Why did Sherman's March Start?

Answer 1

Sherman was worried about his long and tenuous supply-line - a single-track railroad all the way back to Nashville, which was always being blown up by Confederate cavalry.

Noting that the Georgia farms had enjoyed a good harvest, he decided he could forget about his supply-line, and live off the land, while conducting punitive raids on the civilian underpinning of the Confederacy.

This ran directly counter to Grant's orders, which were to destroy the Army of Tennessee, but fortunately Sherman carried high credibility with Grant, who eventually approved this new plan, which undoubtedly shortened the war.