Why did Shermans March happened?

Answer 1

After the conquest of Atlanta the Union forces were deployed along a kind of isosceles triangle, having its base at Chattanooga and the top at Atlanta, with a high of about 140 Kilometers.
Sherman's situation was now critical than that of Grant had been near Vicksburg, because the backbone that supported the salient was given only by the thin strip of the rails exposed to be everywhere attacked and destroyed and not by the course of Mississippi, which was completely dominated by the Union gunboats.
So the Commander of Confederate Army of Tennessee Major Gen. Hood, endorsed by Jefferson Davis decided to operate against Sherman's exposed line of communication, in order to force Sherman to withdraw from Atlanta, leaving Georgia practically undefended.
Sherman solved the situation deciding to cut his communication line and with two-third of his army to start an offensive through Georgia, whose final goal was that of reaching the sea at Savannah, taking from the back the Confederate Eastern Front. As well known, Sherman's Armysupported the offensive byplundering the country and scorching the earth behind them.