 At the start of the Civil War , McClellan 's knowledge of what was called " big war science " and his railroad experience suggested he might excel at military logistics . This placed him in great demand as the Union mobilized . The governors of Ohio , Pennsylvania , and New York , the three largest states of the Union , actively pursued him to command their states ' militia . Ohio Governor William Dennison was the most persistent , so McClellan was commissioned a major general of volunteers and took command of the Ohio militia on April 23 , 1861 . Unlike some of his fellow Union officers who came from abolitionist families , he was opposed to federal interference with slavery . So some of his Southern colleagues approached him informally about siding with the Confederacy , but he could not accept the concept of secession . 
