burning of Atlanta was a deliberate attempt to undermine Southern morale, to make Southerners realize that there was now no way they could win the war. Though it inflicted enormous suffering on the civilian population, Sherman's total war strategy was brutally effective in breaking the back of Southern resistance and hastening the Confederacy's final defeat.