This is a fairly broad question. But, you can note that the Union had a lot of turnover. Several generals were placed in charge of the Union forces. Winfield Scott is too old and proves ineffective. George McClellan is criticized for being too cautious. Robert E. Lee was unavailable for the Union because he led the Confederate forces. The South did not have the same instability. However, Lee's defeat at Gettysburg proved crippling. No one was there to fill the void. Also, the wounding of Stonewall Jackson took away Lee's "right arm." At the end, Grant proved effective because he was bold and so was Sherman. Grant was able to make progress because he played the numbers game. The Union had more men so Grant racked up heavy casualties for both sides but the casualties had greater consequences for the Confederates. It was sort of like Reagan playing the spending game with the Soviet Union in the 1980's. In both cases, the losing side could not keep pace.