To give a little backstory, Sherman was engaged with Joseph Johnston's Army of the Tennessee as he slugged his way to Atlanta in 1864. Upon reaching Atlanta, there was a change in leadership for the Army of the Tennessee that removed Johnston, a defensive minded General, and replaced him with John Hood, an offensively minded General brought up in the hard knocks school under R.E. Lee.

After Sherman rousted Hood from Atlanta he fought him all the way to Alabama; he was unable to destroy the Army, however. And he wasn't satisfied with this type of fighting. Sherman said, "It will be a physical impossibility to protect the [rail]roads now that Hood, Forrest, and Wheeler, and the whole batch of devils are turned loose, without home or habitation. By attempting to hold the roads, we will lose a thousand men monthly and will gain no result." Instead Sherman wanted to cut away from his supply lines, a dangerous as well as bold move, and march through Georgia and make them "howl" the whole way through as he so famously said.

When Sherman presented this plan to his leadership, Grant and Lincoln weren't supportive. But through time and his built up influence with his friend, he was able to convince U.S. Grant who in turn swayed Lincoln in favor of the bold action. Sherman was smart and left the Rock of Chickamauga, General George Thomas, in Nashville to contest any efforts Hood would make. Hood would eventually fight two decisive battles at Franklin and Nashville that would ultimately destroy his Army.

With Hood off pursuing his own course and R.E. Lee under siege at Petersburg, there was nobody left besides Joseph Wheeler's cavarly of 3,500 soldiers and several thousand Georgia militia to put up a mild resistance to the 62,000 "hardened campaigners" that Sherman had under his command. "The militia attacked a rear-guard Union infantry brigade on November 22, but after suffering 600 casualties to the Yankees sixty they made no more such efforts." The march, by all accounts from Sherman's men, was quite pleasant. It's obvious the residents of Georgia felt otherwise.

Sherman's boldness was rewarded in this regard and I'd say somewhat emulated, in a larger scope, Grant's decision at Vicksburg to break away from his supply lines and have his troops live off the land in Mississippi.

Source:

J. McPherson. "Battle Cry of Freedom." Pgs 807-815.