After his defeat at manassas, general mcdowell was replaced by general: custer mcclellan grant jackson

The answer is General McClellan.  Though he made many reforms in the army, he did not made much progress in terms of military campaigns.  Later he and President Lincoln were at odds with each other that Lincoln relieved him of his command.  It would take several replacements before General Grant was appointed commander of the army thereby leading them to victory.

go up to your profile picture in the top right hand. click on it. then click on edit profile. there you shall see delete account