William T. Sherman was one of Lincoln's two main Generals by the end of the war, and although he was very eccentric and many in the Union Army and government did not like or trust him, he was a very effective fighter. He is perhaps best remembered for changing the nature of war, or at least realizing that the nature of war had changed.
He introduced "Total War" tactics that made no distinction between soldiers in the field and civilians and cities that supported them. Therefore, he destroyed cities, crops, houses - anything that would help his enemy. His feeling was that the more cruel war was, the sooner it would be over, in the end saving lives.
We still use Sherman's tactics today, and teach his military principles at our war colleges.