They're plan initially failed and Grant was not in full command at the time of the begging of the plan nor did he begin using total war that was Sherman. Although the plan to take the Mississippi worked the initial invasion of Virginia was a failure and only worked after the south lost 2 major battles in the North and saw hard fighting in Virginia as well. Overall the North had more man power and more industry, but the South had veterans and military school graduates thus the strong officer corp of the armies.
