I think the answer to original question, why Sherman is considered by BH Liddel Hart to be the first modern general, is because of his March to the Sea through Georgia. This was a case of total war; Sherman sought to break the Confederacy's will to fight by marching through weakly-defended heartland. Not only would it strike fear in the hearts of civilians, but it would also cause concern among soldiers on the front for the wellbeing of their families at home. It also sought to rob the Confederacy of a great deal of resources. Rather than turning his army completely loose on the civilian population, Sherman drafted specific orders for how his men were to conduct themselves

Theory is Sherman was among the first Generals to implement the Total War concept, so warfare would not be limited to fight the enemy army and secure territorial gains. He would take war to other layers of the enemies society, this is attacking their industrial centers, destroying and/or appropriating their sources of supply like farms, stock, crops, This did not only created a significant logistical pressure for the field troops but deprived the local populations of resources, even far in the rear. Therefore the battlefield was no longer limited to the front line where the troops clashed but to the rear including civilians although not directly attacked. This should have impacted the will to resist as war was no longer seen from the distant and secure rear waiting for news from the front, now the front was at their home gates. In the case of the Civil War slave also became the target of Sherman, by freeing them it took away labour & manpower from the Confederacy so even if they try to rebuild the farms, crops etc they would not have the hands to do it.